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Surface and interface magnetism in nanostructures and thin films

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Surface and interface magnetism in nanostructures and thin films
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Frey, Natalie A
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Transverse susceptibility
Magnetic nanoparticles
Multilayer thin films
Exchange bias
Magnetic anistropy
Dissertations, Academic -- Physics -- Doctoral -- USF   ( lcsh )
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non-fiction   ( marcgt )

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Summary:
ABSTRACT: Nanostructured systems composed of two or more technologically important materials are useful for device applications and intriguing for the new fundamental physics they may display. Magnetism at the nanoscale is dominated by size and surface effects which combined with other media lead to new spin dynamics and interfacial coupling phenomena. These new properties may prove to be useful for optimizing sensors and devices, increasing storage density for magnetic media, as well as for biomedical applications such as drug delivery, MRI contrast enhancement, and hyperthermia treatment for cancer. In this project we have examined the surface and interface magnetism of composite nanoparticles and multilayer thin films by using conventional DC magnetization and AC susceptibility as well as transverse susceptibility, a method for directly probing the magnetic anisotropy of materials.Au and Fe3O4 synthesized together into three different nanoparticle configurations and ranging in size for 60 nm down to 9nm are used to study how the size, shape, and interfaces affect the most fundamental properties of magnetism in the Au-Fe₃O₄ system. The findings have revealed ways in which the magnetic properties can be enhanced by tuning these parameters. We have shown that by changing the configurations of the Au and Fe₃O₄ particles, exotic behavior can be observed such as a large increase in anisotropy field (HsubscriptK ranging from 435 Oe to 1650 Oe) and the presence of exchange bias. Multilayer thin films have been studied as well which combine the important classes of ferromagnetic and ferroelectric materials. In one case, barium hexaferrite/barium strontium titanate thin films, the anisotropic behavior of the ferromagnet is shown to change due to the introduction of the secondary material.In the other example, CrO₂/Cr₂O₃ bilayers, exchange coupling is observed as Cr₂O₃ is an antiferromagnet as well as a ferroelectric. This coupling is manifest as a uniaxial anisotropy rather than the unidirectional anisotropy associated with exchange biased bilayers. Not only will such multifunctional structures will be useful for technological applications, but the materials properties and configurations can be chosen and tuned to further enhance the desired functional properties.
Thesis:
Dissertation (Ph.D.)--University of South Florida, 2008.
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Includes bibliographical references.
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by Natalie A. Frey.
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Document formatted into pages; contains 157 pages.
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usfldc doi - E14-SFE0002434
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SurfaceandInterfaceMagnetisminNanostructuresandThinFilmsbyNatalieA.FreyAdissertationsubmittedinpartialfulllmentoftherequirementsforthedegreeofDoctorofPhilosophyDepartmentofPhysicsCollegeofArtsandSciencesUniversityofSouthFloridaMajorProfessor:HariharanSrikanth,Ph.D.WeiChen,Ph.D.ArunKumar,Ph.D.ShyamMohapatra,Ph.D.DavidRabson,Ph.D.SarathWitanachchi,Ph.D.DateofApproval:April3,2008Keywords:transversesusceptibility,magneticnanoparticles,multilayerthinlms,exchangebias,magneticanistropycCopyright2008,NatalieA.Frey

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DEDICATIONTomyhusband,Dean.Thankyouforbeingbymysidenomatterwherelifetakesus.

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ACKNOWLEDGMENTSFirstIwouldliketothankDr.HariharanSrikanthforbeingmyadvisorandfriendforthepastsixyears.Iamextremelygratefulfortheunwaveringsupportandpatienceofmyparents,family,friends,andofcoursemylovinghusband.ThankyoutomyIGERTco-advisorsDr.ShyamMohapatraandDr.ArunKumarforworkingwithmeonthisprojectthelasttwoyears,andalsotomyothercommitteemembersDr.WeiChen,Dr.DavidRabson,andDr.SarathWitanachchi.ThankyoutoDr.VinayGuptafortakingtimetochairmydissertationdefense.IwouldliketothankDr.NancyDudneyofOakRidgeNationalLaboratoryandDr.HarryEdelmanofSeagateTechnologyforhostingmeonmytwosummerinternshipsandcontinuingtobeinuentialtome.Thankyoutoallmyco-workersandlabmatesovertheyears,notablyJamesGass,Dr.JeSanders,Dr.PankajPoddar,Dr.RankoHeindl,MarienetteMoralesVega,MelodyMiner,andDr.Manh-HuongPhan.AspecialthankstoDr.SrinathSanyadanamwhoseguidancewhileapartofourlabhasbeeninvaluabletome.Ialsowouldliketoacknowledgemycollaborators,especiallyDr.ArunavaGuptafromtheUniversityofAlabamaMINTCenterandDr.ShouhengSunfromBrownUniversityforprovidingmewithexcellentsamplestomeasureforthisdissertation.ThankyoutoalltheUSFPhysicsDepartmentstafortheirhelpduringmytimeasagraduatestudent.Iamverygratefulfora2-yearNSF-IGERTfellowshipfromtheUSFSKINSprogramthroughNSFIGERTGrantNo.DGE-0221681,andinparticularthankyoutoMr.BernardBatsonforhishelpduringmytwoyearsasanIGERTfellow.

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TABLEOFCONTENTSLISTOFTABLESivLISTOFFIGURESvLISTOFABBREVIATIONSxABSTRACTxiiCHAPTER1INTRODUCTION11.1LiteratureReview41.1.1MagneticNanoparticlesforBiomedicalApplications41.1.2Au-Fe3O4CompositeParticles51.1.3CrO2EpitaxialThinFilmsandBilayerCr2O3ThinFilms61.1.4BariumHexaferrite/BariumStrontiumTitanateMultilayerThinFilms71.2DissertationOutline7CHAPTER2AREVIEWOFMAGNETISMINMATERIALS92.1Nomenclature92.2Diamagnetism102.3Paramagnetism112.4MagneticOrdering112.4.1Ferromagnetism142.4.2Antiferromagnetism162.4.3FerrimagnetismandFerrites162.5MagneticAnisotropy182.5.1MagnetocrystallineAnisotropy182.5.2ShapeAnisotropy202.5.3SurfaceAnisotropy202.6MagneticNanoparticlesandSuperparamagnetism212.7ExchangeCouplinginNanostructures22CHAPTER3MEASUREMENTTECHNIQUES263.1DCMagnetizationMeasurements273.2ACSusceptibility28i

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CHAPTER4TRANSVERSESUSCEPTIBILITY324.1TheoryandHistoricalBackground324.2MeasuringtheTransverseSusceptibilityUsingaTunnelDiodeOscillator38CHAPTER5Fe3O4ANDAu-Fe3O4NANOPARTICLESFORBIOMEDICALAPPLICATIONS455.1Introduction455.1.1TargetedDrugDelivery465.1.2HyperthermiaTreatmentforMalignantCells475.1.3MRIContrastEnhancement495.1.4TransverseSusceptibilityasaBiosensor495.2NanoparticleSynthesis505.3DCMagneticPropertiesofFe3O4andAu-Fe3O4Nanoparticles515.3.1Fe3O4Nanoparticles515.3.2Au-Fe3O4Nanoparticles545.4TransverseSusceptibilityMeasurements565.4.1Fe3O4Nanoparticles565.4.2Au-Fe3O4Nanoparticles585.5NanoparticleTransfection625.6TransverseSusceptibilityMeasurementsofCellswithAu-Fe3O4Nanoparticles645.7Conclusion67CHAPTER6DUMBBELL"ANDFLOWER"Au-Fe3O4COMPOSITENANOPARTICLES696.1Introduction696.2NanoparticleSynthesis706.3DCMagneticMeasurements726.4ExchangeBiasandTrainingEectinFlower-ShapedNanoparticles776.5ACSusceptibilityMeasurements806.6TransverseSusceptibilityMeasurements846.7MemoryEectinFlowerandDumbbellNanoparticles926.8OriginsofEnhancedAnisotropyandExchangeBiasinFlowerNanoparticles956.9Conclusion98CHAPTER7SINGLELAYERCrO2ANDBILAYERCrO2/Cr2O3THINFILMS1007.1Introduction1007.2ThinFilmGrowth1017.3DCMagneticCharacterization1037.4TransverseSusceptibilityMeasurementsforCrO2Films1077.5TransversesusceptibilityMeasurementsforCrO2/Cr2O3Bilayers1117.6OriginsofExchangeCouplinginCrO2/Cr2O3Bilayers1167.7Conclusion120ii

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CHAPTER8BARIUMHEXAFERRITE/BARIUMSTRONTIUMTITANATEMULTILAYERTHINFILMS1218.1Introduction1218.2MultilayerThinFilmGrowth1248.2.1FilmsGrownatOakRidgeNationalLaboratory1248.2.2FilmsGrownattheUniversityofCentralFlorida1258.3MultilayerCharacterization1278.4MagneticPropertiesofBaMandMultilayerThinFilms1288.4.1MagneticPropertiesofFilmsGrownattheUniversityofCentralFlorida1298.4.2MagneticPropertiesofFilmsGrownatOakRidgeNationalLaboratory1308.5CorrelatingtheCoercivitywithMicrostructureinBaMandBaM/BSTOMultilayerFilms1378.6Conclusion138CHAPTER9CONCLUSIONANDFUTUREWORK1399.1MagneticNanoparticlesforBiomedicalApplications1399.2Au-Fe3O4CompositeNanostructures1409.3CrO2EpitaxialThinFilmsandBilayerCrO2/Cr2O3ThinFilms1419.4BaMThinFilmsandBaM/BSTOMultilayerThinFilms142REFERENCES143APPENDICES151AppendixAListofPublications152AppendixBListofConferencePresentations155ABOUTTHEAUTHOREndPageiii

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LISTOFTABLESTable7.1MagneticpropertiesofCrO2lmsatroomtemperatureandlowtemperature109Table7.2MagneticpropertiesofCrO2/Cr2O3bilayerlmsofdierentCr2O3contentmeasuredatroomtemperature116Table7.3MagneticpropertiesofCrO2/Cr2O3bilayerlmsofdierentCr2O3contentmeasuredatlowtemperature116Table8.1RFMagnetronsputteringparametersforBSTO/BaMmultilayersgrownbytheauthoratOakRidgeNationalLaboratory125Table8.2RFMagnetronsputteringparametersforBSTO/BaMmultilayersgrownattheUniversityofCentralFlorida126Table8.3CoercivityvaluesformultilayerlmsgrownatTheUniversityofCen-tralFlorida130Table8.4CoercivityvaluesformultilayerlmsgrownatOakRidgeNationalLaboratory134iv

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LISTOFFIGURESFigure2.1Thespinsinaparamagnetintheabsenceofamagneticeld11Figure2.2Magneticorderinginaferromagnet15Figure2.3DiagramofamagnetizationversuseldM-Hcurveofaferromagneticmaterial16Figure2.4Magneticorderinginanantiferromagnet17Figure2.5Magneticorderinginaferrimagnet17Figure2.6Exchangebiassystemsandassociatedphenomena23Figure2.7Schematicdiagramofthespincongurationofanantiferromagnet/ferromagnetbilayeratdierentstagesi-vofanexchangebiasedhysteresisloop24Figure3.1PhysicalPropertiesMeasurementSystemPPMS26Figure3.2ZeroeldcooledZFCandeldcooledFCmagnetizationversustemperaturecurvesforNiFe2O4nanoparticles28Figure4.1Geometricalconstructofatransversesusceptibilitymeasurement33Figure4.2TheoreticaltransversesusceptibilityTandparallelsusceptibilityPcurvesasafunctionofreducedeldhh=HK/HDCascal-culatedbyAharonietal.36Figure4.3SchematicoftheTDOcircuitandsamplespaceleftandcomputeraideddrawingdrawingoftheinductancecoilwhichservesasthesampleholderright40Figure4.4UnipolartransversesusceptibilityscanofNiFe2O4nanoparticles42Figure4.5DetailedbipolartransversesusceptibilityscanofNiFe2O4nanoparticles42Figure5.1ZeroeldcooledandeldcooledcurvesforFe3O4nanoparticles52Figure5.2ZeroeldcooledandeldcooledcurvesforFe3O4nanoparticlessus-pendedinaparanwaxmatrix53v

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Figure5.3MagnetizationversuseldcurveforFe3O4nanoparticlestakenat300K53Figure5.4MagnetizationversuseldcurveforFe3O4nanoparticlestakenat2K54Figure5.5MagnetizationversuseldcurveforFe3O4nanoparticlessuspendedinparanwaxtakenat2K55Figure5.6ZeroeldcooledandeldcooledcurvesforAu-Fe3O4nanoparticles56Figure5.7MagnetizationversuseldcurveforAu-Fe3O4nanoparticlestakenat300K57Figure5.8MagnetizationversuseldcurveforAu-Fe3O4nanoparticlestakenat2K57Figure5.9LowtemperaturebipolartransversesusceptibilityscanofbareFe3O4notsuspendedinparanwax.59Figure5.10TransversesusceptibilityscanstakenatseveraldierenttemperaturesforFe3O4particles60Figure5.11LowtemperatureKtransversesusceptibilitybipolarscanofFe3O4particles61Figure5.12RoomtemperatureKtransversesusceptibilitybipolarscanofFe3O4particles61Figure5.13TransversesusceptibilityscanstakenatseveraldierenttemperaturesforAu-Fe3O4particles62Figure5.14LowtemperatureKtransversesusceptibilitybipolarscanofAu-Fe3O4particles.63Figure5.15RoomtemperatureKtransversesusceptibilitybipolarscanofFe3O4particles64Figure5.16TEMimageofAu-Fe3O4particlescircledinsideofHEKcells65Figure5.17TransversesusceptibilitymeasurementsofHEKcellswithvaryingcon-centrationsofAu-Fe3O4nanoparticles66Figure6.1TEMimageofadumbbellAu-Fe3O4particle.71Figure6.2TEMimageofaowerAu-Fe3O4nanoparticle.72Figure6.3HighresolutionTEMimageofadumbbellAu-Fe3O4nanoparticle.73Figure6.4ZeroeldcooledandeldcooledcurvesfordumbbellAu-Fe3O4nanoparticles.74vi

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Figure6.5ZeroeldcooledeldcooledcurvesforowerAu-Fe3O4nanoparticles.74Figure6.6Valuesofmagnetizationat2KfortheAu-Fe3O4compositeparticlestakeninthezeroeldcooledconditionfordierentvaluesofH75Figure6.7MagnetizationversuseldcurvefordumbbellAu-Fe3O4nanoparticlestakenat2Kand75K.76Figure6.8MagnetizationversuseldcurveforowerAu-Fe3O4nanoparticlestakenat2K.77Figure6.9MagnetizationversuseldcurveforowerAu-Fe3O4nanoparticlesaf-tercoolingina5Teslaeld79Figure6.10HEandHCasafunctionoftemperatureforowerAu-Fe3O4nanopar-ticlesaftercoolingina5Teslaeld80Figure6.11Trainingeectinower-shapedAu-Fe3O4nanoparticles81Figure6.12ACsusceptibilitydatafordumbbellAu-Fe3O4nanoparticles82Figure6.13ACsusceptibilitydataforowerAu-Fe3O4nanoparticles83Figure6.14ACsusceptibilitydatattedtotheNeel-Arrheniusmodel85Figure6.15ACsusceptibilitydatattedtotheVogel-Fulchermodel86Figure6.16UnipolartransversesusceptibilitycurvesofthedumbbellAu-Fe3O4nanoparticlestakenatseveraldierenttemperatures87Figure6.17LoweldportionofatransversesusceptibilityscanofthedumbbellAu-Fe3O4nanoparticlestakenat30K88Figure6.18UnipolartransversesusceptibilitycurvesoftheowerAu-Fe3O4nanopar-ticlestakenatseveraldierenttemperatures89Figure6.19LoweldportionofatransversesusceptibilityscanoftheowerAu-Fe3O4nanoparticlestakenat30K90Figure6.20ValuesofHK1andHK2asafunctionoftemperatureforthedumb-bellAu-Fe3O4nanoparticles91Figure6.21ValuesofHK1andHK2asafunctionoftemperaturefortheowerAu-Fe3O4nanoparticles91Figure6.22ZeroeldcooledandeldcooledtransversesusceptibilitycurvesfortheowerAu-Fe3O4nanoparticlestakenat30K93Figure6.23MemoryeectindumbbellAu-Fe3O4nanoparticles94vii

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Figure6.24MemoryeectinowerAu-Fe3O4nanoparticles95Figure6.25SchematicofpossibleinteractionsinowerAu-Fe3O4leadingtoanoma-lousbehavior97Figure7.1Cross-sectionalhighresolutionSTEMmicrographofheteroepitaxialCrO2/Cr2O3bilayer102Figure7.2Hysteresisloopsof21.5nmthickCrO2lm104Figure7.3Hysteresisloopsof725nmthickCrO2lm105Figure7.4HysteresisloopsforCrO2/Cr2O3bilayersofvaryingCrO2content.In-setshowsthehysteresisloopsforthe100%200nmCrO2lmtakenalongthebandcaxes.106Figure7.5Variationofcoercivityasafunctionof%CrO2inCrO2/Cr2O3bilayerlms107Figure7.6UnipolartransversesusceptibilitydataforCrO2lmsofvaryingthick-nessestakenatroomtemperature108Figure7.7UnipolartransversesusceptibilitydataforCrO2lmsofvaryingthick-nessestakenat10K109Figure7.8TemperaturedependenceofHKforvariousthicknessesofCrO2110Figure7.9Unipolartransversesusceptibilityscansof200nmCrO2lmforseveraltemperaturesshowinganisotropypeaksemergingasthetemperatureisdecreased112Figure7.10RoomtemperatureunipolartransversesusceptibilityscansofCrO2/Cr2O3bilayersfordierentCrO2percentages114Figure7.11KeversusCrO2thicknessforthelmsstudiedhereandinref[55]115Figure7.12HKversustemperaturefortheCrO2/Cr2O3bilayers117Figure7.13LowtemperatureKunipolartransversesusceptibilityscanof64%CrO2bilayer117Figure7.14Schematicofspin-opcouplingmodel119Figure8.1CrystalstructureofBariumHexaferrite122Figure8.2CrystalstructureofBariumStrontiumTitanate123Figure8.3X-raydiractionscanofBaM/BSTOmultilayersgrownonAl2O3atOakRidgeNationalLaboratory126viii

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Figure8.4X-raydiractionscanofBaM/BSTOmultilayersgrownonAl2O3attheUniversityofCentralFlorida127Figure8.5Cross-sectionalimageofaSi/SiO2/BSTO/BaM2multilayerlmafterannealing128Figure8.6SEMimageoftheBaMsurfaceofamultilayerlm129Figure8.710Kand300KhysteresisloopsforBaMonAl2O3grownattheUni-versityofCentralFlorida131Figure8.810Kand300KhysteresisloopsformultilayersonAl2O3grownattheUniversityofCentralFlorida132Figure8.910Kand300KhysteresisloopsformultilayersonAl2O3grownatOakRidgeNationalLaboratory133Figure8.1010Kand300KhysteresisloopsformultilayersonSigrownatOakRidgeNationalLaboratory133Figure8.1110KhysteresisloopsformultilayersonSitakenwithHin-planeandout-of-plane135Figure8.1210KhysteresisloopsformultilayersonAl2O3takenwithHin-planeandout-of-plane136ix

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LISTOFABBREVIATIONSACAlternatingCurrentAuGoldBMagneticInductanceBaMBarium-Iron-Oxide,Barium-Hexa-Ferrite,BaFe12O19BSTOBarium-Strontium-TitanateBa0.5Sr0.5TiO3DCDirectCurrentemuMagneticMomentUnitElectromagneticUnitFe3O4MagnetiteHMagneticFieldHKAnisotropyFieldHCCoerciveFieldCoercivityJJouleKKelvinMDC-MagnetizationMRRemanentMagnetizationMSSaturationMagnetizationM-HMagnetizationvs.AppliedMagneticFieldOeOerstedPPMSPhysicalPropertiesMeasurementSystemRFRadio-FrequencyssecondSEMScanningElectronMicroscopex

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TTeslaTEMTransmissionElectronMicroscopeXRDX-RayDiractionTSTransverseSusceptibilityxi

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SURFACEANDINTERFACEMAGNETISMINNANOSTRUCTURESANDTHINFILMSNatalieA.FreyABSTRACTNanostructuredsystemscomposedoftwoormoretechnologicallyimportantmaterialsareusefulfordeviceapplicationsandintriguingforthenewfundamentalphysicstheymaydisplay.Magnetismatthenanoscaleisdominatedbysizeandsurfaceeectswhichcom-binedwithothermedialeadtonewspindynamicsandinterfacialcouplingphenomena.Thesenewpropertiesmayprovetobeusefulforoptimizingsensorsanddevices,increasingstoragedensityformagneticmedia,aswellasforbiomedicalapplicationssuchasdrugde-livery,MRIcontrastenhancement,andhyperthermiatreatmentforcancer.Inthisprojectwehaveexaminedthesurfaceandinterfacemagnetismofcompositenanoparticlesandmul-tilayerthinlmsbyusingconventionalDCmagnetizationandACsusceptibilityaswellastransversesusceptibility,amethodfordirectlyprobingthemagneticanisotropyofmate-rials.AuandFe3O4synthesizedtogetherintothreedierentnanoparticlecongurationsandranginginsizefor60nmdownto9nmareusedtostudyhowthesize,shape,andinter-facesaectthemostfundamentalpropertiesofmagnetismintheAu-Fe3O4system.Thendingshaverevealedwaysinwhichthemagneticpropertiescanbeenhancedbytuningtheseparameters.WehaveshownthatbychangingthecongurationsoftheAuandFe3O4particles,exoticbehaviorcanbeobservedsuchasalargeincreaseinanisotropyeldHKrangingfrom435Oeto1650Oeandthepresenceofexchangebias.Multilayerthinlmshavebeenstudiedaswellwhichcombinetheimportantclassesofferromagneticandfer-roelectricmaterials.Inonecase,bariumhexaferrite/bariumstrontiumtitanatethinlms,theanisotropicbehavioroftheferromagnetisshowntochangeduetotheintroductionofxii

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thesecondarymaterial.Intheotherexample,CrO2/Cr2O3bilayers,exchangecouplingisobservedasCr2O3isanantiferromagnetaswellasaferroelectric.Thiscouplingismanifestasauniaxialanisotropyratherthantheunidirectionalanisotropyassociatedwithexchangebiasedbilayers.Notonlywillsuchmultifunctionalstructureswillbeusefulfortechnologicalapplications,butthematerialspropertiesandcongurationscanbechosenandtunedtofurtherenhancethedesiredfunctionalproperties.xiii

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CHAPTER1INTRODUCTIONNanostructuredsystemscontainingtwoormoretechnologicallyimportantmaterials,referredtoasmultifunctional"materials,arebecomingincreasinglypopularasthetrendtowardsdeviceandsensorminiaturizationcontinues.Spatialconstraintsarepromptingscientistsandengineerstothinkcreativelyaboutbringingmultifunctionalitytonewelec-troniccomponentsaswellastoexploitthepropertiesthatoccurasaconsequenceoftheinterfacesthatareformedbythedirectcontactofdierentmaterials.Inmagneticmaterials,shrinkingoneormoredimensionstothenanoscalehasprofoundimplications.Fornanoparticleswithdiameterstoosmalltoformmagneticdomains,ther-maluctuationscanleadtodestabilizationofthemagneticstate.Thishasforcedtheharddriveindustrytolookatwaysofincreasingthemagneticanisotropyofsmallparticlessothatstoragedensitiescancontinuetoincreasewithoutcompromisingthestabilityofthemedia.ThemoststraightforwardmethodofincreasingthemagneticanisotropyistousematerialswithexceedinglyhighmagnetocrystallineanisotropysuchastheL10phaseofFePt.Whileprogressinmakingmonodisperse,smallparticleswithuniformmagneticpropertieshasbeenslow,peopleareturningtoothermethodsofincreasinganisotropysuchasusingexchangecouplednanostructuresorcapitalizingonshapeandsurfaceanisotropy.Whilesuperparamagnetismhasbecomeahinderanceintheharddriveindustry,biomed-icalengineershaveembracedthenotionofhigh-response,remanence-freeparticlesforexter-nalmanipulationafterinjectionintothehumanbody.Applicationsforsuchnanoparticlesincludetargeteddrugdelivery,MRIcontrastenhancementandlocalizedheatingforkillingcancerouscells.Fortheseapplications,thesizeoftheparticlemustcoincidewiththeabil-ityofcertaincellstoallowpassagethroughthecellmembrane.Sincethissizeisdependent1

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uponthepartofthebodytargeted,theabilitytotuneparticlesizeiscrucial.Forsomeoftheseapplications,notablyhyperthermia,tuningthemagneticpropertiessuchasblockingtemperatureandfrequencyaswellascoercivityintheferromagneticstatebecomesim-portantaswell.Providingbetterfunctionalitythroughtheuseofametalcoatingallowsforalargervarietyofmoleculestobedeliveredtospecicsitesandcanalsominimizeinter-particleinteractions.Inmagneticnanoparticles,theabilitytotunetheanisotropytomeettheneedsofvastlydierentapplicationsisachallenge.Inthisdissertation,itisdemonstratedthatthesametwomaterials,namelyAuandFe3O4,canbesynthesizedindierentcongurationstoachieveabroadrangeofmagneticcharacteristics.Au-Fe3O4inthecore-shellcongura-tionisextremelyusefulforbiomedicalapplicationsbecausetheFe3O4componentcanbemanipulatedbyexternalmagneticeldsandtheAusurfacecanbeusedfortheattachmentofavastarrayofbiologicalmolecules.Here,weshowhowthesizeoftheparticlescanbemanipulatedtomeettherequirementsforbiocompatibilitywhilesimultaneouslyoptimiz-ingthepropertiesforcertainbiomedicalapplications.Thiscanbedonebymakingsurethecompetitionbetweentheanisotropyenergyoftheparticleandthethermalenergynec-essarytodemagnetizetheparticlestrikeaprecariousbalancewhichallowsonetousethefrequencyofthemagneticeldtoswitchbetweenthemagneticstates.Weshowhowthismaximizesthefunctionalityoftheparticleforseveralapplicationsincludingdrugdeliveryandhyperthermia.Furthermore,wedemonstratehowsimplychangingthearrangementoftheAuandtheFe3O4candrasticallyalterthefunctionalityoftheparticleandgiverisetonovelmag-neticbehavior.WhentheFe3O4isgrownonthesurfaceofanAuparticle,theresultisananostructuremadeoftwoparticlessharingoneinterfacedumbbell"particle.ThecompositeparticlethenhasgreaterversatilityastheFe3O4canstillbemanipulatedexter-nally,butnowbothsurfacesareavailableforfunctionalization.WhentheFe3O4isallowedtogrowonmultiplefacetsoftheAuparticlesviaslightlydierentsynthesisconditions,adierenttypeofcompositeparticleisformedower"particle.ThisFe3O4cluster-type2

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geometryresultsinsurprisingmagneticpropertiesincludingexchangebias,trainingeect,andanomalousrelaxationbehavior.Ourstudiespointtocomplexandcompetinginterac-tionsbetweentheFe3O4particlessharingthesameAuparticle.Thesignicantincreaseinanisotropyachievableintheseparticlesindicatesthatacluster-typegeometryofeasilysynthesizednanoparticlescouldbeaviablepathtobeatingthesuperparamagneticlimitinmagneticrecording.Inmagneticthinlmsystems,asinnanoparticles,themagneticpropertiesaregreatlyinuencedbysurfaceandinterfaceeects.Thedierencesinmagnetismbetweenthinlmsandbulkmaterialsareconsequencesoflmthickness,substratematerialandinterfaceswithotherlayersifthesystemisabilayerormultilayer.Ineachofthesecases,theanisotropyofthelmisaectedbyinterfacialstrainaswellasexchangecouplingtotheothermaterialspresent.Usingexchangecouplingtopinmagneticlayersortoincreasethecoercivityhashadalargeimpactondeviceapplicationsfrommagneticreadandwriteheadstospinvalvesandsensors.Combiningtheeectsofexchangecouplingwithmagnetoelectricormultiferroicmaterialshasonlyrecentlybeguntobeexplored[9]andwilllikelyplayaroleaspeoplecontinuetomaximizefunctionalityinnanostructures.Theinterfacesformedbetweenmagneticthinlmshavebeenexploredformanyyears,althoughsurprisinglymagnetismintheepitaxialinterfacebetweenCrO2anditsnativeoxideCr2O3hasnotearnedmuchattention.Thesetwomaterialsareinterestingtechnolog-icallyduetoCrO2beingaspin-polarizedferromagnetandCr2O3beingamagnetoelectricantiferromagnet.Inthisdissertationwepresentevidenceforinterfacialcouplingbetweenthetwomaterials,whichismanifestinanenhancedanisotropymeasuredusingtransversesusceptibility.Theanomalousanisotropyobservedinthissystemislikelyduetocontribu-tionsfromexchangecouplingbetweentheferromagneticandantiferromagneticphasesaswellasstrainpresentfrombothCrO2interfaces.Wealsoexaminehowthemagneticpropertiesofpolycrystallinebariumhexaferrite,amagneticmaterialpossessingveryhighmagnetocrystallineanisotropy,changewhengrownasamultilayerwithbariumstrontiumtitanate,aferroelectric.Wedemonstratehow3

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thefunctionaldependenceofthecoercivitywithrespecttotemperatureisalteredbythepresenceofthebariumstrontiumtitanate,whichcouldbeusefulfortuningtheanisotropyofbariumhexaferriteinmultilayerthinlms.Inallofthesystemsdescribedabove,itisclearthatasolidunderstandingofthemagneticanisotropyfromallcontributionsmustbereachedbeforematerialscanachievetheirfullpotential.Thismeansmeasuringtheeectiveanisotropyofthesystemandguringouthowthesurfacesandinterfacesofnanoscalematerialsmayaecttheresult.Transversesusceptibilityhasbeenshownovertheyearstobeanexcellentmethodofmeasuringtheanisotropyinsystemsfromthinlmstonanoparticles[79,12,66,67].Itisadirectmeasureoftheanisotropyeldinasampleandcanbeusedtoextracttheeectiveanisotropyconstant.Whilethusfarminimalworkhasbeendoneusingtransversesusceptibilitytomeasuretheexchangecouplinginmultilayerthinlms[80],oneofthemostimportantoutcomesofthisdissertationistoshowthatitisquiteavaluabletechniqueforunderstandingthecomplexmagneticbehaviorexhibitedinmultifunctionalmaterials.1.1LiteratureReview1.1.1MagneticNanoparticlesforBiomedicalApplicationsPankhurstetal.[62]andBerryetal.[7]providedrecenttopicalreviewsofapplicationsofmagneticnanoparticlesinbiomedicinethoughbothworksstoppedshortofinvokingthespecialneedforne-tuningthesizeandmagneticpropertiesofnanoparticles.WhilefunctionalizationofFe3O4wasdiscussed,theuseofAuasacoatingwasnotcoveredindetail.Sincethen,therehaveappearedseveralarticlesoutliningthesynthesisandcharacterizationofAu-Fe3O4nanoparticlesforbiomedicalapplicationsincludingbutnotlimitedtoMandaletal.[49],Lyonetal.[46],Gangopadhyayetal.[20],andLuetal.[44].ThesestudiesfocusmainlyonthesynthesisandDCpropertiesbutfailtodirectlyexaminehowtheAu-coatingreducedinter-particleinteractions.Inchapter5weprovidedirectevidenceofthedecreaseininter-particleinteractionswhenAuisusedasacoating.4

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Oneofthesestudies[20]proposedAuasacatalystforlaser-inducedhyperthermia.HyperthemiausingjustFe3O4or-Fe2O3nanoparticleshasreceivedattentioninseveralpublications[5,20,61,17].ThesepublicationsaddresstheneedtotapintobothrelaxationprocessesNeelandBrownian,butonlytherecentlypublishedEggemanetal.[17]suggeststuningtheblockingfrequencytoutilizehystereticlossesinDCsuperparamagneticparticles.WhileweareunabletomeasuretheAChysteresisloopsofourAu-Fe3O4particles,weshowedthattheseparticles,whilesuperparamagneticinDCelds,displayatransversesusceptibilitysignalconsistentwithferromagneticparticles.ThiswouldimplythatinACeldsofhighenoughfrequency,hystereticlossescancontributetoparticleheatinginoursamples.1.1.2Au-Fe3O4CompositeParticlesThesynthesisofdumbbell"-shapedAu-Fe3O4particleswasreportedbyourcollab-oratorsatBrownUniversity[93]forthepurposeofusingtheAuandFe3O4surfacessimultaneouslyforbiomedicalapplications.Toourknowledgethisistheonlysuchin-stanceofsuggestingacompositeparticlewithmorethanonesurfacefordrugdelivery.Thissamepaperalsoreferencedthesynthesisofnovelower"-shapedAu-Fe3O4parti-clesastheresultofslightchangesinsolventpH.Inthisdissertationweperformawidearrayofmagneticmeasurementsontheseparticlestocompareandcontrasthowthege-ometricalcongurationandinterfacesaectthemagneticproperties.Thesedierencesresultinfascinatingmagneticpropertiesassociatedwiththeowerparticles,whichcanbeattributedtoincreasedintra-particleinteractionandmagneticfrustration.ThoughtheAuatthecenteroftheseclustersmaybediculttofunctionalizeforsomebiomedicalapplications,theanisotropyassociatedwiththeseparticlesandtheabilitytotunethesizeofeachcomponentcouldmakethemattractiveforhyperthermia.Theincreasedanisotropyoftheowerparticlescouldalsobeviewedinlightofndingwaystoovercomethesuperparamagneticlimitinmagneticrecording.Thediscussionofnovel-shapedparticlesandtheirsurfacesforthispurposeisfairlylimited,thoughAlbrecht5

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etal.[3,48]havemadesomerecentadvancesinthisregardsbycappingpolystyrenenanoparticleswithCo/Ptmultilayerssothatthemagneticnanocapshavenovelshapeandinterfaceanisotropy.Otherlithographicallypatternedexchange-biasednanostructuresaredescribedinNoguesetal.'stopicalreviewofexchangebiasinnanostructures[60].Whilesuchpatternedmediabylithographyisgettingattentionformagneticrecording,chemicalsynthesisofnovelanisotropicparticlesshouldnotberuledoutbecauseofitseaseandcost-eectiveness.Fromafundamentalphysicspointofview,surfaceanisotropyinnanoparticleshasbeenexploredinseveralpublications,mostnotablyKodamaetal.'sworkonNiFe2O4[38].Sincethen,theissuesofexchangebiasfromsurfacedisorderhasbeenexploredforFe3O4[92,22]and-Fe2O3aswellasmodeledbyKachkachiandDimian[37]andBdkeretal.[8].However,exchangebiasbehaviorincluster-typenanoparticleswithcompetinginteractionscontributingtotheeectiveanisotropyincludingspinfrustrationandmultipleinterfacesisatopicthathasnotyetbeenaddressedinthemagnetismcommunity.1.1.3CrO2EpitaxialThinFilmsandBilayerCr2O3ThinFilmsExchangebiasinthinlmshasbeenobservedinseveralsystemsdatingbackover50yearstoMeiklejohnandBean[53],althoughthereisstillnocomprehensivetheorythatcanexplainalloftheaspectsassociatedwithit.ThephenomenonofauniaxialexchangeanisotropywithoutaunidirectionalanisotropybetweenanantiferromagnetandaferromagnethasbeenreportedinrecentyearsbyLeightonetal.[42,41],andthespin-opexplanationforitwasproposedbyShulthessandButler[72].TheCrO2/Cr2O3systemitselfhasbeenstudiedonlyminimallyinthinlmformforthepurposeofmeasuringtheoxidelayerasatunnelbarrier[14]andinnanoparticleformforexchangebiaseects[95].TheclosestcomparableworktotheCrO2/Cr2O3systemisthe[Co/Pt]5/Cr2O3bilayersystempublishedbyBorisovetal.[9],inwhichthemagnetoelectricpropertiesofCr2O3wereusedtoswitchthesignoftheexchangebiasofthemultilayers.ExchangecouplingbetweenCrO2andCr2O3inthinlmformhasnotbeenreported,andouruse6

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oftransversesusceptibilitytomeasurethissystemisacontributiontotheunderstandingofuniaxialexchangeanisotropyinferromagnetic/antiferromagneticbilayersandtherstsuchmeasurementofaferromagnetic/magnetoelectricbilayer.1.1.4BariumHexaferrite/BariumStrontiumTitanateMultilayerThinFilmsFinally,mostoftheworkthathasbeendoneontheBaM/BSTOsystemhasbeenreportedsincethecompletionoftheauthor'smaster'sthesis.ThishascomefromworkdonebyDr.RankoHeindl,whoreceivedhisPh.D.asaresultofhisworkonepitaxialBaM/BSTObilayers[29].Hewasabletodemonstratethedualtunabilityofthepermittivityandpermeabilityofthissystemforradiofrequencyapplications.ThegrowthoptimizationandmagneticcharacterizationofpolycrystallineBaM/BSTOmultilayerlmswasanimportantadvancementtowardsthehigh-qualitylmsthathavebeengrownsincethisworkwasoriginallyperformedandpublished.1.2DissertationOutlineChaptertwogivesabriefoverviewofmagnetisminmaterialswhichservesasafoundationforthemagneticpropertiesdiscussedthroughoutthedissertation.Be-sidesaddressingthemaintypesofmagnetismfoundinmaterialswediscussthephenomenonofsuperparamagnetismaswellastheexchangecouplingthatcanexistattheinterfaceofferromagneticandantiferromagneticmaterials.Chaptersthreeandfourdescribethemeasurementtechniquesusedinthiswork.Chapterthreediscussesthetraditionalmeasurementtechniquesusedinthemag-netismcommunity.Chapterfourdiscussesthemethodoftransversesusceptibility,alesser-knowntechniquewhichisadirectmeasurementoftheanisotropyeldofamaterial.Specialattentionispaidtothismethodasitisanintegralmeasurementusedforthedissertation.7

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ChaptersveandsixcontainmeasurementresultsforAu-Fe3O4nanoparticlesindierentcongurations.Chapterveisdedicatedspecicallytocore-shellAu-Fe3O4nanoparticlesforbiomedicalapplicationsandincludesworkdoneinconjunctionwiththeUSFCollegeofMedicineinfulllmentoftherequirementsoftheNationalSci-enceFoundationIntegratedGraduateEducationResearchandTraineeshipIGERTFellowship.ChaptersixexplorestwoothergeometricalcongurationsoftheAu-Fe3O4system,thedumbbell"andower"congurationwhichareinterestingforfundamentalphysicsandavarietyoftechnologicalapplicationsfrombiomedicinetomagneticstorage.ChaptersevenfocusesonthemagneticanisotropyofCrO2/Cr2O3thinlms,amul-tifunctionalsystemcomposedofaferromagnetandanantiferromagnetwhichisalsoamagnetoelectricmaterial.ThischapterdiscussesindetailthepropertiesofCrO2singlelayerthinlmsaswell.Chaptereightisasummaryoftheauthor'sworkforhermaster'sthesiswhichisthegrowthandcharacterizationofbariumhexaferrite/bariumstrontiumtitanatemultilayerthinlms.Chapternineconcludesthedissertationandproposesnewdirectionsforthefutureofthisresearch.8

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CHAPTER2AREVIEWOFMAGNETISMINMATERIALS2.1NomenclatureEverymaterialwhichissubjecttoamagneticeld,H,acquiresamagneticmoment.Themagneticmomentofanatomhasthreesources:electronspin,electronorbitalmomen-tumaboutthenucleus,andthechangeintheorbitalmomentuminducedbyanappliedmagneticeld.Thedipolemomentperunitvolumeisdenedasthemagnetization,M.Inmostmaterials,MisproportionaltotheappliedeldH,suchthatM=H.1whereisthemagneticsusceptibility.Thesusceptibilityindicateshowresponsiveama-terialistoanappliedmagneticeld.ThemagneticinductionisusuallywhatispresentedinMaxwell'sequationsandisrelatedtoMandHbythefollowingB=0H+M.2where0ispermeabilityoffreespace.Hereisaaconstantthatdependsuponwhichsystemofunitsisbeingused.InSIunits,=1,inGaussianandcgsunits,=4.Thisdissertationwillusecgsunitsforwhich0=1.Bisthesameasthedensityofmagneticuxinsidethematerial.IftherelationshipbetweenMandHislinear,thenBcanalsobewrittenasB=H.39

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where=0+=0r.4isknownasthemagneticpermeability.FormaterialsthathavealinearrelationshipbetweenMandH,theirmagneticbehaviorcanbeclassiedintermsofandr.However,somematerialsdonothavealinearrelationshipbetweenMandH,andinfactMmaynotevenbeasingle-valuedfunctionofHbutmayinsteaddependonthehistoryoftheappliedeld.Thispointwillbediscussedmuchmoreextensivelyinlatersections.2.2DiamagnetismAllmaterialsexhibitdiamagnetism.Itisaveryweakeect,andifthisistheonlymag-neticresponsetheydisplay,theyarereferredtoasdiamagnets.Indiamagneticmaterials,<0andr<1.Thisindicatesthatthemagneticuxinsideofadiamagnetislessthantheuxoutside,andthemagnetizationofadiamagnetdecreasesinmagnitudeinresponsetotheappliedeld.Diamagnetismarisesfromthechangeinorbitalmotionofelectronsinamaterialinresponsetoanappliedeld.Whileorbitalmotionofelectronscanonlybecorrectlyde-scribedusingquantummechanics,diamagnetismisoftenderivedusingasemi-classicalapproachwhichyieldsthesameresult.Ifweconsideranelectronmovinginacircularorbit,itfeelsacentripetalforceduetotheCoulombattractiontothenucleus.WecanexplaindiamagnetismasoccurringfromthechangeinvelocityoftheelectronthatarisesoncetheLorentzforcefromthemagneticeld,e=cvB,isaddedtothethecentripetalforcealreadyactingontheelectron.Thisforcedecreasesthevelocity,whichinturnde-creasesthecurrentcausedbytheorbitingcharge,andtheresultisadecreasedmagneticmoment.Itisthedecreaseinmagneticmomentthatisobservedasthediamagneticeect.Thisexplanationissemi-classicalinthatitassumesacircularorbitusingtheresultsof10

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quantummechanics,whichexplainhowelectronsareabletoeectivelymoveaboutthenucleus.[1]2.3ParamagnetismWhilediamagnetismisaresultofelectronorbitalmotion,paramagnetismistheresultofelectronspinsinteractingwithanappliedmagneticeld.Paramagnetismischaracterizedbyapositivesusceptibility>0andamagneticpermeabilityrgreaterthan1.Itcorrespondstothemagneticbehaviorfoundinmaterialsinwhichlocalizedmagneticmomentsarepresentbutinwhichnonetmacroscopicmagnetizationexistsinzeroappliedeld.Thisisbecausethemagneticmomentsareonlyweaklycoupledtoeachother,sothermalenergycausesrandomalignmentofthemoments.Whenamagneticeldisapplied,themomentsstarttoalign,butonlyasmallfractionisdeectedintotheelddirectionforpracticaleldstrengths.Aschematicofthemagneticspinsinaparamagneticmaterialisshowningure2.1. Figure2.1.Thespinsinaparamagnetintheabsenceofamagneticeld.Figureadaptedfromreference[30].2.4MagneticOrderingMagneticorderingcomesaboutwhenspinsoftheatomsinamaterialareallowedtointeractwithoneanotherviaexchangeinteractions.Theseinteractionsoccurbetweenthe11

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spinsoftheionsatthelatticesitesandarecausedbytheoverlapoftheelectronicwavefunctions.Ifweconsiderasimplemodelwithtwoelectronswhichhavespatialcoordinatesr1andr2,thenthewavefunctionforthesystemcanbewrittenasaproductofthesingleelectronstates.Iftherstelectronisinstatear1andthesecondelectronstateisinstatebr2,thenthejointwavefunctionisar1br2.Thisproductstatedoesnotobeyexchangesymmetrysinceinterchangingthetwoelectronsyieldsar2br1whichisnotamultipleoftheoriginalwavefunction.Theonlystatesallowedaresymmetrizedorantisymmetrizedproductstates.ForelectronstheoverallwavefunctionmustbeantisymmetricsothespinpartofthewavefunctionmusteitherbeanantisymmetricsingletstateSS=0orasymmetrictripletstateTS=1.ForthewavefunctioninthesingletcaseS=1 p 2[ar1br2+ar2br1]S.5andinthetripletcaseT=1 p 2[ar1br2)]TJ/F21 10.909 Tf 10.909 0 Td[(ar2br1]T.6TheenergiesofthetwopossiblestatesareES=ZSHSdr1dr2.7andET=ZTHTdr1dr2.8TheenergydierencebetweenthestatescanthenbeexpressedasES)]TJ/F21 10.909 Tf 10.909 0 Td[(ET=2Zar1br2Har2br1dr1dr2.912

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TheHamiltonianoftwointeractingspinsisusuallyexpressedasS1S2.10whereS1andS2arethespinoperators.ForthesingletstateS1S2=)]TJ/F15 10.909 Tf 8.485 0 Td[(3=4whileforthetripletstateS1S2=1=4.TheHamiltoniancanbewrittenasan'eectiveHamiltonian'H=1 4ES+3ET)]TJ/F15 10.909 Tf 10.909 0 Td[(ES)]TJ/F21 10.909 Tf 10.909 0 Td[(ETS1S2.11Thersttermisaconstant,butthesecondtermappearsinequation2.9.Theintegralpartofequation2.9istheexchangeintegralandcanbedenedasJ=ES)]TJ/F21 10.909 Tf 10.909 0 Td[(ET 2=Zar1br2Har2br1dr1dr2.12Sothespin-dependenttermintheeectiveHamiltoniancanbewrittenasHspin=)]TJ/F15 10.909 Tf 8.485 0 Td[(2JS1S2.13IfJ>0,ES>ET,andthetripletstateS=1isfavored.IfJ<0,ES
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Apositiveexchangeinteractioncausesspinstoalignparalleltoeachother,leadingtoferromagnetism.Anegativeexchangeinteractioncausesspinstoalignantiparalleltoeachotherandleadstoantiferromagnetism.Theseexchangeinteractionsarewhatcausespon-taneousmagneticmomentsinmaterialswhichinturnresultinthecomplicatednonlinearandmulti-valuedrelationshipsbetweenMandH.2.4.1FerromagnetismWhentheexchangeinteractionbetweentwoneighboringspinsispositive,theyaligninthesamedirectionregardlessofwhetherthereisanexternaleldornot,andthisgivesrisetoaspontaneousmagnetizationgure2.2.Thistypeoflong-rangemagneticorderingisknownasferromagnetism.However,mostmacroscopicferromagneticsamplesarenotmagnetizedpossessinganetmagnetizationunlessanexternalmagneticeldisapplied.Thisisbecauseitismoreenergeticallyfavorableforthecoupledspinstobreakintomagneticdomains,orsmallregionswherethespinsarealigned.Eachdomaincanhaveamagneticmomentwhichisorientedinadierentdirectionfromthatofitsneighbor.Whiletheexchangeenergyfavorsthealignmentofneighboringspins,themagnetostaticenergyishighestandthereforeunfavorablewhenamacroscopicsamplehasallofitsspinsaligned.Thus,thelowestenergystateofaferromagnetisonemadeupofdomainswhosemagnetizationvectorsarepointingindierentdirectionsandthedomainsthemselvesaremadeofstronglycoupledspinsalignedinthesamedirection.Applyinganexternalmagneticeldtoaferromagnetcausesthedomainstoorientinthesamedirectionandanetmagnetizationtoarise.Thisresponseisnonlinearandisoneofthemaincharacteristicsofferromagnets.Themagnetizationrapidlyincreaseswithappliedelduntiltheferromagnetissaturated,i.e.thedomainsareallalignedinthedirectionofthemagneticeld.ThismaximummagnetizationinresponsetoanappliedeldisreferredtoasthesaturationmagnetizationMS.Whentheeldisdecreasedbacktozero,themagnetizationasafunctionofelddoesnotfollowthesamepathasitdidwithincreasingeld.Infact,whentheeldgoesallthewaybacktozero,there14

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Figure2.2.Magneticorderinginaferromagnet.Figureadaptedfromreference[30].remainsanetmagneticmomentcalledtheremanentmagnetization,MR.Inorderforthematerialtoreturnbacktozeronetmagneticmoment,i.e.themomentsofthedomainstoberandomized,amagneticeldneedstobeappliedintheoppositedirection.Themagneticeldneededtoaccomplishthisiscalledthecoerciveeld,HC,orcoercivity.Thecoercivityofamaterialisaveryimportantpropertywhosevalueisheavilydependentonthecrystalstructureandgrowthconditionsofthesample.Thesefactorswillbediscussedfurtherinsection2.5.Iftheeldisdecreasedevenfurther,thedomainswillreorientintheoppositedirectionandalignatthenegativesaturationmagnetization.Thereisalsoanegativeremanentmagnetizationandcoerciveeldwhenbringingtheappliedeldbacktozeroanduponincreasingitagaininthepositivedirection.OnefullcycleofthismagnetizationversuseldfromzerotopositivesaturationtonegativesaturationandbacktopositivesaturationiscalledanM-Hcurveorahysteresisloop.ItispresentedschematicallylabeledwithMS,MRandHCingure2.3.FerromagnetsundergoaphasetransitionatacriticaltemperaturecalledtheCurietemperature,TC.Thistemperatureisdierentforeverymaterialanddependsuponthestrengthoftheexchangeinteractionbetweenspins.Whenthethermalenergyishighenoughtoovercometheexchangeenergy,thespinsarenolongercoupledtoeachotheroveralongrange,andthematerialeectivelybecomesparamagnetic.15

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Figure2.3.DiagramofamagnetizationversuseldM-Hcurveofaferromagneticmate-rial.ThecharacteristicstonotearesaturationmagnetizationMS,remanentmagnetiza-tionMRandcoercivityHC.Figureadaptedfromreference[28].2.4.2AntiferromagnetismInantiferromagneticmaterials,theexchangeinteractionisnegativeandalignsthespinsantiparalleltoeachother.Antiferromagnetscanbethoughtofascontainingtwointerpenetratingandidenticalsublatticesofmagneticions.AlthoughonesetofmagneticionsisspontaneouslymagnetizedbelowsomecriticaltemperatureinthiscasecalledtheNeeltemperaturethesecondsetisspontaneouslymagnetizedbythesameamountintheoppositedirectiongure2.4.Asaresult,antiferromagnetshavenonetmagnetization,andtheirresponsetoexternaleldsissimilartothatofparamagneticmaterials-themagnetizationislinearintheappliedeldandthesusceptibilityissmallandpositive.2.4.3FerrimagnetismandFerritesFerrimagnetsareaspecialclassofantiferromagnetsinthattheexchangecouplingbetweenadjacentmagneticionsleadstoantiparallelalignmentbuttheyhaveanetspon-taneousmagneticmomentsimilartoferromagnets.Thisnetmomentoccursbecausethe16

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Figure2.4.Magneticorderinginanantiferromagnet.Figureadaptedfromreference[30]. Figure2.5.Magneticorderinginaferrimagnet.Figureadaptedfromreference[30].magnetizationofonesublatticeisgreaterthanthatoftheoppositelyorientedsublatticegure2.5.Themosttechnologicallyimportantclassofferrimagnetsareferrites,whicharemixedmetaloxidescontainingthecompoundFe2O3.Twoclassesofferrites,cubicspinelferritesandhexagonalferrites,arethesubjectofmuchoftheresearchpresentedinthisdissertation.CubicferriteshavethegeneralformulaMOFe2O3,andhexagonalferriteshavethegeneralformulaMO6Fe2O3.McanbeadivalentionsuchasMn,Ni,Zn,Co,Mginthecaseofcubicferrites;inhexagonalferritesMisusuallyBaorSr.Chapters5and6focusonmagnetite,Fe3O4orFeOFe2O3andchapter8discussesBaFe12O19orBaO6Fe2O3.17

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Ferritesareelectricallyinsulatingandndapplicationsinsituationswheretheelec-tricalconductivitypresentinmostferromagneticmaterialswouldbedetrimental.Theyarewidelyusedinhighfrequencyapplications,becauseanACelddoesnotproduceundesirableeddycurrentsinaninsulatingmaterial.2.5MagneticAnisotropyMagneticanisotropyreferstothedependenceofthemagneticpropertiesonthedi-rectioninwhichtheyaremeasured.Themagnitudeandtypeofanisotropyaectmanypropertiesofthematerialincludingcoercivityandblockingtemperatureinnanoparticles.Asaresult,studyingtheanisotropyanddeterminingwhatfactorscontributetoincreasedordecreasedanisotropyleadstocreatingbettermaterialsforaparticularapplication.Mag-neticanisotropyisatopicthatwillbediscussedfrequentlythroughoutthisdissertationandthissectiondescribesafewofthesourcesofanisotropyencounteredinthesubsequentchapters.2.5.1MagnetocrystallineAnisotropyMagnetocrystallineanisotropyisthetendencyofthemagnetizationinamaterialtoalignitselfalongapreferredcystallographicdirection.Thepreferreddirectionsarecalledtheeasy"axessinceitiseasiesttomagnetizeanddemagnetizeasampletosaturationiftheexternaleldisappliedalongapreferreddirection.Whenexamininghysteresisloopswiththeeldappliedalongeasyandhardaxes,acoupleofdierencescanbeseen.Whentheeldisappliedintheeasydirection,theriseofthemagnetizationtosaturationismorerapidandMSisreachedatamuchlowereld.Thisindicatesthatasmallereldwasrequiredtoalignthespinsalongtheelddirection.Also,whenthesampleismagnetizedalonganeasyaxis,itretainsmoreofitsmagnetizationwhentheeldisremoved:MRishigher.Thesquarenessratioofacurve,denedasS=MR/MS,isameasureofhoweectiveamaterialisatstayingmagnetizedintheabsenceofaeld.Formaterialsmagnetizedintheireasydirection,Sismaximized.Alongthehardaxisofmagnetization,ittakesamuch18

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largereldtoreachMS.Whentheeldisreversedthemagnetizationdoesnotgobacktotheremanentmagnetizationthatitwouldalongtheeasyaxis.Formanymaterials,themagnetizationiscompletelyunstablealongthehardaxisandreturnstozerooncetheeldisremoved.Thesquareness,S,isminimizedforloopstakenalongthethehardaxis,becauseMRhasbeenminimized.Thesaturationmagnetizationremainsthesameindependentofelddirection,eventhoughtheeldrequiredtoreachitisdierent.Theanisotropyenergyistheenergyrequiredtorotateaspinsystemofadomainawayfromtheeasydirection.Thisisactuallyjusttheenergyrequiredtoovercomethespin-orbitcoupling,becausereorientingthespinalsorequiresreorientationoftheelectronorbit.Theorbitisstronglycoupledtothelattice,whichiswheremostoftheresistancetorotationoriginates.Thereforethestrengthofthespin-orbitcouplinginamaterialdeterminesthemagnetocrystallineanisotropy.Forcubicstructures,themagnetocrystallineanisotropyisexpressedasaseriesex-pansionofthedirectioncosinesiofthesaturationmagnetizationrelativetothecrystalaxes[1]:E=K12122+2223+2321+K2212223+:::.15whereK1,K2,etc.aretheanisotropyconstants.Thistypeofmagnetocrystallineanisotropyiscalledcubicanisotropy.Forhexagonalstructures,theanisotropyissaidtobeuniaxialbecausethethereisjustoneeasyaxisofmagnetization,andsotheanisotropyisdenedonlybytheangleoftheappliedeldwiththeeasyaxis.Theexpressionforuniaxialanisotropyis[1]E=K1sin2+K2sin4+:::.16Itisimportanttorememberthattheanisotropyconstantsaretemperaturedependentanddecreasewithincreasingtemperatureduetothethermalenergycontribution.19

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2.5.2ShapeAnisotropyIfasampleispolycrystalline,thenitwillshownonetcrystallineanisotropybecausetherewillbecrystallitespointedinalldirections.Howeveronlyifthesampleisexactlysphericalwillthesameeldmagnetizeittothesameextentineverydirection.Ifthesampleisnotspherical,thenitwillbeeasiertomagnetizeitalongalongaxis.Thisisknownasshapeanisotropy.Theoriginofshapeanisotropyisthedemagnetizingeld.Whenasampleismagnetizeditwillproducemagneticchargesorpolesatthesurface.Thissurfacechargedistributionisitselfanothersourceofamagneticeld,calledthedemagnetizingeldanditactsinoppositiontothemagnetizationthatproducesit.Demagnetizingeldsarecomplicatedtocalculateandaresolelyafunctionoftheshapeofthesample.Thedemagnetizingeldandthusshapeanisotropyconstantincreasesastheaspectratioofthesampleorparticleincreases.2.5.3SurfaceAnisotropyThelasttypeofanisotropytobediscussedissurfaceanisotropy.Aspinonthesurfaceofasampleorparticlehasanearestneighborononesideofit,butnottheother.Thereforetheexchangeenergyatthesurfacecannotbethesameasinthebulk.Inmacroscopicsamples,theroleofthesurfacespinsisnegligiblecomparedtothebulkbehavior.However,asthesurface-area-to-volumeratioincreasesfornanoparticles,alargerpercentageofthespinsresideonthesurface,contributingalargeramounttotheoverallmagneticresponse.Theresultisalargercoercivityasalargereldisneededtoreversethesurfacespins,whichinananoparticlepointradiallyoutwardsinsteadofaligningwiththeinteriorspins[8].Whilesurfacespinshavemissingnearestneighbors,spinsattheinterfaceoftwoma-terialsalsohaveenvironmentsthataredierentfromthebulk.Thisinterfaceanisotropybecomesmoreimportantasthinlmnanostructuresandnanocompositesbecomemoreprevalentindeviceapplications.Theinterfacebetweenaferromagnetandanantiferro-magnetleadstoexchangecoupling,whichisdiscussedinalatersection.20

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2.6MagneticNanoparticlesandSuperparamagnetismWhenthesizeofamagneticparticleissmallenoughthattheformationofdomainsisnotfavorable,theparticleissaidtobesingledomain.Thisusuallyoccursonthenanometerscale,andtherealizationofmagneticnanoparticleshasprofoundconsequencesforallsortsoftechnologicalapplicationsfromdrugdeliverytomagneticrecording.Ingeneral,magneticnanoparticlesshowsuperiormagneticproperties,suchasanenhancedsaturationmagnetizationandremanentmagnetization.Whentheparticleisintheferromagneticstate,thecoercivityismuchlargeraswell.Whenaparticle'ssizeisonthesingledomainscale,aphenomenonknownassuper-paramagnetismispossible.Thisoccurswhenthethermalenergyisenoughtodemagnetizetheparticleintheabsenceofanappliedeld.Foranarrayofparticles,itmeansthatthenetmomentassociatedwitheachparticleeasilyalignswithanappliedeldbutisfreetorotateoncetheeldisremoved.Thissituationisanalogoustoaparamagnet,onlyinsteadofeachindividualspinaligningwiththeeldandthenrandomizingaftertheeldisremoved,thereareparticlescomposedofroughly105spinsthatcanalignwiththeeld.Thisresultsinamuchhighersusceptibilityandbettermagneticresponsethanatraditionalparamagnet,butthereisnocoercivityorremanentmagnetization.Magneticnanoparticlescanexhibitsuperparamagnetismonlywhenthetemperatureishighenoughtocausedemagnetizationbyovercomingtheanisotropyenergyintheabsenceofaeld.Whenthetemperatureisloweredandthethermalenergyisnotenoughtodemagnetizetheparticles,theyagainbehaveassingledomainparticleswithmagnetichys-teresis.ThiscriticaltemperatureiscalledtheblockingtemperatureTB,anditdependsontheparticle'sintrinsicpropertiesandsize.Theconditionsgivenaboveforsuperparamagnetismallassumethattheparticlesde-magnetizeduetothermalenergysimultaneouslywiththeremovalofaeld,andonthetimescaleofmostDCmeasurementsthisappearstobetrue.However,thereisanitetimescalefortheparticlestodemagnetize,andwhenthemagneticpropertiesareprobed21

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withanACeld,theblockingtemperaturecanincrease.Thusmeasurementfrequencyisalsoafactorwhendescribingtheblockingbehaviorofasuperparamagneticsystem.Superparamagneticparticlescanbeveryusefulformanyapplications.Becausetheydonotmaintainapermanentmoment,theydon'tagglomerate,sotheyareusefulforsuspendinginmagneticuidsorferrouidsthatareusedforbiomedicalimagingandtargeteddrugdelivery.Theirlackofcoercivitymeanstheyhavenomagneticlosses,whichmakesthemidealforACapplicationsliketransformers.However,theirlackofcoercivityandstabilitymakesitimpossibletousesuperparamagneticnanoparticlesformagneticstorage.Infact,"beatingthesuperparamagneticlimit"hasbecomeapriorityforharddrivecompanieswhowantgreaterarealdensitywhilestillmaintainingmagneticstability.2.7ExchangeCouplinginNanostructuresWhenmaterialswithferromagnetic-antiferromagneticinterfacesarecooledthroughtheNeeltemperatureoftheantiferromagnet,anexchangeanisotropyisinducedintheferro-magnet.Thistypeofanisotropywasdiscoveredin1956byMeiklejohnandBeanwhilestudyingCoparticlesembeddedinaCoOmatrix[53].Exchangeanisotropyisduetothecouplingoftheinterfacespinsandcanbeobservedinantiferromagnetic/ferromagneticthinlms,core-shellnanoparticles,andantiferromagnetic/ferromagneticcompositesofparticlesinamatrix.Aftercoolinganexchange-coupledsamplefromaboveTNbutbelowtheTCoftheferromagnet,thehysteresisloopoftheantiferromagnet/ferromagnetsystemcanbeshiftedalongtheeldaxisgenerallyinthedirectionopposite`negative'tothecoolingeld.Thisresultsintheabsolutevalueofthecoerciveeldbeingdierentfortheincreasinganddecreasingelds.Thisphenomenonisknownasexchangebias,HE.ThehysteresisloopswillalsohaveanincreasedcoercivityHCwhich,alongwithHE,disappearsatornearTN,conrmingthatitistheantiferromagnetthatisresponsibleforthisbehavior.Figure2.6isadiagramsummarizingexchange-biasedmaterialsandthemodiedhysteresisloopduetoexchangeanisotropy.22

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Figure2.6.Exchangebiassystemsandassociatedphenomena.Theshiftinhysteresisloopcanbequalitativelyexplainedbytheantiferromagneticinterfacespinsinducingaunidirectionalanisotropyintheferromagnet.WhenaeldisappliedaboveTN,theferromagneticspinslineupwiththeeld,whiletheantiferromag-neticspinsremainrandomizedgure2.7i.WhencoolingthroughTNinthepresenceofaeldduetotheinteractionattheinterface,theantiferromagneticspinsnexttotheferromagneticspinsalignferromagnetically.Theotherspinplanesintheantiferromag-netremainalignedantiferromagneticallysotheantiferromagnetstillhasnonetmomentgure2.7ii.Whentheeldisreversed,theferromagneticspinsbegintorotate.Forsucientlylargeantiferromagneticanisotropy,theantiferromagneticspinsdonotrotategure2.7iii.Theantiferromagneticspinsattheinterfaceexertatorqueontheferro-magneticspinstokeepthemintheiroriginalposition.Therefore,theferromagneticspinshaveonestableconguration,i.e.theanisotropyisunidirectional.Theeldneededtocompletelyreversetheferromagneticlayerwillbelarger,becausealargereldisneededtoovercomethetorque.However,oncetheeldisrotatedbacktoitsoriginaldirection,theferromagneticspinswillstarttorotateatasmallereldduetotheinuenceoftheantiferromagneticspinswhichnowexertatorqueinthesamedirectionastheeldgure23

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2.7v.Thematerialineectactsasiftherewereaninternalbiasingeld,hencethetermexchangebias". Figure2.7.Schematicdiagramofthespincongurationofanantiferromagnet/ferromagnetbilayeratdierentstagesi-vofanexchangebiasedhysteresisloop.Figureadaptedfromreference[59].Therearesomeexchange-coupledsystemsforwhichanincreaseincoercivityisobservedaftereld-cooling,butthereisnoshiftinhysteresisloop.Insuchasituation,theantifer-romagneticanisotropyistoolowtocreateaunidirectionalanisotropyintheferromagnet.Aftersaturationoftheferromagneticspinswhentheeldisreversed,theantiferromagneticspinscanbedraggedbythespinsintheferromagnet.Itisenergeticallyfavorablethatthespinsinboththeferromagnetandtheantiferromagnetrotatetogether.Theresultofdraggingtheantiferromagnetspinsinbothdirectionsresultsinauniaxialanisotropyandanincreaseincoercivity,HC.24

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Exchangecoupledthinlmsareusedinnumeroustechnologicalapplicationssuchasspin-valvesensors,magnetictunneljunctionreadheadsforharddiskdrives,andmagneticrandomaccessmemory[48].Theenhancedcoercivityandanisotropythatareobservedinthesetypesofsystemshaveledtoexchange-couplednanoparticlesandlithographicallypatternedstructuresbeingproposedtobeatthesuperparamagneticlimitinmagneticme-dia.25

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CHAPTER3MEASUREMENTTECHNIQUESAllofthemeasurementtechniquesdescribedinthischapterwereperformedusingacommercialPhysicalPropertiesMeasurementSystemPPMSfromQuantumDesign.ThePPMSconsistsofaliquidheliumdewarwitha7Teslalongitudinalsuperconductingmagnetandatemperaturecontrollerintherange2Kto350Kgure3.1.Magnetizationversuseld,magnetizationversustemperature,andACsusceptibilitymeasurementswereallperformedusingtheAC/DCMagnetometrySystemACMS,afeatureofthePPMS.ThetransversesusceptibilitymeasurementswereperformedusingamodiedQuantumDesignMultifunctionalProbe. Figure3.1.PhysicalPropertiesMeasurementSystemPPMS.26

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3.1DCMagnetizationMeasurementsThemostcommonDCmagneticmeasurementisthemagnetizationversuseldM-Hcurve,whichwasdiscussedingreatdetailinchapter2.ThismeasurementyieldsthesaturationmagnetizationMS,remanentmagnetizationMR,andcoerciveeldHC.ItcanbeanindicatorofanisotropybecausetheshapeandsquarenessratioisdierentwhenHisappliedalongtheeasyandhardaxesofmagnetization.Lastly,whentakenaftercoolinginaeld,ashiftalongthehorizontalaxisalongwithanincreaseinHCisanindicationofexchangecoupling.Thissectiondiscussesmagnetizationversustemperaturemeasurementswhichareoftenusedtocharacterizemagneticmaterials.ThisisusuallydonebytakingzeroeldcooledZFCandeldcooledFCmagnetizationcurves.MZFCTisdeterminedbycoolingasamplewithH=0tolowtemperature,sothatthemagneticmomentsoftheparticleshaverandomorientations.Asmall,constanteldtypicallyaround100Oeisappliedsothatthereisameasurablemagnetizationasthetemperatureisincreased.MZFCincreasesasthethermalenergyisraised,andthereissucientenergytoaligntheparticlemomentsparalleltotheeld.MZFCdropsagainathightemperaturewhenthermaluctuationsareabletodemagnetizethesampleandasharpdropoisseenattheCurietemperature,TC,whenthemagnetizationisdestabilizedduetothermaluctuations.Formagneticnanoparticles,theZFCmeasurementisespeciallyusefulfordeterminingtheaverageblockingtemperature,TB.IntheZFCinitialstateatlowtemperature,thenetmagnetizationisideallyzero.Whenaeldisapplied,andthemagnetizationmeasuredasthetemperatureisraised,onlyparticleswithTBlessthanthemeasuringtemperaturecontribute.Therefore,theaverageblockingtemperatureforthearrayofparticlesisseenasamaximumintheZFCcurve.TheFCmagnetization,MFCT,ismeasuredbyrstapplyingasmalleldagainaround100Oeatroomtemperature.Asthesampleiscooled,themagnetizationrisesasthermaluctuationsbecomelessimportant.UnlikeMZFC,theFCmagnetizationsaturatesatlowtemperature.ThepointwheretheZFCandFCcurvesmeetisoftenreferredtoasthefreezingtemperatureTF,andindicatestheonsetofirreversiblemagnetizationatthe27

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eldinwhichthemeasurementistaken.Formagneticnanoparticles,thisoftencoincideswiththeblockingtemperature,TB.AsamplegraphofaZFCcurveplottedalongwithanFCcurvethisisthetypicalrepresentationforasampleofNiFe2O4nanoparticlesisshowningure3.2.Theseparticlesweremadebyanothergraduatestudentinthelabandthemagneticpropertieswerestudiedextensively[21]. Figure3.2.ZeroeldcooledZFCandeldcooledFCmagnetizationversustemperaturecurvesforNiFe2O4nanoparticles.3.2ACSusceptibilityWhileDCmagneticmeasurementsgenerallymeasuretheequilibriummagneticprop-ertiesofasample,ACmagneticmeasurementscanprobethetimescaleatwhichmanymagnetizationprocessesoccur,andprovidevaluableinformationaboutparticleinterac-tions,spindynamics,andthepresenceofmagnetictransitions.Insteadofapplyinga28

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largeDCmagneticeld,asmallACmagneticeldisused,whichcausesatime-dependentmomentinthesample.Atlowfrequencies,wherethemeasurementismostsimilartoaDCmeasurement,themagneticmomentofthesamplefollowstheMHcurvethatwouldbemeasuredinaDCexperiment.AslongastheACeldissmall,theinducedACmomentisMAC=HACsin!t.1whereHACistheamplitudeofthedrivingeld,and!isthedrivingfrequency.=dM/dHisthesusceptibilityaswellastheslopeoftheMHcurveatverysmalleldswherethemagnetizationisstillreversiblei.e.hysteresishasnotyetsetin.HACisusuallyaround10Oewherethelinearsusceptibilityassumptionisstillvalid.OneadvantageofACsusceptibilityisthatthemeasurementisverysensitivetosmallchangesinmagnetization.SincetheACmeasurementissensitivetotheslopeofMH,andnottotheabsolutevalue,smallmagneticshiftscanbedetectedevenwhenthetotalmomentislarge.Athigherfrequencies,theACmomentofthesampledoesnotfollowalongthereversiblepartoftheDCmagnetizationcurveduetodynamiceectsinthesample.Essentially,therotationofthemagneticmomentcannotkeepupwiththealternatingmagneticeld.Inthishigherfrequencycase,themagnetizationofthesamplemaylagbehindthedriveeld,aneectthatisdetectedbythePPMS.Thus,theACmagneticsusceptibilitymeasurementyieldstwoquantities:themagnitudeofthesusceptibility,,andthephaseshift,,relativetothedrivesignal.Alternatively,onecanthinkofthesusceptibilityashavinganin-phase,orreal,component0andanout-of-phase,orimaginary,component00.Thetworepresentationsarerelatedby0=cos.200=sin.3=p 02+002.429

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Inthelimitoflowfrequency,wheretheACmeasurementismostsimilartotheDCmeasurement,therealcomponent0asdiscussedabove.Theimaginarycomponent,00,indicatesdissipativeprocessesinthesample.Inconductivesamples,thedissipationisduetoeddycurrents.Inferromagnets,anonzeroimaginarysusceptibilitycanindicateirreversibledomainwallmovement,orabsorptionduetoapermanentmoment.Also,both0and00areverysensitivetothermodynamicphasechanges,andareoftenusedtomeasuretransitiontemperatures.Inthisdissertation,weusethefactthatrelaxationinsuperparamagneticparticlesgivesrisetoanonzero00.Graphsofboth0and00showmaximaattheblockingtemperatureforthetransitionfromferromagnetismtosuperparamagnetism.RecallthatTBdependsonthetimescaleofthemeasurement.ACsusceptibilityisapowerfultoolbecauseitusesatimescalewherethiseectcanbeclearlyseenasashiftinTBwithfrequency.TheNeel-Arrheniusrelationdescribesthemagnetizationreversalofanon-interactingsingledomainparticleinalocalminimumoverananisotropybarrier,Ea)]TJ/F19 7.97 Tf 6.586 0 Td[(1=)]TJ/F19 7.97 Tf 6.586 0 Td[(10expEa=kBT.5whereisthereversalrate,TisthetemperatureandkBisBoltzmann'sconstant.Here,0istheattemptfrequencyandiscomparabletotheLarmorprecessionfrequency[47].Generallythisvaluesfallsintherange10)]TJ/F19 7.97 Tf 6.587 0 Td[(10)]TJ/F15 10.909 Tf 10.909 0 Td[(10)]TJ/F19 7.97 Tf 6.586 0 Td[(9s.Bymakingaplotof1/TBversuslnf,theNeel-Arrheniusrelationcanbeusedtoextracttheattemptfrequencyy-interceptoftheplotandtheanisotropyenergybarrierslopeoftheplot.However,asmentionedbefore,thisrelationisonlyvalidfornon-interactingnanoparticlesystems,andattimesunphysicalvaluesofeachorbothoftheseparametersmayobtained.Inthatcase,theVogel-Fulcherrelation)]TJ/F19 7.97 Tf 6.586 0 Td[(1=)]TJ/F19 7.97 Tf 6.586 0 Td[(10expEa=kBT)]TJ/F21 10.909 Tf 10.909 0 Td[(T0.630

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mustbeusedwhichaccountsforweakdipolarinteractionsbetweenparticlesbytheuseofathirdparameter,T0.Theresultofdipolarinteractionsistoslowdownthemagneticresponsebecausetheparticlesmustalsoovercomethelocalenergypresentfromneighboringparticles.31

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CHAPTER4TRANSVERSESUSCEPTIBILITYTransversesusceptibilityTisameasureofthemagneticsusceptibilityinonedirec-tion,whileanexternalmagneticeldisappliedperpendiculartothedirectionofmeasure-ment.Inaseminal1957paper[2],Aharonietal.calculatedTasafunctionofHDCforacollectionofStoner-Wohlfarthparticles[86].Theseareellipsoid,singledomain,ferromag-neticparticleswithuniaxialanisotropy.AccordingtoAharoni'stheory,measurementofTwithrespecttoHDCappliedalongthehardaxisofmagnetizationshouldyieldpeaksatthepositiveandnegativeanisotropyelds,HK.TheanisotropyeldofamaterialistheeldneededtosaturatethemagnetizationofamaterialintheharddirectionandisrelatedtotheeectiveanisotropyKeviathefollowing:HK=2Ke=MS.1Thusitcanalreadybeseenthatdirectlymeasuringtheanisotropyeldofamaterialsgivesvaluableinformationabouttheeectiveanisotropyofasample.4.1TheoryandHistoricalBackgroundTransversesusceptibilityalongwiththeparallelsusceptibilityareactuallythediagonalcomponentsofthe33reversiblesusceptibilitytensor.TheparallelsusceptibilityisthesusceptibilityasafunctionofHDCmeasuredinthedirectionoftheappliedeld,andthetransversesusceptibilitycomponentscorrespondtothesusceptibilitywithHDCappliedalongeitherofthetwodirectionstransversetothemeasurement.ForHDCalongthe32

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z-direction,thetransversesusceptibilityiswrittenasT=dMx dHzHx=0;Hy=0.2Expressingsusceptibilityasasecondranktensorallowsfortheresponseofasampletobemeasuredinadirectiondierentfromtheeld.Asaresult,twoeldsarerequiredtodoatransversesusceptibilitymeasurement:asmallACmagneticeldHAC,forwhichvariationwithsamplemagnetizationcanberelatedtothesusceptibility,andanexternalDCmagneticeldHDCthatcanbevariedoveralargerange.Figure4.1showsthegeometryinvolvedinatransversesusceptibilitymeasurement. Figure4.1.GeometricalconstructofatransversesusceptibilitymeasurementincludingtheDCeldHDC,theACeldHAC,themagnetizationvectorM,andtheeasyaxisE.A..Alsoincludedaretherelevantanglesusedinthetransversesusceptibilitycalculation.Fig-ureadaptedfromreference[81].Here,theaxisofanisotropy,K,isdenotedE.A.foreasyaxis,anddenedbythesphericalpolaranglesKandK.Similarly,themagnetizationvector,M,hascoordinatesMandM.TheappliedeldHDCisalignedalongthez-axis.ChoosingthecoordinatesystemsothatH=033

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thenHx=HsinHHz=HcosHMx=MSsinMcosMUsingtheexpressionforTequation4.2itfollowsthatT=3 20limK!0dsinMcosM dHsinH.3where0=M2s=3K.Forthemagneticmomenttolieinanenergyminimum,itisnecessarytoimposethefollowingconditions:E M=0;E M=0.4ForaStoner-Wohlfarthparticle,theenergyisgivenbythesumoftheanisotropyenergyandtheenergyofinteractionwiththeappliedmagneticeldZeemanenergy[86]E=EK+EH.5Intermsofthesphericalcoordinatesystemdenedabove,equation4.5becomesE=)]TJ/F21 10.909 Tf 10.909 0 Td[(KsinKcosKsinMcosM.6+sinKsinKsinMsinM+cosKcosM2)]TJ/F21 10.909 Tf 10.909 0 Td[(HMSsinHsinMcosM+cosHcosM)]TJ/F21 10.909 Tf 10.909 0 Td[(HMScosMUsingtheseexpressions,Aharonietal.[2]arrivedattheexpressionfortheeld-dependenttransversesusceptibilityofasingleStoner-Wohlfarthparticlebyminimizingequation4.634

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usingtheconstraintsgivenbyequation4.4T=3 20cos2Kcos2M hcosM+cos2M)]TJ/F21 10.909 Tf 10.909 0 Td[(K+sin2sinK)]TJ/F21 10.909 Tf 10.909 0 Td[(M hsinK.7wherehisthereducedeld,HK/HDC.Foranarrayofparticleswithrandomlyorientedanisotropyaxes,assumingtheparticlesareidenticalandthatinter-particleinteractionsarenegligible,theaverageTbecomesT=1=2Z20Z=20TsinKdKdK.8AfterintegratingoverKandsubstitutingbackequation4.7,wearriveattheexpressionT=3 40Z=20cos2M hcosM+cos2K)]TJ/F21 10.909 Tf 10.909 0 Td[(M+sinK)]TJ/F21 10.909 Tf 10.909 0 Td[(M hsinKsinKdK.9Usingthisrelation,alongwiththeonefortheparallelsusceptibilityP=3 20Z=20sin2sinK hcos+cos2)]TJ/F21 10.909 Tf 10.909 0 Td[(KdK.10Aharonietal.calculatedseveralvaluesofTandPforvariousvaluesofreducedeldh,andmadeaplotofhowTandPshouldbehaveafterreducingtheeldfromsaturationgure4.2.TheypredictedthatPshoulddivergeattheswitchingeld,HS,andthatTshouldshowthreecusps:twoatthepositiveandnegativeHKvalues,andoneatHS.Theshapeofthetransversesusceptibilitycurveisheavilyinuencedbythoseparticleswhoseeasyaxisisalignedat90toHDC.Therstexperimentalconrmationofthistheorywasin1987byParetiandTurilli[63]whoshowedthepresenceofthepeaksatHKandHS.Theirexperiment,onbariumferriteparticles,resultedinpeaksratherthancusps.ThiswasattributedtotheinhomogeneitiesinparticleshapeanddimensionsresultingindistributionsofHKandHS.Overtheyears,transversesusceptibilityhasattractedexperimentalinterestlargelyfromthemagneticstoragecommunity,whohavemeasuredtransversesusceptibilityon35

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Figure4.2.TheoreticaltransversesusceptibilityTandparallelsusceptibilityPcurvesasafunctionofreducedeldhh=HK/HDCascalculatedbyAharonietal.Figureadaptedfromreference[2].particulaterecordingmedia[31,69,13].Theperformanceofmagneticmediaisheavilydependentontheanisotropicproperties,andtransversesusceptibilityasadirectprobeofHKhasbecomeaninvaluabletoolforassessingnewmaterials.However,duetothelimitingassumptionsoftheStoner-Wohlfarthmodel,somemod-icationshavebeenmadetoAharoni'soriginaltheory.Notably,Hoareetal.[31]addedaweightedanisotropyaxisdistributionfunctiontoaccountfortexturedmedia,i.e.2-dimensionalarraysofparticlesthatarepreferentiallyalignedwiththeireasyaxesintheplaneofthearray,anadditionusefulforrecordingtape.AbigadvanceintransversesusceptibilitytheorywasmadebySpinuetal.[82]whentheywereabletousemicromagneticsimulationstomodeltransversesusceptibilityusingtheLandau-Lifshitz-Gilbertapproach,whichallowedthemtoaccountforthesecondorderanisotropyconstantK2.Formanymaterials,thevalueofK2isappreciableorevennegativeasisthecaseforCo,andneglectingthistermcanleadtoinaccuracyinK1.Later,36

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theLLGapproachwasusedbyStancuandSpinutocalculatethetransversesusceptibilityforasingledomainparticlewithcubicanisotropy[85].In2006,Matarranzetal.[51]returnedtotheStoner-Wohlfarthmodelinabidtoincorporateinter-particleinteractionsintotransversesusceptibilitytheory.LikeHoareetal.[31],theytootriedtoaccountforparticletexturingbyintroducingadouble-Gaussiandistributionfunctionofeasyaxes.Tomodelthedipolarinteractions,theyusedthemean-eldmodelreplacingthereducedeldhrecallh=HK/HDCbyhe=h+m.Herem=M/MSisthereducedmagnetization,andisaninter-particleinteractionparameter.ThemostimportantthingaboutthismodicationoftheStoner-WohlfarthmodelistheabilitytoreproducesomeofthefeaturesseeninphysicalnanoparticletransversesusceptibilitymeasurementssuchasbroadenedpeakslocatedasymmetricallyaroundHDC=0,themergingoftheHSpeakwiththe-HKpeak,andaprominentasymmetryinpeakheights.Theexperimentalexistenceofthesephenomenawillbeaddressedinthenextsection.Inthepastfewyears,focusintransversesusceptibilitytheoryhasturnedtothecomplextransversesusceptibility[16,15,36].LiketraditionalACsusceptibility,transversesuscep-tibilityalsohasanout-of-phasecomponent.Ananalysisofthein-phaseandout-of-phasetransversesusceptibility,undertakenbyPapusoi[36]in2000,revealedsomeveryimportantconclusionsaboutcomplextransversesusceptibility.First,whentheDCeldisdecreasedfromHKdowntozero,theparticlerelaxationtimedistributionshiftsfromzerotoveryhighvalues.IntheDCeldrangewheretheparticlerelaxationtimebecomesofthesameorderofmagnitudeasthereciprocaloftheACeldfrequency,thetransversesusceptibilityisstronglyinuencedbytheparticlevolumedistribution.ThisimpliesthattheHDCincomplextransversesusceptibilitymeasurementsplaysananalogousroletothetempera-tureinACsusceptibilitymeasurements.ThisworkpromptedCimpoesuetal.[16,15]todevelopamicromagneticmodelthataccountsforthefrequencyoftheperturbingACeld,whichuntilthenhadbeenneglected.Itwasfoundthatthecomplextransversesusceptibil-itywithrespecttoHDCcontainsmultiplepeaks,whichcanbecorrelatedwithanisotropyandvolumedistributions.37

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Finally,in2006,itwasreportedthatanewtransversesusceptibilitytheorybasedonmagnetizationvectordynamicsasdescribedbytheLandau-Lifshitzequationofmotionledtotheconclusionthatwhatwemeasureastransversesusceptibilityisactuallythezerofrequencylimitofferromagneticresonance[78].ItcaneasilybesaidthatthewidespreaduseofmicromagneticsimulationswithbetterandfastercomputersinrecentyearshasallowedtransversesusceptibilitytheorytoevolvefromthesimpliedviewofAharoniover50yearsagotoasophisticatedseriesofcalculationsusedtogaininvaluableinformationontheanisotropyofmagneticmaterials,andexplaintherichvarietyofbehaviorobservedinphysicaltransversesusceptibilitymeasurements.4.2MeasuringtheTransverseSusceptibilityUsingaTunnelDiodeOscillatorAllofthetransversesusceptibilitydatapresentedinthisdissertationweretakennotwithatraditionalsusceptometer,butwithaself-resonanttunneldiodeoscillatorTDO.Resonantmethodshavetheadvantageofprecisionandhighsensitivitywhenitcomestodetectingchangesinthephysicalpropertiesofmaterialsasafunctionoftemperatureandmagneticeld.Thisisduetothefactthatfrequencycanbemeasuredwithahighdegreeofaccuracy.InatypicalresonanttechniquebasedonanLCtankcircuit,thecapacitororinductorcouplestothematerialunderstudy,andactsasatransducerofphysicalparameters.Anychangeinmaterialpropertieswillinduceachangeinthecapacitanceorinductance,whichinturnresultsinashiftintheresonantfrequency.Thus,measurementofthefrequencyshifttranslatestodirectlyprobingtheelectronic,dielectric,ormagneticresponseofthematerialtotheoscillatingsignal.Tunneldiodeoscillatorswhichoperatebasedonthisprinciplehavebeenusedinthepasttostudyawidevarietyofmaterialproperties[83].TheprincipleoftheTDOcanbeexplainedasfollows.AnLCtankcircuitismain-tainedataconstantamplituderesonancebysupplyingthecircuitwithexternalpowertocompensatefordissipation.Thispowerisprovidedbyatunneldiodethatisforward-biasedwithavoltageintheregionofnegativeslopeofitscurrent-voltageI-Vcurve,or38

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negativeresistanceregion".Suchanarrangementmakesitaself-resonantcircuitasthepowersuppliedbythediodemaintainscontinuousoscillationoftheLCtankoperatingatafrequencygivenbytheexpression!=1 p LC.11Whenasampleisinsertedintotheoscillatortankcoil,thereisasmallchangeinthecoilinductanceL.IfL/L<<1,onecandierentiateequation4.11andobtaintheexpression! !)]TJ/F15 10.909 Tf 21.195 7.38 Td[(L 2L.12Theinductancechangeisrelatedtomaterialproperties.Inthecaseofamagneticmaterial,thisisproportionaltotherealpart,0,ofthecomplexpermeability=0)]TJ/F21 10.909 Tf 10.909 0 Td[(i00.13Theinductancecoilinthisexperimentalsetupservesasthesamplespaceinwhichagelcapcontainingthesamplecant.ThisentirecoilisinsertedintothesamplechamberofourPPMSusingacustomizedradiofrequencyRFco-axialprobegure4.3.TheDCmagneticeldHDCisvariedusingthePPMS.TheoscillatingRFeld,HRF,producedbytheRFcurrentowinginthecoilwindings,isorientedperpendiculartoHDC,andthisarrangementsetsupthetransversegeometrydescribedintheprevioussection.WhenHRFisperpendiculartothevaryingHDC,thechangeininductanceisactuallydeterminedbythechangeintransversepermeability,T,ofthesample.Thus,wecanderiveanabsolutevalueforthetransversesusceptibility:T=T)]TJ/F15 10.909 Tf 10.909 0 Td[(1.1439

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Figure4.3.SchematicoftheTDOcircuitandsamplespaceleftandCADdrawingoftheinductancecoilwhichservesasthesampleholderright.ThepercentchangeintransversesusceptibilitythencanbedenedasT T%=jTH)]TJ/F21 10.909 Tf 10.909 0 Td[(satTj satT100.15wheresatTisthetransversesusceptibilityatthesaturatingeldHsat.Thisquantity,whichrepresentsagureofmerit,doesnotdependongeometricalparametersandisusefulforcomparingthetransversesusceptibilitydatafordierentsamples,orforthesamesampleunderdierentconditions.Despitethefactthatthistechniqueonlygivesyouthepercentchangeintransversesusceptibility,themostimportantfeaturesoftransversesusceptibilitywithrespecttotemperature,namelytheHKandHSpeaks,arestillpresent,whichallowustodrawimportantconclusionsaboutthesample'sanisotropy.Moreover,sincethisisaresonantmethod,weareabletousethehighdegreeofsensitivity10Hzin10MhztolookatverysmallsamplesthatoftentimesdonothaveahighenoughmomenttobepickedupbythemagnetometerinthePPMS.Thisfeaturewillbehighlightedinchapter5,wherewediscusstheuseoftheTDOmethodtosensenanoparticlesinsideofhumancells.40

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Figure4.4isasampletransversesusceptibilityscanofNiFe2O4nanoparticlestakenfrompositivesaturationtonegativesaturation.Henceforth,thistypeofscanwillbereferredtoasaunipolarscan,andonegoingfrompositivetonegativesaturationandthenbacktopositivewillbereferredtoasabipolarscan.Figure4.5isadetailedviewofabipolarscanofthesameNiFe2O4sample.Thearrowscorrespondtothemeasurementsequencesothatthesetsofpeaksarisingfromthepositivetonegativescancanbeeasilydistinguishedfromthenegativetopositivescan.AcoupleoffeaturescanbeseenintheseguresthataredierentfromAharoni'stheoreticalcurvegure4.2.First,therearepeaksseenattheanisotropyelds,butinthiscase+HK6=)]TJ/F15 10.909 Tf 8.485 0 Td[(HK.Intheunipolarscan,the+HKvalueis365Oeandthe)]TJ/F15 10.909 Tf 8.485 0 Td[(HKvalueis)]TJ/F15 10.909 Tf 8.485 0 Td[(390Oe.Inothernanoparticlesystems,thetwoHKvaluescandivergebyquiteabitmore,aswillbeseeninchapters5and6,andindeedinthebipolarscan,thedisparityisalittlemoreapparent.Inalmosteverycase,thepeakclosesttosaturationhasasmallerHKvaluethantheonethatoccursafterpassingthroughH=0.Second,thereisnopeakcorrespondingtoHS.Lastly,thereisadierenceinpeakheightbetween+HKand)]TJ/F15 10.909 Tf 8.485 0 Td[(HK,withthepeaksnearesttosaturationbeinghigherinamplitude.Thus,+HKishigherinamplitudeandsmallerinvalue,and)]TJ/F15 10.909 Tf 8.485 0 Td[(HKissmallerinamplitudeandlargerinvaluecomingfrompositivesaturationgoingtonegativesaturation.Conversely,fromnegativesaturation,the)]TJ/F15 10.909 Tf 8.484 0 Td[(HKpeakishigherinamplitudeandsmallerinvalue,whilethe+HKpeakissmallerinamplitudeandlargerinvalue.Fortheremainderofthisdissertation,wewilldistinguishthetwosetsofpeakswiththeterminologyHK1andHK2,whereHK1isalwaystherstpeakthatoccursaftersaturationineitherdirection.NotethatjHK1j
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Figure4.4.UnipolartransversesusceptibilityscanfrompositivesaturationtonegativesaturationofNiFe2O4nanoparticles. Figure4.5.Detailedbipolartransversesusceptibilityscanfrompositivetonegativesatu-rationandbacktopositivesaturationofNiFe2O4nanoparticles.42

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eledbyintroducinganorientationdistributionfunctionandamodiedreducedeld.Whilenoneofthenanoparticleswemeasuredshouldpossesstexturingi.e.themomentsarecom-pletelyrandomlyaligned,sizedispersionandinter-particleinteractionsshouldbothhaveimportantcontributionstotransversesusceptibilitymeasurements.Sizedispersion,whichalsowillleadtoanHKdispersion,canexplainwhyweseepeaksinsteadofthecuspsseeningure4.2.Whenitcomestointer-particleinteractions,wefeelthatqualitativelythedeviationsfromtheorycanbeexplainedmoreeectivelybyconsideringtheenergylandscapeoftheparticlescontributingtothetransversesusceptibilityatdierentvaluesofHDC.Forin-stance,HK1alwaysoccursaftersaturation,whentheZeemanenergytheenergyassociatedwiththemomentsaligningwiththeeldishighest.Sincetheparticlesarebeingheavilyinuencedbythechangingeld,therotationofthemomentsismorecoherentcausingasharperpeakwithahighermagnitudeatHK1.AtthetransversesusceptibilityminimumoccurringafterH=0,themomentshaveessentiallyrandomizedandarenolongeralignedwiththeeld,consistentwithaminimizingofZeemanenergy.Atthispoint,theinter-particleinteractionsaredominatingthemagneticresponse.StronginteractionsshouldleadtoHK2beingclosertosymmetricwithHK1,sincetheeldeachparticleexperiencesfromitsneighborsshouldhaveasimilarresponsetoanappliedeld.Thus,minimizingtheZee-manenergyshouldnothaveaslargeaneectonthecollectiveresponseofthesystem.Iftheinter-particleinteractionsareweak,thentheoverallmagneticresponseoftheparticlesintherandomizedstatetoanincreasingeldwillbemuchsmaller,leadingtopeakswithsmallerheightthanthoseoccurringaftersaturation.Webelievethatiswhyinthinlms,thepeaksaresymmetric.Wecanthinkofthinlmsasbeingthehighlyinteracting"limitofthepicturedescribedabove,whereinsteadofdipolarinteractionsproducingaeldinternaltothesystem,theexchangeinteractionbetweenspinscausingthecrystaleldisresponsibleforthesimilarbehavioroneachsideofH=0.Innanoparticles,weusuallyseethecompletedisappearanceofthepeakassociatedwiththeswitching.ThisislikelyduetothedispersioninHKandHSassociatedwithadistributioninparticlesizes.These43

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dispersionslikelyoverlapleadingtoasingle,broadenedpeakwhichisalsoconsistentwiththevalueofHK2beinglargerthanHK1.BecauseofthevaryinginteractionsanddispersionsinHKandHS,theHK2peakcanvarygreatlyinheightandeldvalue.Forthesereasons,KeisalwayscalculatedusingHK=HK1,thoughoftenbothHKvalueswillbeplottedversustemperaturetoexaminetheanisotropyevolutioninamaterial.Andnally,whentheKevaluesarecalculatedfromtransversesusceptibilityusingequation4.1,weareactuallycalculatingK1.Inchapter2theanisotropyenergywasoftenexpressedasaseriesexpansion,whereonlythersttwotermswereusedcontainingtherstandsecondorderanisotropyconstants.IncalculatingKe,weareneglectingthesecondorderandhigherterms,keepingonlyK1.WhileK2isnon-negligibleandevennegativeinsomematerials,wehavefoundthatforourpurposes,namelyprobingnanoparticlesdominatedbyuniaxialanisotropyandsurfaceeects,calculatingK1onlycapturesthemainphysicsofthesystemstoasatisfactorydegree.44

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CHAPTER5Fe3O4ANDAu-Fe3O4NANOPARTICLESFORBIOMEDICALAPPLICATIONSInthespringof2006,theauthorwasawardedaNationalScienceFoundationIntegrativeGraduateEducationResearchandTraineeshipIGERTfellowship.TheIGERTprogramwasdevelopedtoeducatePh.D.scientistswithinterdisciplinarybackgrounds,deeperknowl-edgeinchosendisciplines,andtechnical,professional,andpersonalskillstomeettheneedsofachangingscienticlandscape.TheIGERTresearchperformedbytheauthorwasincollaborationwithProfessorShyamMohapatraandDr.ArunKumaroftheUniversityofSouthFloridaCollegeofMedicine.Prof.MohapatraandDr.Kumarhaveresearchinter-estsinmagneticnanoparticlesforbiomedicalapplications.Thisresearchwasundertakeninordertohelpcharacterize,aswellasoptimizethemagneticpropertiesofthenanoparticlestheyuseforcelltransfection,inlinewiththegoalsoftheIGERTprogram.5.1IntroductionInrecentyears,magneticnanoparticleshavebecomeatopicofinterestforawiderangeofmedicalapplicationsduetotheirdimensionsbeingsmallerthanorcomparabletoseveralbiologicalentitiessuchascells-100m,viruses20-450nm,andproteins-50nm[62].Theabilityofmagneticparticlestobemanipulatedbyanexternalmagneticeldmakethemespeciallyattractiveforlocalizedtreatmentoptionssuchastargeteddrugdeliveryandhyperthermia,aswellasdiagnosticslikeenhancingexistingMRItechniquesandsensorsbasedonthedetectionofamagneticsignal.Inthissection,wedescribethesetreatmentanddiagnosticapplications,aswellasproposehowtransversesusceptibility45

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couldbeusedforcancercelldetection.TherestofthechapterdescribesthemagneticpropertiesofFe3O4andAu-Fe3O4particlessynthesizedspecicallyfortheseapplications.5.1.1TargetedDrugDeliveryThebiggestproblemassociatedwithsystemicdrugadministrationisthatapharmaceu-ticalbecomesevenlydistributedthroughoutthebody,whichresultsinalackofspecicityfortheareaofinterest[7].Therefore,highlevelsofadrugmustbeadministeredtoachievethedesiredconcentrationintheaictedarea.Bothnon-specicityandthetoxiclevelsneededtotreatillnessoftenleadtounwantedandharmfulside-aects.Nowhereisthiscurrentlymoreevidentthaninchemotherapy,wheresignicantdamageisdonetotheentirebodyinthehopesofkillingoftenlocalizedcancercells.Targeteddrugdeliveryaimstoalleviatetheseissuesbyimmobilizingadrugontoabiocompatiblemagneticnanoparticle,whichactsasacarrier.Thedrug/carriercomplexes,whichareusuallyintheformofasuspensionofnanoparticlesferrouidinabiocompatibleliquid,canbeinjectedintothepatientviathecirculatorysystem.Aftertheparticlesenterthebloodstream,external,high-gradientmagneticeldscanbeusedtoconcentratethecomplexataspecictargetsitewithinthebody.Oncethedrug/carrierisconcentratedatthetarget,thedrugcanbereleasedeitherviaenzymaticactivity,orchangesinphysiologicalconditionssuchaspH,osmolality,ortemperature.Fe3O4superparamagneticnanoparticleshavebeenexaminedfortargeteddrugdeliveryduetotheirbiocompatibility,magneticpropertieshighsaturationmagnetization,andtheirabilitytobefunctionalized[25,24,58].However,ifthesurfaceisleftuntreated,agglomerationcanoccur,andthenaturalhydrophobicityofthesurfacecausestheparticlestobetakenupbythebody'ssystems,mainlythekupercellsintheliver[25].Usually,Fe3O4particlesmustrstbecoatedwithanamphiphilicpolymericsurfactantsuchaspolyethyleneglycolPEGtokeepthemfromagglomerating,andtominimizeunwantedproteinadsorptionontonanoparticles.Thesubsequentcoatingcanthenbefunctionalizedbyattachingcarboxylgroupsorothermolecules.46

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CoatingtheFe3O4particleswithanoblemetal,suchasgoldAu,canserveasimilarpurpose,butinthiscaselinker"moleculeswithfunctionalitiesatbothendsandananityforAucanbeusedtoaidinfunctionalization.Awell-knownexampleisthioladsorption,inwhichalkanedithiolsarecovalentlyattachedtotheAusurface[56].Thisopensupagreaterrealmofpossibilitiesasthiolscanbebondedwithproteins,peptides,carbohydrates,lipids,andDNA[49].ItiswidelythoughtthatgreaterfunctionalityofFe3O4nanoparticlescanbeachievedthroughcoatingthemwithAuandexploitingAu-thiolchemistry.5.1.2HyperthermiaTreatmentforMalignantCellsThepreferentialkillingofcancercellswithoutdamagingnormaltissuehasbeenoneofthemaingoalsofcancertherapyformanyyears[7].Thepotentialofhyperthermialocalizedheatingasatreatmentforcancerwasrstpredictedfollowingobservationsthatseveraltypesofcancercellsweremoresensitivetotemperaturesinexcessof41Cthantheirnormalcounterparts[35].Theuseofmagneticnanoparticlesforhyperthermiainvolvesdispersingtheparticlesthroughoutthetargettissue,andthenapplyinganACmagneticeldofsucientamplitudeandfrequencytocausetheparticlestoheat.Thisheatconductsintotheimmediatelysurroundingdiseasedtissuewhereby,ifthetemperaturecanbemaintainedatthethresholdof41Cfor30minutesormore,thecancerouscellisdestroyed.Magneticnanoparticlesinasuspensioncanbeheatedviafourdierentmechanisms:Brownianrotation,Neelrelaxationlosses,eddycurrentlossesiftheparticleisalsoconducting,andhystereticlosses.Nomatterwhattypesoflossescontributetotheheating,ithasbeensuggestedthattheproductofthefrequencyandmagnitudeoftheappliedeldnotexceedHf=6106OeHz[5].BrownianrotationandNeelrelaxationlossesbothoccurasaconsequenceofamagneticparticleexposedtoanACmagneticeld.Brownianrotationreferstothephysicalrotationoftheparticleinsuspensionanddependsontheparticlesizeandviscosityoftheuid.ItisthedominantmodeofrelaxationatlowerfrequenciesfinthekHzrange.Neelrelaxationreferstothemovementofthemagneticmomentinresponsetothemagnetic47

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eldanddominatesathigherfrequencies.BothtypesofrelaxationcanbeprobedbyACsusceptibilitymeasurementssection2.2.Generally,theuseoffrequenciesintherangef=0:05)]TJ/F15 10.909 Tf 11.672 0 Td[(1:2MHzisconsideredsafeforhumans[62],soinprinciplebothrelaxationmodescouldbeusedtocontributetoheating.Ifthemagneticparticleisalsoconducting,eddycurrentscanformwithintheparticle,andcontributeinductiveheating.However,Fe3O4,whichisaninsulator,hasbeenlookedatthemostforhyperthermiabecausethemagneticpropertiesandbiocompatibilityfaroutweighthebenetsofheatingviaeddycurrentsinhyperthermia.Metallicnanoparticles,suchasFe,areoftentoxicandhighlypyrophoric,andspecialcareneedstobetakentosafelycoatthesurface.Usinganalreadysafematerialwithhighsaturationmagnetization,likeFe3O4,andcoatingitwithAuasproposedintheprevioussubsection,wouldaidinhyperthermiaapplicationsaswellbecausetheFe3O4couldcontributeBrownianandNeellosses,whileeddycurrentsaregeneratedontheAusurface,addingtotheoverallheatingabilityoftheparticles.Thusfar,wehaveassumedthatthemagneticnanoparticlesinsuspensionaresuper-paramagnetictoavoidagglomeration,andaidineasierpassagethroughthecellmembrane.Largely,superparamagneticparticlesarepreferredbecausetheystayinsuspension,thusallowingeasiermanipulationtothespecicsite.However,oncetheparticleshavebeentakenupbyatumorcellandanACmagneticeldhasbeenapplied,thisisnolongeranissue.Infact,aparticlethatissuperparamagneticatroomtemperatureandinDCeldsbutacquiresacoercivityinanACeldwouldbeverybenecialforhyperthermia.Theamountofheatgeneratedperunitvolumeisgivenbythefrequencymultipliedbytheareaofthehysteresisloop:PFM=0fIHdM.1Recently,Eggemanetal.[17]studiedthesizeandconcentrationeectsofironoxidenanoparticlesthatexhibitedhysteresisathigherfrequencies,andfoundthathystereticlossesintheseparticlesaresignicant.Therefore,beingabletotunetheblockingtem-48

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peratureandblockingfrequencytooptimizefunctionalityandhystereticlossesisaviableroutetoimprovingpropertiesforhyperthermia.5.1.3MRIContrastEnhancementMagneticresonanceimagingMRIiscurrentlyoneofthemostcommonimagingtech-niquesforsofttissuestructureofthemusculoskeletalsystem.Inbrief,itmeasureschangesinthemagnetizationofhydrogenprotonsinwatermoleculessittinginamagneticeldafterapulseofradiofrequencywithHACperpendiculartoHDCispropagatedthroughthesample.Protonsfromdierenttissuereactdierently,givingapictureoftheanatomicalstructures.Theseimagescanbeenhancedbyaddingcontrastagents,whichsharpenthecontrastbyaectingthebehavioroftheprotonsintheirvicinity.InstandardclinicalMRIscans,contrastagentstravelthroughthebloodstreamandtissues,increasingcontrastev-erywhere.ThemostcommonlyusedMRIcontrastmediaaregadoliniumchelates,whichtendtobenon-specicwithrapidaccumulationintheliverandonlyallowashortwindowforimaging.Dextran-coatedsuperparamagneticironoxidenanoparticlesforMRIenhancementarebecomingincreasinglycommon.Theyareselectivelytakenupbyreticuloendothelialsys-tem,thecellsthatlinebloodvessels,whosefunctionistoremoveforeignsubstancesfromthebloodstream.NanoparticlesusedforMRIcontrastenhancementrelyonthedieren-tialuptakeofanatomicalregions,andnanoparticlesizeplaysasignicantroleinwhichcellsselectivelyuptakethem.Largerparticlesd>30nmaretakenupbytheliverandspleen,smallerparticlesd<10nmarenoteasilydetectedandpropagatethroughthebloodstreamandreticuloendothelialcells,includingbonemarrowandlymphnodes.Smallparticlescanalsobeusedtoimagethevascularsystemandcentralnervoussystem.5.1.4TransverseSusceptibilityasaBiosensorOnecanthinkofanynumberofwaystouseanexternaleldtosensecellsthathavetakenupnanoparticles.Recently,thelargechangeinmagnetoimpedanceofamorphous49

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magneticribbonswasusedtodetectthepresenceofcellsloadedwithmagneticnanopar-ticles.Thismethod,inwhichtheimpedanceofamagneticconductorchangeswithHDC,reliesonthefringeeldsassociatedwiththemagneticnanoparticlesinsidethecellstogiveachangeinmagnetoimpedancefromthenormalresponseexhibitedbytheribbons[40].Weproposedthattransversesusceptibilitymaybeusedtosenseparticlesthathavebeentakenupbycellsaswell.Recallthatthistechniqueishighlysensitivefortwodistinctreasons:anysusceptibilitymeasurementisameasureofthederivativeofthemagneticre-sponsewithrespecttoeld,andourmethodofmeasurementisaresonanttechniquewhichcandetectchangesinfrequencyontheorderof10Hzin10MHz.Therefore,itispossiblethatevenasmallsampleofcellsthathavetakenupnanoparticleswhenplacedinsidethesamplespaceofthetransversesusceptibilityprobecouldyieldasignalcharacteristicofthemagneticnanoparticles.5.2NanoparticleSynthesisBothFe3O4nanoparticlesandAu-coatedFe3O4weresynthesizedattheUniversityofSouthFloridaCollegeofMedicinebyDr.ArunKumarfollowingtheprocedureoutlinedbyMandaletal.[49]usingamicellarmethod.First,astocksolutionwasmadebydissolvingferricammoniumsulfate.128MwithrespecttotheFeIIIionandferrousammoniumsulfate.064MwithrespecttotheFeIIionin100ml0.40Maqueoussulfuricacid.Aseparatesolutionof1.0MNaOHwasaddedto0.01MpolyoxyethyleneisooctylphenyletherTX-100tomakeaconcentrationof0.01MTX-100.Ofthissolution,25mlwasaddeddropbydropto0.01MTX-100.Thissolutionwasheatedto70-80C,and25mloftheironstocksolutionwasaddeddropbydropwhilestirring.Heatingandstirringcontinuedfor30minuteswhileFe3O4nanoparticleswereformed.Theparticleswerecentrifugedtoseparatethemfromsolutionandwashed.TheresultingFe3O4particlesizesweremeasuredusingTEMandfoundtohaveanaveragediameterof60nmwithamoderatesizedispersion.Thissizeofparticleisdesirablebecausethecellstobeusedinthisexperimentpreferentially50

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takeupparticlesof60-70nm.ThestaticanddynamicmagneticpropertiesoftheFe3O4particleswerestudiedandarepresentedinthenextsection.TheprocedureoutlinedabovewasfollowedtoobtaintheFe3O4particles,whichwerethencoatedwithgold.Forthisstep0.5gofglucosewasaddedtoasolutionof1:1molarratioFe3O4toHAuCl4.Thesolutionwassonicatedfor15minutesandthenheatedinawaterbathfor1hour.Mandaletal.reportedinreference[49]thattheglucosehelpspromoteAu-Fe3O4adhesionandmaintainAuthicknessuniformity.Themagneticprop-ertiesoftheAu-Fe3O4particleswerealsostudiedandpresentedinthefollowingsection.DuetothedualfunctionalityoftheAuandFe3O4,theseparticleswerechosenforcelltransfection,andsubsequentlytestedfordetectionusingtransversesusceptibility.5.3DCMagneticPropertiesofFe3O4andAu-Fe3O4Nanoparticles5.3.1Fe3O4NanoparticlesZeroeldcooledandeldcooledcurvesweretakenoftheFe3O4particlesinanexternaleldof100Oegure5.1.TheZFCcurveisconsistentwithapolydispersesampleofnanoparticles,withabroadblockingtemperatureTBthatoccursaround267K.ThistemperaturematcheswellwiththatreportedbyGoyaetal.[22]for50nmFe3O4particles.TheyalsoreportabumpintheZFCcurveat16K,whichtheyattributetotheVerwaytransition,awell-studiedstructuraltransitionwhichoccursat120KinbulkFe3O4.TheVerwaytransitionhasbeenshowntobehighlytemperaturedependentinnanoparticles,shiftingtolowertemperatureastheparticlediameterdecreasesuntilitcannotbeseeninparticlesbelowabout40nm[22].TheZFCcurveingure5.1doesshowasmallbumpat16Kaswell,whichre-enforcesthenotionthattheseparticlesarerelativelylarge.ThenearlylinearrelationshipbetweenmagnetizationandtemperatureintheZFCcurveindicatesthattheinter-particleinteractionsintheseparticlesarestrong,whichistobeexpectedsincetheseparticleswerenotcoatedwithasurfactant,sowerefreetoagglomerate.Particleswithfewerinteractionstendtoshowamagnetizationthatrisesfasterthantheincreaseintemperature,resultinginamorecurvedzeroeldcooledmagnetizationversus51

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Figure5.1.ZeroeldcooledandeldcooledcurvesforFe3O4particles.temperature.WealsoperformedDCmagneticmeasurementsontheFe3O4particlesafterdispersingtheminaparanwaxmatrixtoseeifthatwouldaecttheoverallshapeoftheZFCandFCcurvesgure5.2.ThiswasdonebytakingadrysampleofFe3O4andseveralpelletsofwaxinavial,andsonicatinginahotwaterbathwhilethewaxmelted.Thissuspendedtheparticlesinthewaxmatrix,andpreventedthemfromagglomerating.Ideally,theinter-particledistanceisenoughtopreventtheparticlesfromexperiencingdipolarinteractionsfromtheirneighbors.TheZFCcurvedidshowbetterdenitionwithasteeperriseofmagnetization,moreconsistentwithnon-interactingparticles.However,thefreezingofresidualwaterinthesamplemadeitverydiculttoobservethemagneticbehaviorrightaroundtheblockingtemperature.Figures5.3and5.4arethe300Kand2KmagnetizationversuseldcurvesrespectivelyfortheFe3O4powder.Thelackofcoercivityinthe300KM-Hcurveconrmsthattheparticlesaresuperparamagneticat300K.At2K,acoercivityispresentof430Oe.52

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Figure5.2.ZeroeldcooledandeldcooledcurvesforFe3O4nanoparticlessuspendedinaparanwaxmatrix. Figure5.3.MagnetizationversuseldcurveforFe3O4nanoparticlestakenat300K.53

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Figure5.4.MagnetizationversuseldcurveforFe3O4nanoparticlestakenat2K.Thecoercivityofthesystemofparticleswillalsobeaectedbyinter-particleinterac-tions,sothelowtemperatureM-HcurvewasrepeatedaftertheFe3O4particleswereputintoparanwax.Indeed,thecoercivitywasreducedfrom430Oeto230OeindicatingthatwhileinteractionswerepresentinthecaseofbareFe3O4particles,theseinteractionsweregreatlyreducedoreliminatedbyplacingtheminthewaxmatrixgure5.5.5.3.2Au-Fe3O4NanoparticlesDCmagneticmeasurementswerethentakenoftheFe3O4particlescoatedwithAu.AscanbeseenintheZFCcurvegure5.6,theapproachtotheblockingtemperatureissteeperthanfortheFe3O4particles.Thisisconsistentwithalessinteractingsystem,whichcanbeexpectedfortheparticlescoatedwithAu,asopposedtothebareFe3O4.Indeed,oneofthemanyreasonsforcoatingtheFe3O4withgoldistominimizetheinter-particleinteractions.54

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Figure5.5.MagnetizationversuseldcurveforFe3O4nanoparticlessuspendedinparanwaxtakenat2K.TheblockingtemperaturefortheAu-Fe3O4particlesisslightlyhigherthantheFe3O4samples.Thiscouldbeduetoslightdierencesinchemicalbatches,asthetwoweresynthesizedondierentoccasions.Inthissample,liketheFe3O4,thebroadblockingindicatesaparticlesizedispersion.ItislikelythatbothsetsofsampleshaveparticlesizedispersionswiththeAu-Fe3O4haveanaverageFe3O4particlesizeslightlylargerthantheFe3O4particlesmeasuredintheprevioussection.ThelowtemperaturefeaturethoughttobeassociatedwiththeVerwaytransitionispresentinthissampleaswell,occurringat17Kratherthan16K.LikethebareFe3O4particles,the300KM-HcurvesfortheAu-Fe3O4particlesgure5.7conrmthattheyaresuperparamagneticatroomtemperature.At2K,thecoercivityis200Oegure5.8,lowerthanthe430OemeasuredfortheFe3O4particlesalone,andcomparabletotheparticlessuspendedinwax.Again,thiscanbeattributedtoadecreaseininter-particleinteractions.ThisdemonstratesthatsuspendingtheparticlesinwaxandsimplycoatingthemwithalayerofAuachievethesameobjective,thatis,eliminating55

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Figure5.6.ZeroeldcooledandeldcooledcurvesforAu-Fe3O4nanoparticles.inter-particleinteractions.However,whereasmanipulatingnanoparticlesinasolidmatrixisnotveryusefulforbiomedicalapplications,theAu-coatedFe3O4particlesretaintheirversatilitybybeingabletobemanipulatedwhileinabiocompatibleliquidsuspension.5.4TransverseSusceptibilityMeasurements5.4.1Fe3O4NanoparticlesTransversesusceptibilitymeasurementswererstperformedonthebareFe3O4par-ticlesnotinwaxsuspension.Thetransversesusceptibilitydatafortheseparticlesshowindiscerniblefeatures,likelyduetotheinter-particleinteractions.Arepresentativelowtemperaturebipolarscanisshowningure5.9.Therefore,inordertomakeanyquanti-tativeobservationsoftheanisotropyoftheFe3O4particles,welimitthisdiscussiontothetransversesusceptibilitymeasurementsmadeonFe3O4nanoparticlessuspendedinparanwax.Forthisexperiment,thesamewaxsamplewasusedthatwaspresentedinsection4.3.1.56

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Figure5.7.MagnetizationversuseldcurveforAu-Fe3O4nanoparticlestakenat300K. Figure5.8.MagnetizationversuseldcurveforAu-Fe3O4nanoparticlestakenat2K.57

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Figure5.10showsseveralunipolartransversesusceptibilityscansoftheFe3O4particlesinwax.Atlowtemperatures,anisotropypeakscanbeseen.Asthetemperatureincreases,thepeaksshifttolowerelds,andeventuallymergeintoasinglepeakaroundH=0,signalingtheferromagnetictosuperparamagnetictransition.Itcanbeseenfromthesegraphsthatanisotropypeaksarestillpresentat250K,butthetransitioniscompleteby300K,inagreementwiththeZFCcurve.Figure5.11isaloweld,lowtemperature,bipolarscanoftheparticlesclearlyshowingthattwodierentanisotropypeaks,HK1andHK2,arepresent.Recallthatasymmetryinthetwoanisotropypeaksiscommonfornanoparticles,andcanbecorrelatedwithalackofinter-particleinteractionsbecausetheH=0energyenvironmentismuchdierentthantheH=Hsatenvironmentforanon-interactingarrayofparticles.HereHK1415Oe,whileHK2530Oe.Ingure5.12wepresenttheloweld,roomtemperature,bipolartransversesuscepti-bilityscan.Adistinctlackofanisotropypeakscanbeseen.Itcanbeconcludedfromthisscanthatatroomtemperatureand12MHznearlyalloftheFe3O4particlesinthewaxmatrixareinthesuperparamagneticstate.5.4.2Au-Fe3O4NanoparticlesIngure5.13wepresentseveralunipolartransversesusceptibilityscansfortheFe3O4particlescoatedwithAu.Again,anisotropypeakscanclearlybemadeoutatthelowesttemperatures,withthepeaksshiftingtosmallereldsasthetemperatureisincreased.Inthiscase,thereremainsaslightferromagneticsignatureevenatroomtemperature,whichindicatesthatnotalloftheparticleshaveundergonetheferromagnetictosuperparamag-netictransition.ThisisconsistentwiththeAu-Fe3O4particleshavingaslightlyhigherDCblockingtemperatureKversus267KforthebareFe3O4particles.Again,keepinginmindthattheparticlesweresynthesizedontwodierentoccasions,slightdierencesinsizedistributionsisnotsurprising.WhatisimportantthoughisthefactthatbothsamplesaresuperparamagneticatroomtemperatureandinDCelds,whichisnormallyhowmag-58

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Figure5.9.LowtemperaturebipolartransversesusceptibilityscanofbareFe3O4notsus-pendedinparanwax.neticnanoparticlesaremanipulatedinabiologicalenvironment.ByslightlyincreasingtheaveragesizethustheDCblockingtemperature,thesuperparamagneticparticlesbecomeferromagneticinanACeld,inthiscasetheACeldis12MHz.Whilethisfrequencyisslightlyabovethegenerallyacceptedsafefrequencyof1.2MHz,thenotionoftuningthesizetocorrespondwithDCsuperparamagneticbehaviorandACferromagneticbehaviorseemstobeachievablefortheseparticles.Figure5.14isaloweld,lowtemperature,bipolarscanoftheAu-Fe3O4particlestakenat20K.LiketheFe3O4particlessuspendedinwax,theshapeofthetransversesusceptibilityscanfortheseparticlesindicatesanasymmetryinpeakpositionwithHK1415Oe,andHK2535Oe.ThesevaluesmatchupverywellwiththeFe3O4particlesindicatingthattwoofthemostimportantqualitiesoftheFe3O4particlesinparanwaxtheanisotropyeldHK,andthelackofinter-particleinteractionsareintactintheAu-Fe3O4particles.ThisshowsthatcoatingtheFe3O4particleswithAuachievesthe59

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Figure5.10.TransversesusceptibilityscanstakenatseveraldierenttemperaturesforFe3O4particles.60

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Figure5.11.LowtemperatureKtransversesusceptibilitybipolarscanofFe3O4parti-cles. Figure5.12.RoomtemperatureKtransversesusceptibilitybipolarscanofFe3O4particles.61

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importantgoalofseparatingtheparticlesandreducingtheinter-particleinteractions,whilestillmaintainingananopowderformthatcanbesuspendedinabiocompatiblesolvent.Figure5.15showsaloweld,roomtemperature,bipolarscanoftheAu-Fe3O4particlessothattheanisotropypeakscanstillbeseen.Itislikelythatthemajorityofparticlesaretoosmalltocontributetotheanisotropypeaksi.e.aresuperparamagneticevenat12MHz,buttheverylargestparticlesareferromagneticatthisfrequency.Inthenextsections,wedescribehowthetransversesusceptibilitymeasurementwasrepeatedforhumanembryonickidneycellsaftertransfectionwiththeAu-Fe3O4particles. Figure5.13.TransversesusceptibilityscanstakenatseveraldierenttemperaturesforAu-Fe3O4particles.5.5NanoparticleTransfectionHumanembryonickidneyHEK293cellswereobtainedfromtheAmericanTypeCul-tureCollectionATCC.Cellswereculturedonaplasticsubstrateat37Cinminimum62

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Figure5.14.LowtemperatureKtransversesusceptibilitybipolarscanofAu-Fe3O4particles.essentialmediumcontaining10%fetalbovineserumand100U/mleachofpenicillinandstreptomycininanatmosphereof5%CO2/95%air.Au-Fe3O4nanoparticleswereintroducedtothemediumatconcentrationsof0.05,0.1,0.3,0.5,and1mg/mlbuer,wheretheyweretransfectedbythecellsviaphagocytosis.Cellswerethendetachedfromthesubstratebyremovingexcessmedium,rinsingthecelllayerwith0.25%w/vTrypsin-0.53mMEDTAsolutionandaddingTrypsin-EDTAso-lution.Acompletegrowthmediumwasthenaddedtothecellsforincubation.Figure5.16isaTEMimageofacellaftertransfection.Thecircleindicatestheregionwherethenanoparticlesarelocated,andtheparticlesappearasthedark,lament-likestructures.Thenanoparticlescanberecoveredfromthecellsthroughhomogenization.Previousstudiesindicatedthatthepercentageofnanoparticlestransfectedatmaximumconcentra-tion1mg/mlisapproximately70%.63

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Figure5.15.RoomtemperatureKtransversesusceptibilitybipolarscanofAu-Fe3O4particles.5.6TransverseSusceptibilityMeasurementsofCellswithAu-Fe3O4NanoparticlesTransversesusceptibilitymeasurementswereperformedoncellsthathadnotbeentransfectedwithnanoparticles,aswellascellsaftertransfectionofnanoparticlesinalloftheconcentrationslistedintheprevioussection.Foreachoftheseexperiments,asampleofcellswasplacedinsideofaliquid-safe,1mlsampleholder.Aninductancecoilsimilartotheoneusedinthepermanenttransversesusceptibilitysetupwaswoundaroundthesampleholderandheldinplaceoneachendbytwosmallo-rings.Thecoilcontainingthesamplewasthensolderedintothemultifunctionalprobeinplaceoftheregularinductancecoil,andfoundtoself-resonateatthesamefrequencyof12MHz.ThetransversesusceptibilityprobewasthenplacedinsideofthePhysicalPropertiesMeasurementSystemafterthesamplechamberhadbeenwarmedtoambienttemperature,andthepressureinsidethesamplechamberwasmaintainedat1atmosphere.Thisensured64

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Figure5.16.TEMimageofAu-Fe3O4particlescircledinsideofHEKcells.65

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Figure5.17.TransversesusceptibilitymeasurementsofHEKcellswithvaryingconcentra-tionsofAu-Fe3O4nanoparticles.Theblackscanisforthecellswithoutanynanoparticles.thattheparticlescouldstayinsideofthecellsasfreezingofthecellscausesthenanoparti-clestobeexpelled.Applyingalargeenoughpressuregradientmighthavecausedthecellstobetakenfromthesampleholder,anddepositedintothesamplechamberofthePPMS.Low-eldtransversesusceptibilityscansatambienttemperatureandpressureweretakenofthesamples.Figure5.17showstheunipolarscanofthecellswithseveralcon-centrationsoftheparticles,aswellasascanofjustcellsthatwerenottransfectedwithnanoparticles.Itcanclearlybeseenthatthetransversesusceptibilityprobewasabletodetectasignalfromthenanoparticlesinsideofthecells,whereasthecellsbythemselvesleftnosignal.Forthelowestconcentrationofparticles.05mg/ml,notshown,nosignalcouldbeseenandataconcentrationof30mg/mlnotshown,thesignalwastoonoisytoexhibitanisotropypeaks.Asexpected,thehighestconcentrationofnanoparticlesgivesthebestsignal,butitisimportanttonotethatatlowerconcentrationsthesignaloftheparticlescanstillbeseen.66

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TheanisotropypeaksseenforthenanoparticlesinsideofthecellsappearmoredenedthanthoseseenatroomtemperatureforjustthesampleofAu-Fe3O4particles.Inthiscase,thesamebatchofAu-Fe3O4particleswasusedforthemeasurementasforthetransfection.Itiswell-knownthatcertaintypesofcellswillonlytakeupparticlesinaparticularsizerange,whichwaswhytheparticlesweresynthesizedinthisexperimenttobearound60nm.However,whiletheDCmeasurementsandeventhetransversesusceptibilitymeasurementsofjusttheparticlespointedtoamodestsizedispersion,itmaybethatthecellsonlytakeupthebiggestparticles,actingasasizeselectionmechanismforthenanoparticles.Ifmanyofthesmallerparticleswerenottakenupbythecells,theferromagneticsignalat12MHzwouldbestrongerinthissamplethanthepolydispersenanoparticlesamplegure5.15.Thisexperimentdemonstrateshowtransversesusceptibilityasameasurementtech-niquecanactasabiosensorforthepresenceofmagneticnanoparticlesinsideofcells.Thiscouldbeusedinadiagnosticcapacityifthenanoparticlesarefunctionalizedwithabiomarkerspecictoatypeofcancercell.Transfectionofthenanoparticleswouldonlyoccurifthecellswerecancerous,andthentransversesusceptibilitycouldbeusedtode-termineifthecellshadtakenuptheparticles,andthereforeiftheyarecancerous.EventhoughweusedaPhysicalPropertiesMeasurementSystemtoprovidetheHDCitiswellworthnotingthattheeldsneededforthisexperimentarelessthan500Oe,aeldstrengtheasilyachievablewithanelectromagnet.Themeasurementswerealsotakenatroomtem-peratureandnotinavacuum.Realistically,transversesusceptibilityusedinthiscapacitycouldbesetupasatable-topexperimentratherthanintegratedintoacommercialPPMS.5.7ConclusionInthischapter,wepresentedDCmagneticcharacterizationandtransversesusceptibil-itydataforFe3O4andAu-Fe3O4nanoparticlesforbiomedicalapplications.WeshowedthatbareFe3O4particleshavestronginter-particleinteractions,compromisingthemag-neticproperties,andlikelyleadingtoparticleagglomeration.Thisisnotpreferablefor67

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biomedicalapplicationssuchasdrugdeliveryorhyperthermia,whichrequiretheuptakeofsingleparticleswithinaspecicsizerange.Au-Fe3O4particlesshowedalmostidenticalmagneticpropertiestoFe3O4particlesthatweresuspendedinaparanwaxmatrix,showingthatacoatingofAuaroundtheFe3O4actstogreatlydiminishoreliminateinter-particleinteractionsandagglomeration,whilestillkeepingtheversatilityofamagneticnanoparticleinabiocompatiblesuspension.Additionally,theAucoatingisattractivebecauseitoersmorechoicesforfunctionalizationviaAu-thiolchemistry.WhilealltheparticlesstudiedweresuperparamagneticatroomtemperatureandunderDCappliedelds,itwasdemonstratedjusthowsensitivetheferromagnetictosuperpara-magnetictransitionistoappliedACeldsintheparticlesizerangestudied.TheblockingtemperatureoftheFe3O4particleswasslightlylowerthanthatoftheAu-Fe3O4parti-cles,andtheFe3O4particlesshowedroomtemperaturesuperparamagnetismevenat12MHz.SomeoftheAu-Fe3O4particlesappearedtostillbeintheferromagneticstateat12MHz,consistentwiththehigherblockingtemperature,indicatingaslightlyhigheraverageparticlesize.ThischangeinmagneticpropertiesfromsuperparamagneticatDCeldstoferromagneticatACeldsisidealforhyperthermia.Thissizerangealsoallowedforeasytransfectionbythecells,andledtotheemergenceofclearanisotropypeakswhentransversesusceptibilitywasperformedonthecellswiththenanoparticles.68

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CHAPTER6DUMBBELL"ANDFLOWER"Au-Fe3O4COMPOSITENANOPARTICLES6.1IntroductionInthepreviouschapter,itwasdescribedhowFe3O4nanoparticleswithanAushellcanbeusefulforseveralbiomedicalapplications.Whilethemagneticcorecanbema-nipulatedbyDCandACexternalelds,thegoldsurfacecanbefunctionalizedtoallowforattachmentoftherapeuticmoleculesorcontrastagentstobedeliveredtothetissueofinterest.Sofar,thekeyrequirementshavebeenthattheAuhaveasurfaceavailableforfunctionalizationandthattheFe3O4beofasizetopromotecellularuptake,whilemain-tainingdesirablemagneticproperties.Whilemostapplicationsrequirethattheparticlesbesuperparamagnetic,wesawinthelastchapterhowtuningthesizeoftheparticlestohavefrequency-dependentblockingaroundroomtemperaturecanoptimizethemagneticproperties.However,aswasshownbyourcollaborators,becauseofthelatticeconstantsofAuandFe3O4beingnearly1:2inratio,thetwocanbegrownepitaxiallycoupledtoeachother[93].TheyusedthisfacttocreatecompositenanoparticlesofAuandFe3O4growntogetherintheshapeofadumbbell,withthetwosharingacommonfacet.Oneadvantagethismayhaveoverthecore-shellstructureisthatnotjusttheAuwillbeavailableforfunctionalitythroughthiolattachment,buttheexposedFe3O4surfacecanbefunctionalizedusingdif-ferentchemistry,allowingthesamecompositeparticletodelivertwotypesofmolecules.Moreover,itwasproposedthattheuniqueshapeoftheparticleandtheAu-Fe3O4interfacecouldgiverisetodierentmagneticpropertiesthanthecore-shellparticles.69

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ItwasalsofoundthatbyslightlychangingthepHofthechemicalreaction,theFe3O4canbenucleatedonseveralAufacets,creatingacompositestructuresimilartoaowerinappearance,withAumakingupthecore,andFe3O4nanoparticlesmakinguppetals".Inthischapter,wewillshowthatthisresultsinasystemwithvastlydierentmagneticprop-ertiesfromthedumbbellparticles,whichcanbeunderstoodintermsofthecomplexandcompetinginteractionspresentineachcompositeparticle.Whilethedualfunctionalityofthedumbbellparticlesispromisingforbiomedicalapplications,theanomalouslyhighanisotropyoftheowerparticlesshouldbefurtherexploredforhigheranisotropyapplica-tionssuchasbeatingthesuperparamagneticlimitforhighdensitymagneticrecording.6.2NanoparticleSynthesisAllnanoparticlesweresynthesizedatBrownUniversitybyProfessorShouhengSun'sgroup[93].ThedumbbellAu-Fe3O4nanoparticleswerepreparedbydecomposingironpentacarbonyl,FeCO5,overthesurfaceofAunanoparticlesinthepresenceofoleicacidandoleylamine.ThemixturewasheatedtoreuxC,followedbyoxidationinair.TheAunanoparticleswereformedinsitubyinjectingHAuCl4solutionintothereactionmixture.Flower-shapednanoparticlesweresynthesizedbychangingthesolventfromanon-polarhydrocarbontoaslightlypolarizedsolventi.e.diphenylether.ThesizeoftheAuparticlescanbetunedbycontrollingthetemperatureatwhichtheHAuCl4isinjected.ThesizeoftheFe3O4particlescanbetunedbyadjustingtheratiobetweenFeCO5andAu.MoreFeCO5resultsinlargerFe3O4particles.Figure6.1isaTEMimageofasampleofdumbbellAu-Fe3O4nanoparticles.Forthissample,theaverageAusizeisabout8nm,andtheaverageFe3O4sizeisabout9nm.ThedarkparticlesrepresenttheAuduetothehigherelectrondensity,andthelighterparticlesaretheFe3O4.Figure6.2isaTEMimageoftheowernanoparticles.Forcomparisonpurposes,theowersamplehasthesamesizesofAuandFe3O4particlesasthedumbbellsamplenmand9nmrespectively.70

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Figure6.1.TEMimageofadumbbellAu-Fe3O4nanoparticle.CourtesyofBrownUni-versityInthissynthesismethod,theFe3O4isgrownepitaxiallyontotheAuseedparticle,whichispossiblebecauseAuhasanfccstructurewitha=4.08A,whileFe3O4hasaninversespinelcubicstructurewitha=8.35A,whichiswithin3%ofbeingexactlydoublethatofAu.ThisepitaxialgrowthisconrmedbyhighresolutionTEMHRTEMforthedumbbellnanoparticlesgure6.3.Thelatticefringesineachparticlecorrespondtoatomicplaneswithintheparticle,indicatingthatbothtypesofparticlesaresinglecrystalline.ThedistancebetweentwoadjacentplanesinFe3O4wasmeasuredtobe0.485nm,correspondingtotheplanesintheFe3O4.ThelatticefringespacinginAuis0.24nm,resultingfromagroupofplanesintheAu.OncetheFe3O4startstonucleateontheAuinthehydrocarbonsolvent,thefreeelectronsfromtheAumustcompensateforthechargeinducedbythepolarizedplaneattheinterface.AstheAuhasonlyalimitedsourceofelectrons,thecompensationmakesallotherfacetsoftheAunanoparticleelectrondecientandunsuitableformultinucleation,givingonlythedumbbellstructure.Ifthepolarityusedforthesynthesisisincreased,theAunanoparticlecouldcompensatefortheapparentelectrondensitylosswithchargesfromthesolvent,allowingnucleationonmultiplefacets,resultingintheowerstructure.[93]71

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Figure6.2.TEMimageofaowerAu-Fe3O4nanoparticle.CourtesyofBrownUniversity6.3DCMagneticMeasurementsMagneticmeasurementsinastaticeldwerecarriedoutonallsamplesusingthePhys-icalPropertiesMeasurementSystemPPMS.ThesemeasurementsconsistedofzeroeldcooledandeldcooledZFC-FCcurvesandmagnetizationversuseldM-Hcurves.Figure6.4showstheZFC-FCcurvesforthedumbbellnanoparticlestakeninaeldof100Oe.TheZFCcurveshowsapeakataround60K,consistentwiththetransitionfromtheblockedstatetothesuperparamagneticstate.TheZFC-FCcurvefortheowernanopar-ticlesgure6.5showsasharperpeakaround90K.ThereisanadditionalfeatureseenintheFCcurve,whichisatuntilabout65K,andthendropsrapidly.ThisdoesnotlineupwiththepeakseenintheZFCcurve,andisindicativeofanothercharacteristictempera-tureassociatedwiththeowernanoparticles,whichwillbeexploredfurtherinsubsequentsections.SincetheAuandFe3O4componentsarethesamesizeforbothsamples,theincreaseintraditionalblockingtemperatureassociatedwiththeowernanoparticleshastobefromanaddedanisotropycontribution,likelyduetotheunusualcongurationoftheparticles.72

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Figure6.3.HighresolutionTEMimageofadumbbellAu-Fe3O4nanoparticle.CourtesyofBrownUniversityInterestingly,theZFCcurvesforboththedumbbellandtheowerAu-Fe3O4particlesexhibitanegativemagnetizationatthelowesttemperatures.Thistypeofbehaviorhasnotbeenreportedintheliteratureforferritenanoparticlesofanyconguration,andislikelyduetotheuniqueinterfacesthatarepresentinthesesystems.Thisfeatureiscurrentlyunderintenseinvestigation,andrecentexperimentshaverevealedthatforbothtypesofparticlesincreasingtheexternalDCmagneticeldresultsinthelowtemperaturemagnetizationincreasing.ZFCcurvesweretakenforseveraleldstodetermineatwhicheldthe2Kmagnetizationcrossesfromanegativevaluetoapositivevalue.Forthedumbbellparticles,thiseldisaround150Oe,whereasfortheowerparticles,itisaround210Oe.Itisnotsurprisingthatinlightofthemultipleinterfacespresentintheowerparticles,thereisahighermagneticenergyassociatedwithchangingthemagnetizationfromanegativevaluetoapositivevalue.M-HmeasurementsforthedumbbellnanoparticlesweretakenatlowtemperatureKandroomtemperatureaswellassomeintermediatetemperatures.Figure6.7showsthecurvesfor2Kand75K,justabovetheblockingtemperature.Thecoercivityat2Kwasmeasuredtobe750Oe,andat75Kthecoercivityiszero,conrmingthattheparticlesaretheninthesuperparamagneticstate.Fortheowernanoparticlesgure6.8,thelowtemperatureM-HcurverevealedamuchhighercoercivityOe.Inaddition,thecurvewasirreversibledidnotcloseupto1.5Teslaanddidnotsaturateateldsupto273

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Figure6.4.ZeroeldcooledandeldcooledcurvesfordumbbellAu-Fe3O4nanoparticles. Figure6.5.ZeroeldcooledandeldcooledcurvesforowerAu-Fe3O4nanoparticles.74

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Figure6.6.Valuesofmagnetizationat2KfortheAu-Fe3O4compositeparticlestakeninthezeroeldcooledconditionfordierentvaluesofH.75

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Figure6.7.MagnetizationversuseldcurvefordumbbellAu-Fe3O4nanoparticlestakenat2Kblueand75Kblack.Tesla.Thesefeatures,combinedwiththehigherblockingtemperatureandanomalous65KfeatureintheFCcurve,areindicativeofafundamentaldierenceinmagneticresponsebetweenthedumbbellwhichareclosertoconventionalandtheowermoreanomalousnanoparticles.OnecaninferthatthemajordierencebetweenthetwosystemsisthegeometricalarrangementoftheFe3O4clusters,andtheroleofthespinsatthesurfacesaswellasatthemultipleinterfacesbetweenFe3O4andAu.Itisknownthatinterfacialspincongurationcanbegreatlyinuencedincore-shellnanoparticles[60,89,75,64],andanimportantconsequenceistheobservationofexchangebiasinsuchmaterials.ThenextsectiondescribesexperimentsthatweredonetoprobethepresenceofexchangebiasinbothofthesetypesofFe3O4particles.76

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Figure6.8.MagnetizationversuseldcurveforowerAu-Fe3O4nanoparticlestakenat2K.6.4ExchangeBiasandTrainingEectinFlower-ShapedNanoparticlesTotestforthepresenceofexchangebias,bothtypesofparticleswerecooledfromroomtemperatureina5Teslaeld,andmagnetizationversustemperaturemeasurementswereperformed.Whilethedumbbell-shapedparticlesretainedasymmetrichysteresisloopforalltemperaturesaftereld-cooling,theower-shapedparticlesexhibitedalargehorizontalshiftintheirhysteresisloopsgure6.9.Thisexchangebiaswasthentestedforanumberoftemperaturesandshowntopersistuptoabout65K,lowerthantheblockingtemperaturebutconsistentwiththefeatureseenintheFCcurve.Thissuggeststwocharacteristictemperaturesfortheowerparticles,asopposedtoonlyoneforthedumbbellparticlesnamely,theblockingtemperature.Thislowercharacteristictemperaturefortheowerparticlesislikelyduetotheonsetofaunidirectionalanisotropy,whoseorigincanbecorrelatedwithinteractionsbetweenFe3O4particlessharingthesameAuseedparticlehenceforth,thesewillbereferredtoasintra-particleinteractionsasopposedtothemore77

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well-knowninter-particleinteractions,theinteractionsbetweenseparateparticles.InadditiontothehorizontalshiftintheM-Hloops,thereisamarkedincreaseincoercivityaftereld-coolingOeaftereldcoolingasopposedto1270Oeafterzeroeldcooling,whichisanothersignatureofexchangebias.Figure6.10showstheexchangebiaseldHE,ortheamountbywhichtheloopisshiftedandcoercivityHCasafunctionoftemperaturefortheowerparticles.Theshapesofthecurvesaresimilar,andbothhave65KasacharacteristictemperatureHEgoestozero,HCdecreaseslessrapidly,consistentwithmoretraditionalnanoparticlesystems.Thesesimilaritiessuggestthatthetwophenomenaarerelated,andthattheanisotropyintheseparticlesisuniaxialaswellasunidirectional.Exchangebiasinnanostructuresisatopicofcurrentinterest,andhasbeenreportedinseveralcore-shellparticles[60,89,75,64,96].Theseresultsfortheower-shapedAu-Fe3O4particlesrepresenttherstreportofexchangebiasinnanoparticleswithanorderedcluster-typegeometry,displayingbothunidirectionalanduniaxialanisotropy.Furthermore,sinceexchangebiasisn'tseenineitheroftheothertwogeometriesstudiedcore-shellanddumb-bellconguration,thebehaviormustbeassociatedthisparticularconguration.Inmanyexchange-biasedsystems,theexchangeeld,HE,andthecoerciveeld,HC,willdecreaseifthemagnetizationversuseldmeasurementisrepeatedimmediatelyafteraninitialmagnetizationversuseldmeasurementistaken.ThisdecreaseinHEandHCuponsubsequentM-Hcyclesisknownasthetrainingeect[60].Likemanyexchange-biasedsystems,theowernanoparticlesalsoexhibitatrainingeect.Figure6.11isaplotoftheeldcooledhysteresislooptakenfrompositivesaturationtonegativesaturationandbacktopositive.Themeasurementwasthenrepeatedimmediatelywithoutzeroingouttheeldorwarmingthesample.HEwasstillpresentbuttoalesserextent,andHCdecreasedaswell.Themeasurementwasrepeatedforonemorecycle,andHEandHCdecreasedagain,butthedecreasefromsecondtothirdcyclewasnotasmuchasfromthersttosecondcycle.Intotal,HCdecreasedfrom1920Oeintherstcycleto1650inthethirdcycle.Theinsetofgure6.11showsHCasafunctionofM-Hcycle.78

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Figure6.9.MagnetizationversuseldcurveforowerAu-Fe3O4nanoparticlestakenat2Kblueand40Kpurpleaftercoolingina5Teslaeld.Itisinterestingtonotethatboththehorizontalshiftinhysteresisloop,andthetrainingeectseeninthenanoparticles,aredierentfromthoseseeninmanyexchange-biasedthinlms.Inthelattercase,thersteldcooledM-Hloopisalsoaccompaniedbyarstorderreversalasymmetry.Inthesesystems,thehysteresisloophasasharpjumpintherstmagnetizationreversalwithdecreasingeld,whilethesecondreversalwithincreasingeldismoregradual.Incontrast,thesecondhysteresisloopismoresymmetricwithasmallerdecreaseinHEandHC,andsimilarshapesforbothmagnetizationreversals.Anysubsequenthysteresisloopsareunchangedinshapefromtherst.Itissuggestedthatinthinlms,tworeversalmechanismsarepresent,withdomainwallnucleationandprop-agationbeingresponsibleforonereversal,andcoherentrotationofmagnetizationbeingresponsiblefortheother[54].Innanoparticlesystems,thereisonlycoherentmagnetiza-tionrotation,sothehysteresisloopcanonlyshowonetypeofreversal,whichisconsistentwithZhengetal.'smodeloftrainingeectsin-Fe2O3coatedFenanoparticles[96].In79

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Figure6.10.HEandHCasafunctionoftemperatureforowerAu-Fe3O4nanoparticlesaftercoolingina5Teslaeld.thismodel,thetrainingeectandhorizontalshiftinM-HloopshasbeeninterpretedbyamodiedStoner-Wohlfarthmodel,withanadditionalunidirectionalanisotropyenergytermtothetotalenergy.Ifthepresenceofapinnedlayer,whichcausestheexchangebias,isduetoaspin-glass-likesurfacephase,thenthefrozenspinswhichwereoriginallyalignedinthecoolingelddirectionmaychangetheirdirectionsandfallintoothermetastablecongurationsduringthehysteresismeasurements,leadingtoatrainingeect.Inthiscase,itisnotexactlyclearwhetheritisasurfacespineectwhichiscausingthetrainingeect,orthepinningofspinsattheAu-Fe3O4interface.Inthelastsection,wewillpresentaschematicpicturedescribingthevariouspossibleinteractionsintheowerparticles.6.5ACSusceptibilityMeasurementsWemeasuredthetemperaturedependenceofACsusceptibilitybothinphase0Tandoutofphase00Tinthefrequencyrangeof10Hzto10kHzforbothsetsof80

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Figure6.11.Trainingeectinower-shapedAu-Fe3O4nanoparticles.TheinsetshowsthecoercivityHCasafunctionoftheloopcycle.compositenanoparticles.Thedataforboththedumbbellnanoparticlesgure6.12andowernanoparticlesgure6.13showsuperparamagneticbehaviorwithamaximumatatemperatureTm,whichshiftstoahighertemperaturewithincreaseinfrequency.Theout-of-phasecomponentoftheACsusceptibilityfortheowerparticlesshowsashiftinpeakpositionwithanincreaseinfrequency,aswellasadecreaseinpeakmagnitude,againconsistentwithsuperparamagneticparticles.Asdiscussedinchapter2,ThemagnetizationreversalofasingledomainparticleovertheanisotropybarrierEacanbedescribedusingtheNeel-Arrheniuslaw:=0expEa=kBT[22,73].Werstplotted1/TBvs.lnf,andfromthettingoftheexperimentaldatausingtheaboveequation,weobtainedvaluesofEa=kBfortheoweranddumbbellparticlesas4211K.810)]TJ/F19 7.97 Tf 6.587 0 Td[(20Jand2741K.810)]TJ/F19 7.97 Tf 6.586 0 Td[(20Jrespectively.Thettedvaluesof0fortheoweranddumbbellparticleswere4.510)]TJ/F19 7.97 Tf 6.587 0 Td[(17sand3.110)]TJ/F19 7.97 Tf 6.587 0 Td[(15srespectively.Theseattemptfrequenciesareunphysicalandcannotaccuratelyde-81

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Figure6.12.Temperaturedependenceofthein-phase0Tandout-of-phasecomponent00TofthemagneticsusceptibilityforthedumbbellAu-Fe3O4nanoparticles.82

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Figure6.13.Temperaturedependenceofthein-phase0Tandout-of-phasecomponent00TofthemagneticsusceptibilityfortheowerAu-Fe3O4nanoparticles.83

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scribetherelaxationoftheseparticlessincetheaccepted0foranarrayofnon-interactingsuperparamagneticparticlesisbetween10)]TJ/F19 7.97 Tf 6.586 0 Td[(10-10)]TJ/F19 7.97 Tf 6.587 0 Td[(9s.ThispromptedustotrytousetheVogel-Fulchermodeltocalculate0andEa=kBvaluesfortheAu-Fe3O4particlesgure6.15.Recallfromsection2.2thattheVogel-FulcherequationcanaccountforweakdipolarinteractionbyincorporatingtheparameterT0sothat)]TJ/F19 7.97 Tf 6.586 0 Td[(1=)]TJ/F19 7.97 Tf 6.586 0 Td[(10expEa=kBT)]TJ/F21 10.909 Tf 9.108 0 Td[(T0.Thistimethevaluesof0andEa=kBforthedumbbellparticleswere0.8010)]TJ/F19 7.97 Tf 6.587 0 Td[(8sand616Krespectively.Thesearebothreasonablevaluesandindicatethattheweaklyinteractingparticleassumptionisvalidforthedumbbellnanoparticles.ThevalueofT0,whichcanbethoughtofasanactivationenergyforthedipolarinteractioninthissystem,is50K.FortheowerAu-Fe3O4particles,wewerestillunabletoobtainreasonablevaluesfor0andEa=kB.Infact,thevaluesdeviatedevenfurtherfromtheacceptedrangeasweobtaineda0of0.3610)]TJ/F19 7.97 Tf 6.587 0 Td[(27sandanEa=kBof13675K.Wealsoobtainedanonsensicalvalueof)]TJ/F15 10.909 Tf 8.485 0 Td[(105KforT0.NeithertheNeel-ArrheniustsnortheVogel-Fulchertssuccessfullymodeledtherelaxationoftheowerparticles,whichindicatesthattheparticlesareinter-actingsincetheydon'ttNeel-Arrhenius,butthattheinteractionscannotbedescribedasweakanddipolar.Thispointstotheexistenceofamuchstrongerintra-particletypeinteractionintheowerparticles.6.6TransverseSusceptibilityMeasurementsTofurtherstudytheanisotropypropertiesofbothtypesofparticles,wedidtransversesusceptibilitymeasurementsoverabroadrangeoftemperaturesK
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Figure6.14.Plotoflnfagainst1/TBforbothtypesofnanoparticles.ThesolidredlineisthettotheNeel-Arrheniuslaw.85

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Figure6.15.Plotoflnfagainst1/TBforbothtypesofnanoparticles.ThesolidredlineisthettotheVogel-Fulcherrelation.86

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Figure6.16.UnipolartransversesusceptibilitycurvesofthedumbbellAu-Fe3O4nanopar-ticlestakenatseveraldierenttemperatures.broadeningofanisotropypeaksatthelowesttemperaturesisduetotherandomfreezingofspins[66].Athighertemperatures,abovetheblockingtemperature,thepeaksmergeintoasinglepeakatzeroeldwithamonotonicdecreaseintransversesusceptibilityastheeldisincreasedinthenegativeandpositivedirections.Thissignalsthetransitionfromtheblockedstatetothesuperparamagneticstate.Figure6.17showsthelow-eldportionofabipolarscantakenat30Kforthesamesampleofdumbbell-shapedparticlesaspresentedingure6.16.Asisthecasewithmostnanoparticles,twodistinctsetsofpeakscanbeseen,thenarrowerpeakwiththehighermagnitudeHK1occurringaftersaturation,thebroaderpeakwiththelowermagnitudeHK2occurringaftertheeldpassesthroughzero.Asdiscussedinchapter3,thisislikelyduethedierencesintheenergystatesofthesystemduringsaturationwhentheZeemanenergyishighest,andaftertheeldisdecreasedtozero,allowingsomeofthe87

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Figure6.17.LoweldportionofatransversesusceptibilityscanofthedumbbellAu-Fe3O4nanoparticlestakenat30K.Thearrowsindicatethesequenceofmeasurement.spinstorandomizeagain.AlsorecallthataccordingtoAharoni'stheory[2],oneshouldseeaswitchingpeakaswellforaStoner-Wohlfarthparticle.Innanoparticlearrays,theswitchingeldassociatedwitheachparticleisnotawell-denedpeak,andisinsteadadistribution,whichislikelymergedintotheHK2peak.Forthedumbbellnanoparticles,thesetwoseparatepeaks,HK1andHK2,aresymmetricwithrespecttothepositivetonegative+scanandthenegativetopositive)]TJ/F15 10.909 Tf 8.485 0 Td[(scan.Wealsoseenoshiftinpeakpositionaftercoolingina5Teslaeld,showingtheabsenceofexchangebias,andthusconsistentwiththeeldcooledM-Hmeasurements.Ingure6.18,wepresenttheunipolartransversesusceptibilitydatafortheowernanoparticlesatseveraldierenttemperatures.Themostimportantthingtonoteisthedramaticallylargedierenceintheanisotropyeldsfortheower-shapedparticlescom-paredtothedumbbell-shapedparticles.TherstsetofanisotropypeaksfortheowerparticlesHK11650OeforT=10KoccurateldsthreetimeshigherthanthatforthedumbbellparticlesHK1560Oe.Thepositionofthesecondsetofpeaksanisotropy88

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Figure6.18.UnipolartransversesusceptibilitycurvesoftheowerAu-Fe3O4nanoparticlestakenatseveraldierenttemperatures.peaksfortheowerparticlesHK24050OeforT=10KisoversixtimeshigherthanthepositionofthecorrespondingdumbbellparticleanisotropypeaksHK2630Oe.TheHK1andHK2forT=30Kgure6.19are1650Oeand3000Oerespec-tively,dependingonthe+scanor)]TJ/F15 10.909 Tf 8.484 0 Td[(scan.Thesevalues,alongwiththeunipolardata,areconsistentwiththeower-shapedparticleshavinghighershapeandsurfaceanisotropiesthanthedumbbell-shapedparticles.Theasymmetryinthepeakheight,width,andposi-tionintheunipolarscanismuchmorepronouncedfortheowerparticlesaswell.ThefactthatthereisamuchgreaterdiscrepancybetweenHK2peaksofthetwotypesofparticlesthantheHK1peaksisalsoconsistentwiththelargedierenceseenincoercivitybetweenthetwotypesofparticlessincetheHK2peakcanbecorrelatedwiththeswitchingoftheparticles.Itshouldbekeptinmindthatpeakwidthcanalsobecorrelatedwithothereects,suchasshort-rangeinteractionswithinclusters.Theasymmetryofthetransverse89

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Figure6.19.LoweldportionofatransversesusceptibilityscanoftheowerAu-Fe3O4nanoparticlestakenat30K.Thearrowsindicatethesequenceofmeasurement.susceptibilitypeaksbeinghigherintheowerparticlesalsocanbereconciledwiththefactthateachowerparticleisacompactclusterofFe3O4nanoparticlesboundtoasingleAunanoparticle,thusformingageometryfavoringacertainlevelofshort-rangeinteractionsbetweenthespins.Thepresenceofsuchinteractionsinclusterswouldmakeitrelativelyharderandthepeakssharperforcollectiveippingofthespinsastheeldpolarityischangedduringthescans.Therelativelylowerpeakasymmetryinthedumbbellsisconsistentwiththeabsenceofsuchshort-rangeclustertypeinteractions.Figures6.20and6.21showtheevolutionofbothsetsofpeakswithtemperatureforthedumbbell-shapedandower-shapedparticlesrespectively.Theanisotropyeldsofboththesenanoparticletypesdecreasewithincreaseintemperature.ThestrikingdierenceintherstpeakHK1andsecondpeakHK2positionsintheowerparticlesismostevidentinthisgurewhencomparedtothedumbbellparticles,underscoringtheroleofshapeandsurfaceanisotropyintheAu-Fe3O4system.90

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Figure6.20.ValuesofHK1andHK2asafunctionoftemperatureforthedumbbellAu-Fe3O4nanoparticles. Figure6.21.ValuesofHK1andHK2asafunctionoftemperaturefortheowerAu-Fe3O4nanoparticles.91

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Theexchangebiasintheowernanoparticleswasfurtherprobedbytransversesus-ceptibilitymeasurements.IncomparingtheZFCand5TeslaFCtransversesusceptibilitycurvesgure6.22,wecanseethattheshapesofthecurvesandtherelativepositionsofthepeaksremainedthesame,butthecurveswerehorizontallyshiftedasexpected.WhilethenarrowerpeaksmeasuredinthezeroeldcooledconditionwereseenatHK11650OeandHK23000Oe,intheeldcooledconditioncasetheanisotropyeldvalueswereHK1+1100Oe,)]TJ/F15 10.909 Tf 8.485 0 Td[(1500OeandHK2)]TJ/F15 10.909 Tf 20.756 0 Td[(2600Oe,+2250Oerespectivelydependingonthepositiveornegativescan.ItisalsointerestingtonotethedierenceinshapeofthetransversesusceptibilityscansfortheFCversustheZFCcase.TheFCscanshavethenar-rowerpeaks.Recallthatthewidthofthepeaksdependsontherandomfreezingofspins.IntheFCsituation,thespinsarenotrandomlyfrozen;theirdirectionsaredeterminedbytheeldinwhichtheyarecooled,thusnarrowingthepeaks.ThedierenceofHK1'si.e.HK1andHK2'si.e.HK2betweenthe+scanand)]TJ/F15 10.909 Tf 8.485 0 Td[(scanineldcooledtransversesusceptibilityisameasureoftheexchangebias,HE.ThetemperaturedependenceofbothHK1andHK2matchesverywellwiththeHETfromtheM-Hdatashowningure6.10.Ourtransversesusceptibilitystudyofexchangebiasinthesenanoparticlesrepresentstherstsuchstudyinexchange-biasednanostructures.6.7MemoryEectinFlowerandDumbbellNanoparticlesThephenomenaassociatedwiththeintentionaldecayofmagnetizationatonetempera-turewhilecooling,andtheobservationofanimprintofthatdecayatthesametemperatureuponwarmingbackupareoftenreferredasmemory"eects.Thisisbecauseitseemstobethatthesystemretainsamemoryofitsthermalandmagnetichistory.Toperformmemoryeectmeasurementsonbothtypesofsamples,weemployedthesameexperimen-talprocedurethatwasdescribedbySunetal.[87].Theexperimentwasconductedbycoolingthesampleina50OeeldFCfrom300Kataconstantrateof2K/min,andmeasuringthemagnetizationwhilecooling.ThemeasurementwasstoppedatT=90K,70K,50K,20K,10Kand2Kfortheowerparticles,andT=70K,50K,20K,10Kand2K92

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Figure6.22.ZeroeldcooledandeldcooledtransversesusceptibilitycurvesfortheowerAu-Fe3O4nanoparticlestakenat30K.forthedumbbellnanoparticlesforawaittimeof2hours.Duringthatwaittime,theeldwasswitchedo,andthemagnetizationwasallowedtorelax.Thesolidsquaresingures6.23and6.24werecollectedinthismanner.Itcanbeclearlyseenthatduringthewaittime,themagnetizationconsistentlyrelaxedforT>20K,indicatingtheabsenceofanyshortrangeorder.HoweveratT=10Kand2K,afterthewaittimethereisnochangeinmagnetizationeventhoughtheeldwasswitchedo,indicatingthatarobustshortrangeorderhadsetin.Afterreachingthelowesttemperatureof2K,themagnetizationwasmeasuredopencircleswhilewarmingbackupFWto300Katthesamerateof2K/min.TheMTmeasuredduringthiscyclealsoshowsstep-likebehavioratthestoptemperaturesof50K,70Kand90K,indicatingthatthesystemcanretainmemoryofitsthermalandmagnetichistoryatthosetemperatures.Thememoryeectisobservedforboththeoweranddumbbellnanoparticles.Memoryeectshavebeenstudiedextensivelyinspinglasses,andhavebeenreportedinnanoparticlesystemsaswell[87,34,71,94,10].93

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Figure6.23.MemoryeectindumbbellAu-Fe3O4nanoparticles.Blacksquaresshoweldcooling,redcirclesshoweldwarming.Theoriginofthismemoryeectinspinglassesisunderstoodintermsofhierarchalorganizationofthemetastablestatesasafunctionoftemperature.Innanoparticlesystems,thememoryeectisattributedmoretothedistributionofrelaxationtimesarisingduetothedistributionofparticlesizes.Forexample,whenthemagneticeldisturnedoat90K,thoseparticleswithblockingtemperaturesabove90Krelaxeasily,whiletherestremainalignedwiththeeld.Atthenextlowertemperature,smallerparticleswithalowerblockingtemperaturecanrelaxshortlyaftertheremovalofaeld.Thisprocessisrepeatedforthetemperaturesatwhichthememoryeectsareobserved.Uponwarming,thoseparticlesthatrelaxedatthelowesttemperaturesarere-alignedwiththeeld,andsoonuntilallthestoptemperatureshaveonceagainbeenreacheduponwarming.Inthepresentcasewehavearelativelyuniformparticlesize,buttheshapeofthecompositeparticlesalongwithinter-particleinteractionswouldleadtoadistributioninrelaxationtimes.94

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Figure6.24.MemoryeectinowerAu-Fe3O4nanoparticles.Blacksquaresshoweldcooling,redcirclesshoweldwarming.6.8OriginsofEnhancedAnisotropyandExchangeBiasinFlowerNanoparticlesWhatisclearfromtheaboveexperimentsisthatthedumbbell-shapednanoparticlesbehaveinamuchmoreconventionalmannerthantheower-shapednanoparticles,despitethefactthatbothofthesesystemsarecompositenanoparticles.Webelievethatitisthecomplexinteractionsatthemultiplesurfacesandinterfacesoftheowerparticlesthatcanaccountforthedeviationinmagneticpropertiesbetweenthetwosystems.Inthissection,weexaminetheroleofthesurfaceanisotropyandAu-Fe3O4interactions,aswellastheinteractionsbetweenFe3O4nanoparticlessharingthesameAuparticles,andcorrelatetheseinteractionswiththebehaviorobserved.Severalgroupshavefocusedonanomalousmagneticbehaviorinferritenanoparticlesduetosurfacespindisordersuchasexchange-biasedhysteresisloopsandhigheldirre-versibility[38,50,90,92,4].Anexplanationofthisbehavioristhatwhenalargeenough95

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fractionofatomsresideonthesurfaceofaparticle,thebrokenexchangebondsaresu-cienttoinducesurfacespindisorder,thuscreatingacore-shellstructuremadeoftheferritecorewithashellofdisorderedspins.Thesedisorderedspinscantakeonanumberofcong-urations,oneofwhichcanbechosenbyeldcoolingtheparticletoinduceaunidirectionalanisotropyresultinginashiftedhysteresisloop[38].Itisthoughtthatthelowestenergycongurationofsurfacespinsinthezeroeldcooledconditionofasphericalparticleistheoneinwhichthespinspointoutintheradialdirectionfromtheparticle.AlthoughBdkeretal.havedemonstratedviasymmetryargumentsthataperfectsphericalparticleshouldhaveazeronetcontributionfromsurfaceanisotropy[8],itisimportanttonotethatinbothdumbbellandowernanoparticles,thesphericalsymmetryisbrokenresult-inginanetsurfaceanisotropy.However,thefactthatweseenosignaturewhatsoeverofaunidirectionalanisotropyinthedumbbellssuggeststhatsurfaceanisotropyalonecannotaccountforthebehaviorseenintheowerparticles.Itispossiblethatthesurfacespincongurationofthedumbbellparticlescanexplainthespin-glass-likebehaviorseenintheACsusceptibilitydataandthatother,strongerinteractionssuppressthisbehaviorintheowernanoparticles.Intheower-shapedsystem,twoFe3O4particleslocatedatoppositeendsofthestruc-tureareseparatedbyoneAuparticle,andthethreeparticlesasagroupformastructurereminiscentofthoseseeninthinlmsdisplayinggiantmagnetoresistanceGMR.Suchmultilayerlmshavetwometallicferromagneticlayersseparatedbyanonmagneticmetalunlikethesituationdescribedhereinwhichtheferromagneticphaseisinsulating.TheresultisanindirectexchangecouplingbetweenthetwoferromagneticlayersmediatedbytheRKKYinteraction.Thesignandstrengthofthecouplingconstantisdependentuponthespacebetweenthelayers.Inthecaseoftheowernanoparticles,itispossiblethatthedistancebetweenFe3O4particlesi.e.theradiusoftheAuparticlesetsupaanantiferromagneticcouplingbetweentheFe3O4nanoparticlesreminiscentofthistypeofexchangegure6.25.Asimilar,indirectferromagneticcouplinghasbeenobservedforadjacentFenanodotssharingametallicCusubstrate[65].Inthiswork,theexchange96

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Figure6.25.SchematicofpossibleinteractionsinowerAu-Fe3O4leadingtoanomalousbehavior.strengthcouldbetunedbychangingthedistancebetweennanodots.ThistellsusthatitisreasonabletopostulatethatatthedistancesseparatingtheFe3O4nanoparticles,indirectexchangethroughtheAu-Fe3O4interfacescanresultinantiferromagneticcoupling,settinguptheunidirectionalanisotropyneededtoobserveexchangebiaseects.JustasPierceetal.foundthatthemetal-mediatedcouplingdominatedoveranisotropyeects,webelievethattheinterfacialcouplingbetweentheAuandFe3O4dominatesthelowtemperaturebehaviorseen.However,sincetherearemultipleFe3O4particlessharinginterfaceswiththeAuparticle,thisantiferromagneticcouplingcouldhavetheaddedfeatureofspinfrustra-tion.WhilethemagnetizationstatesoftwooppositeparticlesaredeterminedbyindirectexchangeacrosstheAu,theycanstillbedirectlyadjacenttooneanother,creatingacom-petitionbetweenthemetal-mediatedexchangeandtraditionalinter-particleinteractions.Thiscompetitioncouldleavethesystemfrustratedaddingtothelargeanisotropyseen.Itshouldbeemphasizedthattheanomalousmagneticbehaviorseenintheowernanoparticlescanbeobservedbelow65K,whichisstillbelowthesuperparamagnetictran-97

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sitionandconsistentwiththeplateauintheFCcurve.Webelievethischaracteristictem-peratureisduetothermalenergyovercomingtheenergyassociatedwiththeAu-Fe3O4interaction.Fortemperaturesbelowtheinteractionenergy,theantiferromagnetic-typeor-derisrobustandweseetheexchangebiasandtrainingeects.AtthethermalactivationenergyK,thecouplingisbrokenandwehaveasituationwheretheparticlesarestillintheblockedstateand,accordingtothetransversesusceptibilitymeasurements,stillshowquiteabitmoreanisotropythanthedumbbellparticles.At75K,thevaluesofHK1andHK2fortheowernanoparticlesare550Oeand1770Oerespectivelygure6.21,whereasforthedumbbellnanoparticlestheHK1andHK2valuesare130Oeand220Oerespectivelygure6.20.Thisindicatesthatevenaftertheunidirectionalanisotropyisgone,theowernanoparticlesstillpossessahighereectiveanisotropythanthedumbbellparticles.Eventhoughtheantiferromagneticcouplinghaslikelybeenbroken,thespinfrustrationassociatedwithaclustergeometryofFe3O4maystillexistgure6.25.Itisalsopossiblethatratherthanpossessingbrokenexchangebondswhichnormallyleadtosurfacedisorderinsmallferritenanoparticles,thenucleatingFe3O4particlesmakingupthepetals"couldbegrowingtogetherintoalargerstructure,formingexchangebondswitheachother,resultinginmoreofacontinuous,cluster-typeparticle.AfterprojectingathreedimensionalparticleontoatwodimensionalscreenwhentakingaTEMimage,itisdiculttotellfromgure6.2howmanyoftheFe3O4particlesareindirectcontact.6.9ConclusionInthischapter,themagneticpropertiesoftwonewcongurationsofAu-Fe3O4werepresented.Whilethedumbbell-shapedAu-Fe3O4didnotshowbehaviorthatdierentfromtraditionalFe3O4nanoparticles,itisstillanimportantsystemtoexploreforbiomedicalapplicationsastwoseparatesurfacesareavailableforfunctionalization,andboththeAuandFe3O4sizescanbecontrolled.Itwouldbeinterestingtolookatvarioussizecom-binationsofAuandFe3O4toseewhichcombinationperformsbestforfunctionalization,cellularuptake,andmagneticmanipulationwhileinsidethecells.98

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Theower-shapednanoparticlesprovedtobeafascinatingsystemfromafundamentalphysicspointofviewduetothecombinationofcompetinginteractionsgivingrisetoexchangebiasandtrainingeectsinthelowtemperatureregime,whilemaintainingananomalouslyhighanisotropyintheintermediateregimebeforeblocking.Inordertofullydevelopthemodelbrieyproposedbytheschematicingure6.25,itwouldbebenecialtolookatseveraldierentsizesofAuparticlestodeterminethedependenceoftheinteractionstrengthonthedistancebetweenparticles.ChangingthesizeoftheFe3O4,aswellasthenumbernucleatedontotheAupossiblybymakingverysmallchangestothereactionpH,couldhelpformabetterpictureofthespinfrustrationlikelyinvolvedbetweenadjacentFe3O4particles.99

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CHAPTER7SINGLELAYERCrO2ANDBILAYERCrO2/Cr2O3THINFILMS7.1IntroductionCrO2belongstoanimportantclassofoxides.ItisferromagneticwithaCurietem-peratureof395K.Bandstructurecalculationshavepredictedthatitisnearly100%spin-polarized,meaningalloftheelectronsattheFermilevelhavethesamespinorientation,andexperimentssupportthesepredictions[77].ThismakesCrO2anattractivematerialforspintronicdevicessuchasspinvalvesandmagnetictunneljunctions.Cr2O3isanantiferromagnetwithaNeeltemperatureof307K.Itisthemorethermo-dynamicallystableofthetwochromiumoxidephases.Inzeromagneticeld,theCr3+ionsareantiferromagneticallyalignedparalleltotherhombohedralcaxis.Whenastrongenoughexternalmagneticeldisappliedalongthecaxis,thespinsswitchtolieinthebasalplane[57].Radoetal.werethersttoexperimentallystudytheexistenceofbothmagneticandelectriceld-dependentmagnetoelectricMEeectinCr2O3powders[68].ThemagneticallyinducedMEeectismanifestasamagnetic-eldinducedvoltage,whiletheelectricallyinducedMEeectisamagneticmomentthatarisesinthepresenceofanelectriceld.Thisdualmanipulationofonematerialcouldhaveimportantconsequencesinthenextgenerationofdevices.Cr2O3isalsothenativeoxidethatformsonthesurfaceofCrO2lmsandthereisevidencethattheCrO2layermaypolarizetheCr2O3layer[14].Moreover,themultifunctionalityofastructurewithspintronicandmultiferroicprop-ertiescouldbeofconsiderableinterestfrombasicandappliedmaterialsperspectives.Thischapterdescribesthegrowth,characterization,andmagneticpropertiesofCrO2thinlmsandCrO2/Cr2O3bilayerthinlms.Wepresentevidenceforexchangecouplingbetweenthe100

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layersseenasanincreaseinanisotropythatcannotbeexplainedbythevaryingthicknessoftheCrO2phasealone.7.2ThinFilmGrowthHigh-qualityepitaxialCrO2lmsweregrownbyProfessorArunavaGupta'sgroupattheUniversityofAlabamaMINTCenter.AtmosphericpressurechemicalvapordepositionCVDwasusedtogrowCrO2on-orientedTiO2substratesusingchromiumtrioxideCrO3asaprecursor.Inthisreaction,oxygenisusedasacarriergasinatwo-zonefurnacetotransporttheprecursorfromthesourceregiontothereactionzone,whereitdecomposesselectivelytoformCrO2.Thelmsweregrownatasubstratetemperatureofabout400C,withthesourcetemperaturemaintainedat260C,andanoxygenowrateof100sccm[43].ItiswellknownthatformationofanaturalCr2O3layerwilloccurontheCrO2surfacebecauseitisthermodynamicallyamuchmorestablephasethanCrO2[55].Becauseofitsmetastability,bulkCrO2willalsoirreversiblybereducedtoCr2O3attemperatureshigherthanabout425C.TheMINTgrouptookadvantageofthistransformationtogrowCrO2/Cr2O3heterostructuresofvaryingrelativethickness.Forexample,bypost-annealingaCrO2lmat450Cforvaryinglengthsoftime,thelmstartingfromthetopsurfacelayercanbecontrollablyconvertedtoCr2O3.AlllmsstudiedweregrownonTiO2,singlecrystalsubstrates,with55mm2dimension,andofvaryingthicknessesandCr2O3content.Inordertodecoupletheeectofthicknessonmagneticanisotropyfromthatofinterfacecouplinginbilayers,wehavealsoexaminedlmsofCrO2withvaryingthicknessintherangeof20nmto725nm,whereasthetotalthicknessoftheCrO2/Cr2O3bilayerswaskeptconstantat200nmwithdierentproportionsofCrO2andCr2O3.Thisdoesnotaccount,however,forthevariationinvolumearisingduetothedierenceindensitybetweenthetwo.TheCrO2andCrO2/Cr2O3bilayerlmswerestudiedatMINTusingaPhilipsX'Pertx-raydiractometerandelectronmicroscopy.Dr.MariaVarelaandDr.StephenPennycook101

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ofOakRidgeNationalLaboratoryimagedoneofthebilayersusinganaberration-correctedscanningtransmissionelectronmicroscopeSTEM.Figure7.1showsacross-sectionalSTEMimageoftheCrO2/Cr2O3bilayers,whereitcanbeseenthatthetwolayersarewell-alignedandformanabruptinterface.TheCr2O3layeriscrystallineanditisgrownepitaxiallyontopoftheCrO2withveryfewdefects.Thegrainboundarydefect,whichinthegureismarkedbyanarrow,propagatesintotheCr2O3layerdirectlyfromtheCrO2layer.MeasurementsoftheSTEMdoneatOakRidgeNationalLaboratoryindicatedthattheplaneoftheCr2O3withacorundumstructureisparalleltotheplaneoftherutileCrO2.Also,thein-plane[010]and[001]directionsofCrO2arealignedwiththe[1120]and[1100]directionsofCr2O3respectively.ThisepitaxialrelationshipisconsistentwithwhathasbeenobservedforthenaturallyformedCr2O3surfacelayeroncommercialacicularCrO2particles[95]. Figure7.1.Cross-sectionalhighresolutionSTEMmicrographofheteroepitaxialCrO2/Cr2O3bilayer.AgrainboundarydefectthatpropagatesacrosstheCrO2/Cr2O3interfaceisindicatedwithanarrow.ImagecourtesyofOakRidgeNationalLaboratory.102

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7.3DCMagneticCharacterizationMagnetizationversuseldM-HcurvesweredoneattheUniversityofAlabamaus-inganalternatinggradientmagnetometer,andhereattheUniversityofSouthFloridausingourPhysicalPropertiesMeasurementSystem.Figures7.2and7.3showtheM-HcurvesfortheCrO2lmsof21.5nmand725nmrespectivelytakenatroomtempera-ture.Measurementsweredonealongthebaxis[010]direction,redcirclesandcaxis[001]direction,blackcircles.Figure7.4showstheroomtemperatureM-HcurvesfortheCrO2/Cr2O3bilayers.The100%CrO2lmcorrespondstothe200nmCrO2lm,andtheinsetshowsthebaxisandcaxisdataforthissample.Thestepobservedinthehysteresisloopsnearthesaturationeldingure7.3isanartifactoftheAGMmeasurement.Miaoetal.showedinreference[55]thatthemagneticeasyaxisofCrO2changesorientationwithlmthickness.ThisisattributedtothecompetitionbetweenthemagnetocrystallineanisotropyoftheCrO2,andthestraininducedbythelatticemismatchwiththesubstrate.ThemagnetocrystallineanisotropyofCrO2favorsthemagneticeasyaxistoorientalongthein-planecdirection,asobservedbothforbulksamples[70],andalsoforthickerlms[43].Thestrainanisotropyisaninterfaceeect,andstrainedthinlmsgrownonTiO2substratesexhibitmagneticeasyaxisalignmentalongthebdirection,sincethelatticemistislargeralongthebthaninthecdirection.91%vs1.44%.Thickerlmsexhibitinhomogeneousstraindistribution,withthemagneticeasyaxisvectorrotatingfromthebdirectionnearthesubstratetothecdirectionclosertothesurface.Consistentwiththispicture,theM-Hcurvesforthe200nmand725nmlmsexhibitaneasyaxisalongthe[001]direction,andahardaxisalongthe[010]direction.Forthe21.5nmlm,theM-Hdataindicateaneasyaxisalongthe[010]direction,andahardaxisalongthe[001]direction.ThedecreaseinsaturationmagnetizationforthebilayerlmsisduetothedecreaseinferromagneticcontentCrO2byannealingandconversiontoanitferromagneticCr2O3.ThethicknessesoftheCr2O3layerswerededucedfromthedecreaseinsaturationmagneti-zationMSofthebilayerincomparisontothepureCrO2lmastheCr2O3contribution103

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Figure7.2.Hysteresisloopsof21.5nmthickCrO2lmtakenalongthebandcaxes.toMSisnegligible.TheestimatedpercentcontentofCrO2remainingindierentbilayerlmsmeasuredwere64%,50%,and32%.Theannealingtimesfortheselmswere14h%CrO2,24h%CrO2,and34h%CrO2.Forallthebilayers,theroomtem-peratureM-Hcurvesshowedamagneticeasyaxisalongthe[001]directioncaxis,andahardaxisalongthe[010]directionbaxis.Ingure7.5thetemperaturedependenceofthecoercivitywithvariationofCrO2contentinCrO2/Cr2O3bilayersiscomparedtothepureCrO2lmofthesametotalthicknessof200nm.HCofthebilayersincreasesincomparisontothepureCrO2lmdependingonthethicknessofCr2O3.Forthebilayerwith64%CrO2content64nmantiferromagnetthickness,enhancementpersistsevenabovetheNeeltemperatureKupto350K.IncreaseinHCaboveTNisreportedinsinglecrystallineexchange-biasedantiferromagneticFeF2lmswithCo[42]orFe[23]ferromagneticlayers.Thisisinterpretedasbeingduetotheshortrangeorderinducedintheantiferromagnetbytheferromagnet.AstheCrO2Cr2O3contentincreasesdecreasesthevariationinHCdecreasesandwell104

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Figure7.3.Hysteresisloopsof725nmthickCrO2lmtakenalongthebandcaxes.abovetheNeeltemperatureitbecomesequaltothatofthepureCrO2lm.Theinsetofgure7.5showsthevariationofHCwithCrO2contentatroomtemperature.HCincreasesfrom47Oeforthe100%CrO2lmof200nmto174Oeforthebilayerlmwith64%CrO2,andwithfurtherdecreaseinCrO2contentHCdecreases.Thelmswith50%and32%CrO2contenthaveHCvaluesof145Oeand83Oerespectively.HCfortheCrO2lmsaloneisinverselyproportionaltotheferromagneticthicknesstFM.TheenhancementandfunctionaldependenceofHContFMinthebilayerlmsstronglysuggeststheexistenceofacouplingbetweentheCrO2andCr2O3layers.Tofurtherprobethenatureofthecoupling,wemeasuredthehysteresisloopsofthebilayersamplesaftercoolingthemfromabovetheNeeltemperatureinaeldof1Tesla.WedidnotseeanyshiftinM-Hevenat10Kexceptforonelmwith32%CrO2nm,forwhichaverysmallexchangeeldof12Oewasobserved.ThisimpliesthattheexchangecouplingmechanisminthissystemisprimarilymanifestintheenhancementofHC,andnotaccompaniedbyashiftinM-HHE.105

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Figure7.4.HysteresisloopsforCrO2/Cr2O3bilayersofvaryingCrO2content.Insetshowsthehysteresisloopsforthe100%200nmCrO2lmtakenalongthebandcaxes.Theroleofthicknessoftheantiferromagneticandferromagneticlayersinanumberofexchange-biasedsystemshasbeenstudiedindetail.Ingeneraleithertheferromagneticlayerortheantiferromagneticlayersonlyvaried,andthemainresultsofthesestudiesindicatethattheexchangebias,HE,andcoercivity,HC,areinverselyproportionaltothethicknessoftheferromagneticlayer[59].Furthermore,HEandHCareindependentofantiferromagneticthicknesstAFMforthicklms,andHEabruptlydecreasesandgoestozeroforsmalltAFM[59].IntheCrO2/Cr2O3system,thefunctionaldependenceofHEandHContFMandtAFMisrathercomplicatedastFMandtAFMarevaryingsimultaneously.Itisthetotalthickness200nmofthebilayerwhichisheldconstant.Apartfromthis,thetFMfallsintherangeofCrO2thicknesswhereinboththeinhomogeneousstrainandthemagnetocrystallineanisotropycompete,andtheeasyaxisswitcheswithboththicknessandalsotemperature.Thispointwillbefurtherdiscussedinsubsequentsections.106

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Figure7.5.Variationofcoercivityasafunctionof%CrO2inCrO2/Cr2O3bilayerlms.7.4TransverseSusceptibilityMeasurementsforCrO2FilmsIngure7.6wepresenttheunipolarpositivetonegativesaturationeld-dependentchangeintransversesusceptibilityTobtainedfortheCrO2lmswithdierentthick-nessesatroomtemperaturewiththestaticmagneticeld,HDC,appliedalongthehardin-planeaxisofmagnetizationcaxisforthe21.5nmlm,baxisfor200nmand725nmlms.IdenticalanisotropypeaksareseeninTsymmetricallylocatedaroundH=0,followedbyanapproachtosaturationathigherelds.Thisisdierentthanthedatawesawforthenanoparticlesystems,becauseherewecandiscernonedistinctanisotropyeld,whichissymmetricregardlessofdirectionoftransversesusceptibilityscani.e.HK1=HK2=HK.Wealsodonotseeaseparateswitchingpeakalongthehardaxis,becauseastheout-of-planehysteresisloopsindicate,theswitchingeldandanisotropyeldareverynearlyequal.Theanisotropypeakheightincreaseswithincreaseinlmthickness.Anincreaseintheanisotropyeld,HK,withincreaseinlmthicknessisalsoobserved.Using107

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thestandardrelationHK=2Ke=2MS,weextractedtheeectiveanisotropyKeatroomtemperature.CrO2lmsareknowntohaveaprominentin-planeuniaxialanisotropy,lead-ingtotheassumptionthatKeK1[79].Furthermore,transversesusceptibilityperformedonthinCrO2lmsgrownfromthesamegroupin2000revealedthatthemagnetizationrotationcouldbesuccessfullyreproducedusingacoherentrotationmodel,resultinginbehaviorverysimilartoaStoner-Wohlfarthparticle[79].UsingHK=80OeandMS=465emu/cc,wecalculatedKeas1:9104erg/ccforthe21.5nmCrO2lm.Usingthesameprocedure,wecalculatedKevaluesforthe200nmand725nmCrO2lmsas1:1105erg/ccHK=514Oeand2:5105erg/ccHK=1050Oerespectively.TheseresultsagreewellwiththevaluesobtainedbyMiaoetal.intheirthicknessdependentstudyofKeinCrO2lmsusingDCmagneticmeasurements[55]. Figure7.6.UnipolartransversesusceptibilitydataforCrO2lmsofvaryingthicknessestakenatroomtemperature.Scanwastakenfrompositivetonegativesaturation.At10K,theHKandKedon'tshowthesameincreaseinvaluewithincreaseinthicknessgure7.7.The725nmlmstillhasthehighestHKOeandKe:3105erg/cc,butthe21.5nmlmshasahigherHKOeandKe:6105erg/ccthanthe200nmlmOeand1:2105erg/cc.Theroomtemperaturemagneticpropertiesand108

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Figure7.7.TransversesusceptibilitydataforCrO2lmsofvaryingthicknessestakenat10K.Scanwastakenfrompositivetonegativesaturation.lowtemperaturemagneticpropertiesarecollectedintable7.1.ThelowtemperatureMSvaluesweretakenfromreference[55].ThelmsusedinthatstudywerealsogrownbyProfessorGupta'sgroup,sothetwosetsoflmscanbecomparedwithcondence. CrO2 MS HK Ke MS HK Ke thickness emu/cc Oe erg/cc emu/cc Oe erg/cc nm RT RT RT LTRef.[55] LT LT 21.5 465 80 1:9104 640 815 2:6105 200 436 514 1:1105 640 390 1:2105 725 486 1050 2:6105 640 1340 4:3105 Table7.1.MagneticpropertiesofCrO2lmsatroomtemperatureRTandlowtemper-atureLT.Inordertounderstandthiscomplexthicknessdependenceontheanisotropicproper-ties,transversesusceptibilitymeasurementsweredoneoverthefullrangeoftemperaturesandtheHKvaluesareplottedingure7.8.Forthe21.5nmandthe725nmlms,thepeakpositionshiftstohighereldsasthetemperaturedecreases.Thetemperaturevariationisdominatedinthe21.5nmlmbystraineectsfromthesubstrate,whichintroducesa109

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Figure7.8.TemperaturedependenceofHKforvariousthicknessesofCrO2.magnetoelastictermtotheeectiveanisotropy.The725nmlmwillbehavetheclosesttoabulksamplewherethemagnetocrystallineanisotropywilldominate,andanincreaseinanisotropywithdecreaseintemperatureiswhatisexpectedformostbulksystems[70].Forthe200nmlmthereisaslightincreaseinHKwithincreaseintemperature.Thisintermediatethicknesslmshouldhavethemostcompetitionbetweenstrainanisotropyandmagnetocrystallineanisotropy,andinfactitfallswithintherange50-250nmre-portedbyMiaoetal.wherethesecompetinganisotropiesinduceachangeineasyaxisofmagnetization.Thetransversesusceptibilitymeasurementspresentedhereseemtoalsodisplayatemperaturedependentswitchingofeasyaxisduetotheinhomogeneousstraindistribution,resultinginadecreaseinanisotropyeldatlowertemperature.Interestingly,the200nmthinlmalsoshowsadoubleswitchingfeatureconsistentwiththendingsreportedinreference[55],likelyduetothelargepresenceofbothstrainandmagnetocrystallineanisotropiesfavoringdierentmagneticeasyaxes.WhiletheHKmeasurementsforthe200nmlmforthetemperaturesshowningure7.8werealldone110

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withHDCalongthe[010]direction,systematicmeasurementsdonewithHDCalongthe[001]directionshowedthatthecaxiswasaneasyaxisatroomtemperatureconsistentwithgure7.4butthatasthetemperaturewaslowered,itbecameahardaxisofmag-netization.Figure7.9showstransversesusceptibilityscanswithHDCalongthecaxisforseveraldierenttemperatures.Whilejustonepeakattheswitchingeldisseenatroomtemperature,anisotropypeaksbegintoemergeasthetemperaturedecreases.The[001]peaksappeardierentfromthe[010]peaksbecausethe[010]peaksbehaveasexpectedforathinlmmeasuredalongthehardaxis,thatis,theanisotropypeaksaresharpandcoincidewiththeswitchingpeak.The[001]peaksbehavemoreasiftheanisotropyeldshaveaslightdistributionandaseparateswitchingeldmanifestasaseparatepeakclosertoH=0.Thisisconsistentwiththeemergenceofthe[001]beingstrain-relatedandin-homogeneous,sothattheeasyaxisisrotatingthroughoutthethicknessofthelm.ThiscouldgiverisestoslightdistributioninHKwhichisnotequaltoHS.TheincreaseinanisotropyassociatedwiththepresenceofthesepeakswhenthetemperatureisloweredasHDCisappliedalongthecaxiscouldhelptoexplaintheanomalousdecreaseinanisotropywithdecreaseintemperatureasHDCisappliedalongthebaxis.Asexpected,neitherthe21.5nmlmnorthe725nmlmshowedthisbehavioratlowtemperatureswhenHDCwasappliedalongtheirrespectiveeasyaxes.7.5TransversesusceptibilityMeasurementsforCrO2/Cr2O3BilayersTransversesusceptibilitymeasurementswerecarriedoutonallbilayersamplesbyap-plyingtheHDCparalleltothehard[010]axis.First,transversesusceptibilitywasmeasuredonafullydecomposedsampletolookatthesignalduetoCr2O3only.Asexpectedforantiferromagneticmaterials,asinglesharppeakwaspresentatH=0andthecurvesdidnotattenoutathighelds,indicativeofafailuretoreachsaturation.Non-saturatingmagne-tizationisaknownfeatureofantiferromagneticmaterialsthatisalsocommonlyobservedinM-Hcurves.Weobserveadistinctasymmetryintheshapeofthecurvesfornegativeandpositiveeldpolarities.Thiscouldbeassociatedwithslightlydierentresponsesof111

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Figure7.9.Unipolartransversesusceptibilityscansof200nmCrO2lmforseveraltem-peraturesshowinganisotropypeaksemergingasthetemperatureisdecreased.HereHDCisappliedalongthe[001]directionfrompositivetonegativesaturation.thesublatticemagnetizationcomponentsoftheantiferromagneticorderwhentheeldisreversed.WhilethisisgenerallynotseeninDCmagnetizationmeasurementswhichisavolumemagnetizationmeasurement,thetransversesusceptibilitygeometry,whichprobesthetransversecomponentofthemagnetizationvector,ishighlysensitivetotheinuenceofthesublatticemagnetizationenergy.Thetransversesusceptibilitymeasurementsonthebilayersinterestinglyexhibitcom-binedfeaturesassociatedwithboththeferromagneticCrO2anisotropypeaks,andanti-ferromagneticCr2O3peakatH=0,aswellasthenonsaturationandasymmetrydiscussedearlier.ThetransversesusceptibilitydataforallthesamplescontainingdierentamountsofCrO2percentcontentatroomtemperatureispresentedingure7.10.Themostno-ticeablefeatureofthebilayerdataistheshiftintheanisotropypeakstohighereldsasthecontentofCr2O3increases.TheanisotropypeaksarenotassharpasinthecaseforCrO2,butappearasbroadshouldersaboutthecenterpeak.Thebroadeningbecomes112

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morepronouncedwithincreaseinCr2O3untilat32%CrO2,anisotropypeakscanbarelybemadeout.Thereisalsoaprominentstrain-associatedpeakthatappearsandbecomesmoreprominentatlowertemperaturesgure7.13.TheemergenceofpeakswhichcanbeassociatedwithstrainwasalsoseenbySpinuetal.intransversesusceptibilitydataofCrO2thinlms[79].Thisbehaviorcanbeexplainedintermsofaslightdeviationoftheanisotropyaxisfromthehardaxisofmagnetizationandwassuccessfullymodeledbyintroducingamagnetoelastictermintothetotalanisotropyenergy.HKforthe200nm100%CrO2lmis515Oe,whereasforthelmwith32%CrO2itis2100Oe.TheshiftinanisotropypeakstohighereldswithdecreaseinCrO2contentforthebilayersimpliesachangeineectiveanisotropyKe.ItisimportanttoverifyiftheincreaseinHKcorrespondstoanincreaseinKeaftertakingintoaccountthecorrespondingMSofthetFM.ThetFMvaluesbasedonthepercentcontentofCrO2wascalculated,andispresentedintables7.2and7.3.ThecorrespondingKeforthesethicknessesisobtainedbyattoacurvebasedondatainreference[55]gure7.11.WhencomparingtheobservedroomtemperatureKevalueswiththosecalculatedbasedontFM,itisclearthattheobservedKeisconsistentlylargerthanwhatitwouldbeiftheCr2O3werenotpresentinthelms.ThemaximumKe.4105erg/ccwasobtainedforabilayerlmwith50%contentofCrO2,whichismuchlargerincomparisontotheKeextractedfromthecurve.2104erg/cc.Theresultsforallthelmsaregivenintable7.2.ForthepureCrO2lms,HKisproportionaltoCrO2thicknesstable7.1,whileforthebilayers,HKisinverselyproportionaltothetFMtable7.2.TheroleofCr2O3anditsinterfacewithCrO2ismanifestnotonlybytheenhancementofHK,butalsobyitsfunctionaldependencewithtFM.ThetemperaturedependenceofHKforthebilayersisplottedingure7.12.Forallbilayerlms,anincreaseinHKwithdecreaseintemperatureisobserved.ThisindicatesthatthepresenceofCr2O3changesthetemperaturedependenceofthestrainincomparisontothepureCrO2lms,thustheeasyaxisofmagnetizationremainsalongthecaxisthroughoutthetemperaturerange.Thisisalsoevidentfromtheabsenceoftheanisotropy113

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Figure7.10.RoomtemperaturetransversesusceptibilityscansofCrO2/Cr2O3bilayersfordierentCrO2percentages.Scantakenfrompositivetonegativesaturation.114

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Figure7.11.KeversusCrO2thicknessforthelmsstudiedhereandinref[55].ThisisusedforareferencecurvetopredictvaluesofKefortheeectiveCrO2thicknessinthebilayers.peaksthroughoutthetemperaturerangewhenthetransversesusceptibilityismeasuredwiththeappliedeldparalleltotheeasybaxis.Similartotheroomtemperaturemeasurements,HKincreaseswithincreaseinCr2O3contentthroughoutthemeasuredtemperaturerange.WhilethevaluesofKefortheCrO2thinlmsmatchedwellwiththosereportedinreference[55],theKevaluesforthebilayerswereconsistentlylargerthroughoutthetemperaturerange,againindicatingacouplingbetweenthelayers.Aswesawinchapter6,transversesusceptibilitycanbeusedtoprobesystemsshowingexchangebiasduetounidirectionalanisotropy,andtherewasnoshiftinthetransversesusceptibilityforthebilayers,whichisconsistentwiththelackofshiftobservedintheM-Hloops.Table7.3isacollectionofthelowtemperature10KvaluesofHKandKe.Fromgure7.12,wecanseethatHKishighestatlowtemperature,butfromtable7.3itcanbeclearlyseenthatthistranslatesintoanincreaseinKeaswell.Figure7.13showsalowtemperatureKtransversesusceptibilityscanofthebilayerwith64%CrO2.Thestrain-associatedpeakdiscussedaboveismuchmorenoticeableatlowtemperature.For115

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Eective MS HK Keerg/cc Ke HK % thickness emu/cc Oe observed erg/cc Oe CrO2 nm RT TSRT RT calc calc 100 200 436 514 1.1105 1.1105 514 64 128 291 1448 2.1105 4.6104 314 50 99 227 2075 2.3105 6.2104 548 32 64 136 2100 1.4105 7.7104 1136 Table7.2.MagneticpropertiesofCrO2/Cr2O3bilayerlmsofdierentCr2O3contentmeasuredatroomtemperatureRT.ThecalculatedvaluescorrespondtowhattheeectiveanisotropyshouldbeforjusttheCrO2componentaccordingthereferenceingure7.11. Eective HKOe MS Ke % thickness TS emu/cc erg/cc CrO2 nm K K K 100 200 390 660 1.3105 64 128 2130 423 4.5105 50 99 3150 326 5.1105 32 64 3230 212 3.4105 Table7.3.MagneticpropertiesofCrO2/Cr2O3bilayerlmsofdierentCr2O3contentmeasuredlowtemperatureLT.thisparticularsample,thestrainpeakwasnotpresentatroomtemperature.Theothertwosamples,whichdidshowstrainpeaksingure7.10,alsoshowedthestrainpeakevolvingfurtherasthetemperatureisdecreased.7.6OriginsofExchangeCouplinginCrO2/Cr2O3BilayersToobserveanexchangebiasHE,thespinsintheantiferromagnetmustnotberotatedbythemagneticeldapplied.Therefore,theanisotropyenergyoftheantiferromagnetmustbelargerthattheinterfacecouplingenergy,4p AAFMKAFM=264116

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Figure7.12.HKversustemperaturefortheCrO2/Cr2O3bilayers. Figure7.13.LowtemperatureKtransversesusceptibilityscanof64%CrO2bilayer.Thisisaportionofascantakenfrompositivetonegativesaturation.117

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nm,theantiferromagneticspinsshouldbepinningtheferromagneticspins,givingrisetoaunidirectionalanisotropy.Clearlyfromthedatashown,thereisnounidirectionalanisotropy,onlyauniaxialanisotropy.ThebehaviorobservedintheCrO2/Cr2O3bilayerscaninfactbeunderstoodintermsofamodelputforthbySchulthessetal.[72].Themodelappliestosystemswithcompen-satedspinsattheinterface,i.e.whenthereisanequalnumberofpositiveandnegativeexchangeinteractionsacrosstheinterface,whichwouldapplytothenearlyperfectepitaxyseenintheCrO2/Cr2O3bilayers.Insuchasystem,theexchangecouplingbetweentheantiferromagnetandtheferromagnetisperpendicular.Thisperpendicularcoupling,re-ferredtoasspin-op"coupling,tswithinamicroscopicHeisenbergmodelwhere,duetofrustrationofthemomentsattheinterface,theferromagnetminimizestheenergywhenitalignsperpendiculartotheantiferromagneticeasyaxis.ThistypeofcouplingwillnotleadtoaunidirectionalanisotropybecausetheantiferromagneticspinsattheinterfacewillnotpintheferromagneticspinswhencooledfromabovetheNeeltemperatureinthepresenceofaeld.Instead,sincethespinswillbeperpendicular,theantiferromagneticspinswilldrag"theferromagneticones,leadingtoauniaxialanisotropy,andthusanenhancedco-ercivity.Figure7.14isaschematicofthismodel,whichshowsthebulkantiferromagneticspinsbottom,theinterfacialantiferromagneticlmsmiddle,andtheferromagneticspinstop.Theantiferromagneticspinsdonotrotateexceptforsmalldisturbancesveryneartotheinterface.Recallthatboththetotalthicknessofthebilayersnm,andalltheeectiveCrO2thicknessesfallintotheregionofCrO2whichexhibitadoublingswitchingphenomenonduetotheinhomogeneousstraindistributioncausedbythesubstrate.HencethemagnetizationofthebilayersistheresultoftheinhomogeneousstraincausedbythesubstrateatoneendandtheexchangecouplingwiththeantiferromagneticCr2O3attheotherend.Thisislikelytheoriginofthelargestrain-inducedpeakseenforallthebilayerlms.AnotherstrikingfeatureofthecombinedanalysisoftheM-HandtransversesusceptibilityonthebilayersystemisthevariationofHCandHKwithtFM.ThustheCr2O3presenceinthe118

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Figure7.14.Schematicofspin-opcouplingmodel.Adaptedfromreference[72].bilayeraectsthetwomostimportantmagneticproperties,theswitchingeldandtheanisotropyeld,establishingthecouplingbetweentheCrO2andCr2O3layers.ItshouldalsobenotedthatCrO2/Cr2O3systemisanexchange-coupledsysteminwhichthetoplayerisamagnetoelectricantiferromagnet.Recently,magnetoelectricswitchingofanexchangebiaswasshowninCr2O3/CoPt3[9],whereintheCo/PtmultilayersweregrownonCr2O3singlecrystalsof0.7mmthickness.ThedirectionoftheexchangebiasorthehorizontalshiftofM-HcouldbeswitchedbycoolingthesamplefromabovetheNeeltemperatureinanexternalelectriceldeitherparallelorantiparalleltothecoolingDCmagneticeld.Soapartfromtheexchangecoupling,magnetoelectriccouplingofCr2O3totheferromagneticmaycontributetothevariationinHCandHK.TestingformagnetoelectriccouplingwouldrequirelargeelectriceldstobepresentinsidethePPMS,whichisnotsomethingwearecurrentlycapableofdoing.Futureworkwillfocusonthecompletionofaprobedesignedtomeasurecompleximpedance,wherethemagneticanddielectricresponsefunctionscanbesimultaneouslymeasuredinsideofthePPMS.119

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7.7ConclusionWehavepresentedinthischapteracompellingcaseforexchangecouplingofCrO2/Cr2O3bilayersthroughauniaxialanisotropyratherthantheunidirectionalanisotropynormallyassociatedwithexchange-coupledsystems.Thisuniaxialanisotropyismanifestasamod-estincreaseinHCandasubstantialincreaseinHKmeasuredwithtransversesusceptibility.Weweresuccessfullyabletoruleoutasimpleferromagneticthicknessdependencebymea-suringseveraldierentthicknessesofCrO2lms,meanwhileprovidingmoreevidenceofstrain-induceddoubleswitchingintheCrO2lms.Wealsoproposedthatthelackofunidirectionalanisotropyinthebilayersislikelyduetoperpendicularspin-opcouplingbetweentheCr2O3andtheCrO2.120

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CHAPTER8BARIUMHEXAFERRITE/BARIUMSTRONTIUMTITANATEMULTILAYERTHINFILMSThischaptercontainsasummaryoftheauthor'sworkforhermaster'sthesis[18]andthepublicationsthatfollowed[19,84].GrowthofonebatchofthinlmsalongwithsomeoftheirstructuralcharacterizationwasdoneatDr.NancyDudney'slaboratoryatOakRidgeNationalLaboratorybytheauthorasavisitingstudent.AsecondbatchofthinlmswasgrowninDr.KevinCoey'slaboratoryattheUniversityofCentralFlorida.Allpost-annealingandmagneticcharacterizationwasdoneattheUniversityofSouthFlorida.8.1IntroductionAsdiscussedintheopeningchapters,multiferroicmaterials,orthosepossessingbothferroelectricandmagneticordering,haveseenarenewedinterestasoflateduetothedualfunctionalityandinterestingcoupledpropertiestheydisplay.Whilesinglephasemulti-ferroicsarebeingintenselystudiedforfundamentalphysics,itisbecomingmorepopulartosynthesizecompositesandmultilayerstructuresofferroelectricallyandmagneticallyorderedphaseswhereitispossibletoengineerthematerialforadesiredapplicationbymeansofmodifyingitschemicalcomposition,microstructureandlayermorphology.Grow-ingmagnetic/ferroelectricmultilayersispromisingduetothesignicantcontrolonehasoverthegrowthprocess.Thisresultsintheabilitytooptimizegrowthparameterstoachievethedesiredthickness,andmicrostructureincludinggrainsizeandshape.Inthischapter,wepresentresultsonthemagneticpropertiesofmultilayerthinlmsofBaFe12O19andBa)]TJ/F22 7.97 Tf 6.587 0 Td[(xSrxTiO3,twoveryimportantmaterialsusedinmicrowavedevices.121

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ThecompoundBaFe12O19Barium-hexaferrite,BaMisanimportantferrite,andisusedinavarietyofmagneticrecording[33]andhigh-frequencyapplicationssuchasisola-tors,lters,phaseshifters,andcirculators[27].ItbelongstotheM-typeclassofhexag-onalferriteshexaferrites.Thistypeofhexagonalferritehasamagnetoplumbitecrystalstructure,showningure8.1,whichconsistsoffourinterchangingspinelSandSandrhombohedralRandRblocks.Theasteriskmeansthatthecorrespondingblockhasbeenturned180aroundthehexagonalc-axis.Theferrimagneticpropertiescomeentirelyfromthe24Fe3+ions,eachhavingamagneticmomentof5BB:Bohrmagneton,andthetotalmagneticmomentis40B[52].ThelatticeparametersoftheunitcellofBaMarea5.89Aandc23.19A[91].ThemostoutstandingpropertyofBaMisitslargemagneticanisotropy[76].Theeasydirectionofmagnetizationisalongthehexagonalc-axis,andtheharddirectionisalongthehexagonala-axis. Figure8.1.CrystalstructureofBariumHexaferrite.Figureadaptedfromreference[27].122

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Figure8.2.CrystalstructureofBariumStrontiumTitanate.Figureadaptedfromreference[6].Barium-strontium-titanateBSTOisawidelyusedferroelectric,havingtheformulaBa1)]TJ/F22 7.97 Tf 6.587 0 Td[(xSrxTiO3.BSTOisoftenreferredtoasatunable"ferroelectricbecausethepermit-tivitychangeswithappliedelectriceld.Thecompoundwithx=0.5isinaparaelectricstate,withCurietemperaturebelowroomtemperature.TheCurietemperaturecanbeadjustedbychangingthevalueofxBaTiO3isferroelectricandSrTiO3isparaelectricatroomtemperature.ThespontaneouspolarizationcomesfromtherelativedisplacementofBaSrandTiatomstotheOatoms.ThecrystalstructureofBSTOisshowning-ure8.2,withlatticeparameteraround4A.AttheCuriepoint,thelatticeundergoesaphasetransitionfromcubicparaelectrictotetragonalferroelectric.BSTOisaparticu-larlyattractivematerialformicrowaveapplicationsbecauseitpossesesalargepermittivity10;000at0V,hightunability,fastresponsetoelectricelds,highbreakdownelds,lowdielectricleakagecurrents,anditisrelativelyeasytofabricate[6,88].OthergroupshavebroughttogetherthesetwotechnologicallyimportantmaterialsthroughdopingBaMwithBSTO[32]andaformergraduatewhograduatedfromourlabwithaPh.D.hasgrown50%BSTO/50%BaMcompositethinlmsaswellasmultilayers123

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fabricatedbypulsedlaserdepositionPLD[26].Myresearchwastheprimarycontri-butioninourgroup'seortwhichledtotheoptimizedgrowthofsputteredBSTO/BaMmultilayerthinlmsandstudyingtheirmagneticproperties.8.2MultilayerThinFilmGrowth8.2.1FilmsGrownatOakRidgeNationalLaboratoryPureBSTOandBaMlmsaswellasmultilayerlmsweregrownontwotypesofsubstrates:PolishedaluminaAl2O3andthermallyoxidizedsilicon.Depositionwasdoneusinganon-commercialmagnetronsputteringvacuumchamberequippedwithtwogunsholdingtheBSTOandBaMtargetsandattachedtoRFpowersupplies.TheBSTOandBaMtargetswere2-inchceramicsputtertargetswithcopperbackplatesfromSCIEngineeredMaterials.Thetwotargetsweresetupverticallyandamovablesubstrateholdercouldbepositionedovereithertarget.Inaddition,aquartzcrystaloscillatorcouldbeplacedovereitherofthetargetstomeasuredepositionrates.Thepresenceoftwogunsallowedthemultilayerstructuretobegrowninsitubysimplyrotatingthesubstrateholdertobeinpositionabovethedesiredtarget.Theoriginalsubstrateholderwasmodiedbytheauthortoincludeaheaterandthermocouple.Thesubstrateswereheatedduringdepositiontoaround300topromotelmadhesion.Forallmultilayerlms,BSTOwasusedasthebottomlayerduetoitssuperioradhesiontosiliconoverBaM.ThemultilayerlmsgrownatOakRidgeNationalLaboratorywerefourlayers:Substrate/BSTO/BaM/BSTO/BaM.Theoveralllmthicknesswasoptimizedtobearound1.5m.Depositionconditionsarepresentedintable8.1.AlllmsweretakenbacktotheUniversityofSouthFloridaforpost-annealingasthesubstrateheaterwassucientforpromotingadhesion,buttheas-grownlmswerestillamorphous.Postannealingwasdoneinatubefurnaceat1000CinowingO2for10hours.X-raydiractionXRDscanswereperformedattheUniversityofSouthFloridausingaPhilipsPW2-4-ProdiractometeravailablethroughtheCollegeofEngineering.XRDmeasurementsrevealedthepresenceofseveralBaMandBSTOpeaksconsistentwiththe124

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Basepressure=6.810)]TJ/F19 7.97 Tf 6.587 0 Td[(6Torr Layer1 Layer2 Layer3 Layer4 DepositionTemperature350C BSTO BaM BSTO BaM ArgonpressuremTorr 20 20 20 20 Argonowsccm 57.0 56.7 56.6 56.4 PowerW 71 70 71 70 DCBiasV 146 242 148 245 DepositionrateA/min 60.0 39.2 71.6 46.4 Table8.1.RFMagnetronsputteringparametersforBSTO/BaMmultilayersgrownatOakRidgeNationalLaboratory.polycrystallinephasesofeach.ThelmsgrownonSi/SiO2showedimpuritypeakswhichmatchedwellthediractionpatternofSr3Si3O9.SinceBSTOwasthebottomlayer,webelievethattheBSTOlmreactedwiththeSi/SiO2substratecreatinganimpuritylayer.Figure8.3isarepresentativeX-raydiractionscanofthemultilayersonAl2O3.8.2.2FilmsGrownattheUniversityofCentralFloridaMultilayerthinlmsaswellaspureBSTOandBaMlmsweregrowntogetherwithDr.SrinathSanyadanamandDr.RankoHeindlusingacommercialmagnetronsputteringsystembuiltbyAJAInternational,Inc.Thetargetsusedthistimewere3-inchceramictargetsfromSTMC,Ohio.Anewsubstrateholderhadtobebuiltforthissystemaswellbecausethesubstrate-targetdistancewasprohibitivelylargecausingverylowdepositionrates.Evenwiththemodiedsubstrateholder,thedepositiontimewasstillquitelongsothenumberoflayerswaslimitedtojustthree.Aheaterwasnotincludedinthemodiedsubstrateholder,whichcreatedmoreadhesionproblems.ItwasfoundthattheBaMcouldadhereproperlyatroomtemperatureontoAl2O3andallowforannealing.ThusthelmsgrownattheUniversityofCentralFloridawereallAl2O3/BaM/BSTO/BaM.Thedepositiondetailsarepresentedintable8.2.As-grownlmswereagainamorphousandpostannealingwasdoneattheUniversityofSouthFloridausingthesameconditionsdescribedabove.Figure8.4isarepresentativeXRDscanforthemultilayerlmsgrownattheUniversityofCentralFlorida.125

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Figure8.3.X-raydiractionscanofBaM/BSTOmultilayersgrownonAl2O3atOakRidgeNationalLaboratory. Basepressure=7.810)]TJ/F19 7.97 Tf 6.586 0 Td[(6Torr Layer1 Layer2 Layer3 DepositionTemperature:Ambient BaM BSTO BaM ArgonpressuremTorr 4 4 4 Argonowsccm 20 20 20 PowerW 70 70 70 DCBiasV 302 196 302 DepositionrateA/min 38 57 38 Table8.2.RFMagnetronsputteringparametersforBSTO/BaMmultilayersgrownbytheauthorattheUniversityofCentralFlorida.126

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Figure8.4.X-raydiractionscanofBaM/BSTOmultilayersgrownonAl2O3attheUni-versityofCentralFlorida8.3MultilayerCharacterizationFigure8.5showsacross-sectionalscanningelectronmicroscopeSEMimageofoneofthemultilayerthinlmsgrownonSiatOakRidgeNationalLaboratory.Amultilayerstructurewithdistinctinterfacesbetweenlayersisevidentevenafterannealing.Thepicturedepictsalternatelayersof0.3mthickBaMand0.2mthickBSTO.ThebottomlayermayconsistofintermixingofStrontiumwiththeSiliconsubstrate,whichwouldaccountforthepeakscorrespondingtoSr3Si3O9intheXRDspectra.Whiletherewassomeerrorinestimationofdepositionrateleadingtonon-uniformthicknessofindividuallayers,whatitunmistakableisthepresenceofwell-denedanddistinctlyvisiblelayersfromacoarsegrain-structurepointofviewinthenal-annealedlm.Thisispromisingintermsofrealizingcompositemultilayerswithoutsignicantdegradationofmaterialspropertiesattheinterfaces.127

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Figure8.5.Cross-sectionalimageofaSi/SiO2/BSTO/BaM2multilayerlmafteranneal-ing.Figure8.6showsanSEMimageofthesurfaceofthesamesampleusedingure8.5.Theelongatedplatelet-typegrainswhicharetypicalofBaMareclearlyvisiblewithanaveragegrainsizeof1.2mx0.3m.ThishighaspectratiointhegrainsiswhatgivesBaMitswell-knownshapeanisotropy.Thesizeandshapeofthegrainsdependlargelyonthedepositionandannealingconditions,whichinturnaecttheoverallmagneticproperties.Thisisdesirableformicrowavepropertiesasthedielectricconstantandpermeabilityhavebeenshowntoincreasewithincreaseinthegrainsize[32].8.4MagneticPropertiesofBaMandMultilayerThinFilmsFromthegrowthconditionsdescribedinsection8.1,itisnotverypracticaltomakedirectcomparisonsbetweenthelmsgrownatOakRidgeNationalLaboratoryandthosegrownattheUniversityofCentralFlorida.NotonlywerethesubstratetemperaturesdierentCversusroomtemperaturebutalsothemultilayerstructurewasdierentsub/BSTO/BaM2versussub/BaM/BSTO/BaM.Therefore,anycomparisonmadebe-128

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Figure8.6.SEMimageoftheBaMsurfaceofamultilayerlm.tweenthetwocanonlybedonewithoutdecouplingtheeectsofsubstratetemperature,bottomlayerandtotalnumberoflayers.Inthissection,briefsummariesofeachsetoflmsandtheconclusionsthatcanbedrawnfromthemwillbepresentedaswellasabroadoverallsummaryofallsampleswithonlyminimaldiscussionabouttheoriginofthedieringmagneticproperties.8.4.1MagneticPropertiesofFilmsGrownattheUniversityofCentralFloridaWerstexaminedthemagnetizationversuseldloopsofalmofjustBaMonAl2O3at10Kand300Kgure8.7.InalloftheM-Hcurvespresented,anoticeablediamagneticbackgroundcanbeseenatlowtemperatureswhichismanifestasadownwardtiltoftheloop.Thisisduetothemagneticresponseofthesubstrates,whicharediamagneticatlowtemperatureandcontributeaslightparamagneticresponseathighertemperatures,manifestasaslightupwardtilt.TheroomtemperatureHCvalueoftheBaMOeis129

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Sample HCOe HCOe Description 10K 300K BaM/Al2O3 2100 1900 BaM/BSTO/BaM/Al2O3 1800 2100 Table8.3.CoercivityvaluesformultilayerlmsgrownatTheUniversityofCentralFlorida.ingoodagreementwithreportedvaluesofBaMlms[11,74].ForthepureBaMlmonly,wewereabletodeterminethesaturationmagnetizationMSfromthemagneticmomentwithcondencesincetheentirelmthicknessisBaMandthereforethemagneticvolumecanbeestimated.UsingaTencorInstrumentsprolometer,wemeasuredlmthicknessoftheBaMtobe0.45m.Afterndingthesampleareatobe0.324cm2,andthetotalmagneticmomenttobe6.13x10)]TJ/F19 7.97 Tf 6.586 0 Td[(3emuat10Kand1.75x10)]TJ/F19 7.97 Tf 6.586 0 Td[(3emuat300KwecalculatedthevaluesofMStobe420emu/ccat10Kand120emu/ccat300K.ThesevaluesagainmatchwellwithmagneticpropertiesreportedinBaMthinlms[11,74]].Forthemultilayerlms,wedidnotcalculatetheMSvaluesastherewasnowaytoaccuratelydeterminethetotalmagneticvolume.However,forourpurposes,itiscoercivityandtheshapeoftheM-Hcurvesthatwearemostinterestedinasitisthebestindicatorofmagneticanisotropyinthissystem.ForthepureBaMlm,thecoercivityincreaseswithdecreaseintemperatureOeat10K,whichistheexpectedbehaviorinbulkmagneticmaterials.Uponexaminingthe10Kand300Khysteresisloopsofthemultilayerlmsgrownunderthesameconditionsgure8.8itisapparentthemagneticbehaviorisdierent.RatherthanadecreaseincoercivitywithaincreaseintemperaturelikethepureBaMlm,themultilayersexhibitanincreaseincoercivitywithincreaseintemperaturegoingfrom1800Oeat10Kto2100Oeat300K.ThistrendisalsoseeninthelmsgrownatOakRidgeNationalLaboratoryandwillbediscussedinalatersection.ThehighandlowtemperatureHCvaluesforboththeBaMandthemultilayerlmsarecollectedintable8.3.8.4.2MagneticPropertiesofFilmsGrownatOakRidgeNationalLaboratoryMagnetizationversuseldM-Hmeasurementswereperformedat10Kand300Kwithmagneticeldappliedparallelin-planeandperpendicularout-of-planetothesurfaceof130

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Figure8.7.10Kand300KhysteresisloopsforBaMonAl2O3grownattheUniversityofCentralFlorida.thelm.Ingures8.9and8.10thein-plane10Kand300KhysteresisloopsarepresentedforthemultilayerlmsgrownonAl2O3gure8.9andSigure8.10.Onceagainitcanbeseenthatthecoercivityincreaseswithincreaseintemperaturefrom2600Oeto3900OeforthelmonAl2O3,andfrom1460Oeto2300OeforthelmonSi.ThisisthesamebehaviorthatwasobservedforthemultilayerlmsgrownattheUniversityofCentralFloridaandisoppositethetraditionalbehaviorseenforBaMalone,namelyadecreaseincoercivitywithincreaseintemperature.Thispointwillbefurtherdiscussedinthenextsection.Itisalsoapparentthatforbothtemperatures,theHCvaluesforthelmsgrownonSiaremuchlowerthantheHCvaluesforthelmsgrownonAl2O3.Figures8.12and8.11showthe10Kin-planeandout-of-planehysteresisloopsforthemultilayersgrownonAl2O3gure8.12andSigure8.11.ForthemultilayersgrownonSitheperpendicularcoercivityandtheparallelcoercivityat10Kare1250Oeand1450OeandthesquarenessStheratiooftheremanentmagnetizationtothesaturationmag-131

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Figure8.8.10Kand300KhysteresisloopsformultilayersonAl2O3grownattheUniversityofCentralFlorida.132

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Figure8.9.10Kand300KhysteresisloopsformultilayersonAl2O3grownatOakRidgeNationalLaboratory. Figure8.10.10Kand300KhysteresisloopsformultilayersonSigrownatOakRidgeNationalLaboratory.133

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HCOe HCOe HCOe HCOe Sample 10K 10K 300K 300K Description in-plane out-of-plane in-plane out-of-plane Bam/BSTO2/Al2O3 2600 2600 3900 3900 BaM/BSTO2/Si 1460 1250 2300 2200 Table8.4.CoercivityvaluesformultilayerlmsgrownatOakRidgeNationalLaboratory.netization,MR/MSoftheperpendicularandparallelloopsare0.45and0.60respectively.Thedierenceinmagnetizationintwodierentdirectionsofthemultilayersplaneindicatesthatthereisapreferentialorientationofmagnetizationalongafavoreddirection,whichistheeasyaxisofmagnetizationfortheBaMgrains.Ascanbeseeningure8.6,theplatelet-likegrainsintheBaMlayerappeartohaveattainedanin-planetexturingandthemagneticeasyaxisseemstobeinthesamedirection.Incontrast,themultilayersgrownonAl2O3shownosuchdeviationuponchangingorientationsofthemagneticeld.ThepolycrystallineAl2O3substratedoesnotallowgrainorientationandthelackoftexturingcausesnochangeincoercivitywithrespecttoappliedelddirection.ThevalueofHC10Kremainsthesameat2600Oeregardlessofeldorientation.Intable8.4wepresentthein-planeandout-of-planeHCvaluesformultilayerlmsgrownoneachsubstrateat10Kand300K.NotethatthetexturingexiststoalesserextentatroomtemperatureforlmsgrownonSi,andnotatallforthelmsgrownonAl2O3.TheroomtemperatureresultspresentedforthelmsgrownatOakRidgeNationalLaboratoriesonheatedSiareconsistentwiththosereportedforpureBaMlmsdepositedusingpulsedLaserdepositionheatedinsituto900ConSisubstrates[45].Thein-planeandout-of-planecoercivitiesinourcaseisonlyslightlydierentfromthereportedvaluesofLuandSong[45].Whilethedierentlmgrowthtechniquesareexpectedtohavesomeinuenceonthemagneticcharacteristics,substrateheatingintheircasewasmuchhigherandislikelytheprimaryreasonforthemobtainingbettertexture.Also,whilesubstratematerialclearlyhasaneectonthetexturinganorientedsubstrategivestexturedlms,apolycrystallinesubstrategivesisotropiclms,itisimportanttokeepin134

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Figure8.11.10KhysteresisloopsformultilayersonSitakenwithHin-planeandout-of-plane.135

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Figure8.12.10KhysteresisloopsformultilayersonAl2O3takenwithHin-planeandout-of-plane.136

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mindthatneitherlayerofBaMisindirectcontactwiththeorientedSi.ThustheextentofthetexturingthatwouldusuallybeseeninBaMonSimaybelostinthiscaseduetotheintermediateBSTO.ExtensivestudiesofepitaxyandsubstrateinuenceinPLD-grownBaM/BSTOlmshavebeendonebyaformergraduatestudentRankoHeindlandreportedinhisPh.D.thesis[28].8.5CorrelatingtheCoercivitywithMicrostructureinBaMandBaM/BSTOMultilayerFilmsThemostnoticeabletrendintheabovedataisthatwhilethecoercivityoftheBaMdecreaseswithincreaseintemperatureconsistentwithmostotherbulkmagneticmateri-als,themultilayersbehaveintheoppositewaywithincreasingcoercivityastemperatureincreases.WebelievethiscanbeunderstoodintermsofthecompetitionbetweenshapeandmagnetocrystallineanisotropiesinBaM.ThetheoreticalcoercivityofarandomarrayofBaMparticlesisgivenbyKuboetal.[39]HC=0:48Ke=MS)]TJ/F21 10.909 Tf 10.909 0 Td[(NMS.1whereNisthedemagnetizingfactor.Therstterm2Ke/MSisthemagnetocrystallineanisotropyandthesecondtermNMSistheshapeanisotropy.AsMSdecreaseswithincreaseintemperature,thersttermdominatesandHCwillincreasewithtemperature.ItispossiblethatthepresenceoftheBSTOlayersaectthegraingrowthoftheBaM.ItisknownthatthecoercivityofBaMisinverselyproportionaltothegrainsize,withlargergrainsyieldingasmallercoercivity[33].Inthiscase,ifthegraingrowthisinhibited,thecoercivitywillincrease,resultinginasmalleraspectratioandthusasmallershapeanisotropyNMS.Indeed,allthecoercivityvaluespresentedaboveforthemultilayersshowanincreaseincoercivityoverthepureBaM.Incomparingthetwosetsoflms,wecandeducethatthefunctionaldependenceofcoercivityontemperatureisalteredbythepresenceofBSTOandthatheatingthe137

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substratewhiledepositingthemultilayersenhancesthiseectsinceallthelmsgrownonheatedsubstratesdisplaylargerHCvaluesthanallofthosegrownatroomtemperature.HoweveritisimportanttorememberthatthemultilayersgrownatroomtemperaturealsohadlessBSTOpresentthantheonesgrownonheatedsubstrates.Therefore,asmentionedinanearliersection,amoresystematicstudyisneededtodecouplethesetwoeects,namelythenumberofBSTOlayersandthesubstrateheating.Formanytechnologicalapplications,theincreaseinHCwithincreaseintemperatureresultinginhigherroomtemperatureHC,alongwiththeoverallincreaseincoercivitywithBSTOpresentleadstogreaterfunctionalityoftheBaMinthemultilayersthaninBaMalone.WhiletheferroelectricpropertiesofBSTOinmultilayerformwithBaMhavenotbeenexploredextensively,Dr.RankoHeindl,aformerstudentshowedinhisdoctoralthesisthatdualtunabilityofthepermittivityinBSTOandpermeabilityinBaMformicrowaveapplicationsisachievableinBSTO/BaMbilayerthinlms[28].8.6ConclusionWehavegrownBaMthinlmsandBSTO/BaMmultilayerthinlmsandexaminedthestructuralaswellasmagneticpropertiesofboth.TheBaMlmsgrownatroomtemperatureonAl2O3exhibitedbehaviorconsistentwithotherreportsofBaMlmsgrownundersimilarconditions.Themultilayers,grownbothatroomtemperatureand350circConAl2O3andonheatedSishoweddierentbehaviorwithmuchlargerHCvalueswhichincreasewithincreasingtemperature,contrarytowhatisexpectedinmagneticmaterials.ThelmsgrownonSialsoshowtexturingwithdierentHCandsquarenessratiodependinguponthedirectionofappliedmagneticeld.Webelievethatallthedescribedbehaviorcanbeattributedtothedependenceofgrainsizeandshapeonthegrowthconditions,includingthepresenceofBSTOinthemultilayers.Thetrendcanbeexplainedbythetemperaturedependenceonthecompetingmagnetocrystallineandshapeanisotropies.138

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CHAPTER9CONCLUSIONANDFUTUREWORKThisworkpresentedthemagneticpropertiesofseveralmultifunctionalsystemsinclud-ingcore-shellAu-Fe3O4nanoparticles,dumbbell-andower-shapedAu-Fe3O4nanopar-ticles,CrO2andCrO2/Cr2O3thinlmsandBaM/BSTOmultilayerthinlms.Inthesesystemswewereabletoshowhowthemagneticanisotropywasinuencedbykeyparam-etersfromalteringparticlesize,shape,surfacesandinterfacesinAu-Fe3O4particlestotheeectsofinterfacesbetweensubstratesandfunctionallayersinmultilayerthinlms.Inseveraloftheseinstances,weusedtransversesusceptibilityinnewwaystogaindeeperinsightintothephysicsofthesecomplexsystems.9.1MagneticNanoparticlesforBiomedicalApplicationsForFe3O4andAu-Fe3O4nanoparticlessynthesizedspecicallyforbiomedicalapplica-tions,weconclusivelydemonstratedhowcoatingFe3O4particleswithAudecreasesinter-particleinteractionswhileincreasingthefunctionalityoftheparticles.Theparticularsizechosenforthesynthesisoftheparticlescoincideswiththesizerangethatcanbesuccess-fullytakenupbyhumanembryonickidneycells.Thisparticlediameter,about70nm,alsoprovedtobeadvantageousforhyperthermiaapplicationsaswedemonstratedthattheparticlesweresuperparamagneticinDCmagneticeldsandferromagneticatthe12MHzusedintransversesusceptibilitymeasurements.Thisindicatesthathystereticlossescouldalsocontributetotheheatingabilityoftheseparticles.Wewerealsoabletoshowhowthetransversesusceptibilitymeasurementcouldbeusedasasensorforcellsthathavetakenupmagneticnanoparticles.DuetoitshighsensitivityandthelowexternalDCeldsrequired,wedescribedwhythismethodcouldnduseincancerdiagnostics.139

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Remainingquestionswithregardstothisresearchare:Whatothercore-shellcongurationscanbemadewhichmaydisplayevenbettermagneticpropertieswhilemaintainingbiocompatibility?Sincedierentbodilysystemsrequiredierentsizenanoparticlestoachieveuptake,canwetunetheanisotropyinsmaller/largerparticlesofdierentmaterialstodisplaythesamebalanceofDCsuperparamagnetism/ACferromagnetism?Canasmaller,morecompactbiosensorbebuiltbasedontheconceptoftransversesusceptibilityandtheresultsweobtained?9.2Au-Fe3O4CompositeNanostructuresWealsoexaminedthemagneticpropertiesoftwonewcongurationsofAu-Fe3O4.Thedumbbell-shapedAu-Fe3O4particles,whichshowedbehaviorsimilartoFe3O4andevencore-shellAu-Fe3O4nanoparticles,isanimportantsystemtoexploreforbiomedicalapplicationsastwoseparatesurfacesareavailableforfunctionalization.Theower-shapednanoparticlesprovedtobeafascinatingsystemfromafundamentalphysicspointofviewduetothecombinationofcompetinginteractionsgivingrisetoexchangebiasandtrainingeectsinthelowtemperatureregime,whilemaintainingananomalouslyhighanisotropyintheintermediateregimebeforeblocking.AnimportantfeatureofbothofthesesystemsisthefactthatboththeAuandtheFe3O4sizescanbecontrolledforpossibleuseinabroadrangeofapplications.Therearenumerousdirectionsthisresearchcantakeandseveralquestionsthatremainunresolved.Mostnotably:CanweoptimizethesizecombinationsofAuandFe3O4tondwhichcombinationperformsbestforfunctionalization,cellularuptake,andmagneticmanipulationwhileinsidethecells?Whatisthenatureofthelowtemperaturenegativemagnetization?140

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CanwechangethesizesoftheAuintheowerparticlestodeterminethedependenceoftheinteractionstrengthbetweenFe3O4particlesontheintra-particledistance?Similarly,canwechangethesizeoftheFe3O4aswellasthenumbernucleatedontotheAu?ThiscouldhelpformabetterpictureofthespinfrustrationlikelyinvolvedbetweenadjacentFe3O4particles.9.3CrO2EpitaxialThinFilmsandBilayerCrO2/Cr2O3ThinFilmsThethicknessandtemperaturedependenceofCrO2epitaxialthinwasexploredusingtransversesusceptibility.Wewereabletoconrmandtracktheeasyaxisswitchingduetotheinhomogeneousstrainat200nm.Wehavealsopresentedstrongcaseforexchangecou-plingofCrO2/Cr2O3bilayersthroughauniaxialanisotropyratherthantheunidirectionalanisotropynormallyassociatedwithexchangecoupledsystems.ThisuniaxialanisotropyismanifestasamodestincreaseinHCandasubstantialincreaseinHKmeasuredwithtrans-versesusceptibility.WeweresuccessfullyabletoruleoutasimpleferromagneticthicknessdependencebymeasuringseveraldierentthicknessesofCrO2lms.Wesuggestedthatthelackofunidirectionalanisotropyinthebilayersislikelyduetoperpendicularspin-opcouplingbetweentheCr2O3andtheCrO2.Somequestionsthatwouldbeinterestingtoaddressforthesesystemsare:Allthelmshadthesametotalthicknesswhichmeanteachcomponentwasvary-ing.Canweformabetterpictureofthedependenceoftheanomalousanisotropyonthicknessbyvaryingthethicknessesoftheferromagnetandantiferromagnetin-dependently?CananyoftheanisotropicbehavioroftheCrO2/Cr2O3bilayersbeexplainedbyamagnetoelectriccouplingoftheCrO2totheCr2O3?Similarly,couldanexperimentbedoneinthesameveinastheoneusedbyBorisovetal.toseeifappliedelectriceldchangesthenatureoftheexchangecoupling?141

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Aprobethatmeasuresimpedanceinamagneticeldcurrentlybeingbuiltforinte-grationintothePPMScouldhelpanswermanyofthesequestions.9.4BaMThinFilmsandBaM/BSTOMultilayerThinFilmsBaMthinlmsandBSTO/BaMmultilayerthinlmshavebeengrownandthestruc-turalaswellasmagneticpropertieshavebeenexamined.BaMlmsexhibitedbehaviorconsistentwithotherreportsofBaMlmsgrownundersimilarconditions.Themultilay-ersshoweddierentbehaviorwithHCvalueswhichincreasewithincreasingtemperature,contrarytowhatisexpectedinmagneticmaterials.Webelievethatthisbehaviorcanbeattributedtothedependenceofgrainsizeandshapeonthegrowthconditions,includingthepresenceofBSTOinthemultilayers.Thetrendcanbeexplainedbythetemperaturedependenceonthecompetingmagnetocrystallineandshapeanisotropies.ManyofthequestionsthatwereposedatthetimethisresearchwascompletedhavebeenansweredasaresultoftheresearchperformedbyDr.RankoHeindl.Manipulationofthemagneticanisotropyinnanoparticlesandthinlmswillcontinuetobeanimportantaspectinfuturetechnologicalapplications.Thisworkhasshownthatsurfaceandinterfacemagnetismcandramaticallyaltertheoverallmagneticresponseofasysteminunexpectedwaysandtechnologicallyimportantways.142

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[66]P.Poddar,H.Srikanth,S.A.Morrison,andE.E.Carpenter,Inter-particleinterac-tionsandmagnetisminmanganese-zincferritnanoparticles,J.Magn.Magn.Mater.288,443{451.[67]P.Poddar,J.L.Wilson,H.Srikanth,D.F.Farrell,andS.A.Majetich,In-planeandout-of-planetransversesusceptibilityinclose-packedarraysofmonodisperseFenanoparticles,Phys.Rev.B68,214409{1{214409{8.[68]G.T.RadoandV.J.Folen,Observationofthemagneticallyinducedmagnetoelectriceectandevidenceforferromagneticdomains,Phys.Rev.Lett.7,310{311.[69]H.J.Richter,Transversesusceptibilitymeasurementsofparticulatemedia,IEEETrans.Mag.26,1882{1884.[70]D.S.Rodbell,MagnetocrystallineanisotropyofsinglecrystalCrO2,J.Phys.Soc.Jpn.21,1224{1225.[71]M.Sasaki,P.E.Jonsson,H.Takayama,andH.Mamiya,Agingandmemoryeectsinsuperparamagnetsandsuperspinglasses,Phys.Rev.B71,104405{1{104405{9.[72]T.C.SchulthessandW.H.Butler,Consequencesofspin-opcouplinginexchangebiasedlms,Phys.Rev.Lett.81,4516{4519.[73]H.Shim,A.Manivannanm,M.S.Seehra,K.M.Reddy,andA.Punnoose,EectofinterparticleinteractiononthemagneticrelaxationinNiOnanorods,J.Appl.Phys.99,08Q503{1{08Q503{3.[74]S.R.Shinde,R.Ramesh,S.E.Loand,S.M.Bhagat,S.B.Ogale,R.P.Sharma,andT.Venkatesan,Eectoflatticemismatchstrainsonthestructuralandmagneticpropertiesofbariumferritelms,J.Appl.Phys.72,3443{3445.[75]V.Skumryev,S.Stoyanov,Y.Zhang,G.Hadjipanayis,D.Givord,andJ.Nogues,Beatingthesuperparamagneticlimitwithexchangebias,NatureLondon423,850{853.[76]J.SmitandH.P.J.Wijn,Ferrites,JohnWilley&Sons,1959.[77]R.J.Soulen,J.M.Byer,M.S.Osofsky,B.Nadgorny,C.T.Tanaka,J.Nowak,J.S.Moodera,A.Barry,andJ.M.Coey,Measuringthespinpolarizationofametalwithasuperconductingpointcontact,Science282,85{88.[78]L.Spinu,I.Dumitru,A.Stancu,andD.Cimpoesu,Transversesusceptibilityasthelow-frequencylimitofferromagneticresonance,J.Magn.Magn.Mater.296,1{8.[79]L.Spinu,H.Srikanth,A.Gupta,X.W.Li,andG.Xiao,ProbingmagneticanistropyeectsinepitaxialCrO2thinlm,Phys.Rev.B62,8931{8934.[80]L.Spinu,A.Stancu,Y.Kubota,G.Ju,andD.Weller,VectorialmappingofexchangeanisotropyinIrMn/FeComultilayersusingthereversiblesusceptibilitytensor,Phys.Rev.B68,220401{1{220401{4.148

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[81]L.Spinu,A.Stancu,andC.J.O'Connor,Micromagneticstudyofreversibletransversesusceptibility,PhysicaB306,221{227.[82]L.Spinu,A.Stancu,C.J.O'Connor,andH.Srikanth,Eectofthesecond-orderanisotropyconstantonthetransversesusceptibilityofuniaxialferromagnets,Appl.Phys.Lett.80,276{278.[83]H.Srikanth,J.Wiggins,andH.Rees,Radio-frequencyimpedancemeasurementsusingatunnel-diodeoscillatortechnique,Rev.Sci.Inst.70,3097{3101.[84]S.Srinath,N.A.Frey,R.Heindl,H.Srikanth,K.Coey,andN.J.Dudney,GrowthandcharacterizationofsputteredBSTO/BaMmultilayers,J.Appl.Phys.97,10J115{1{10J115{3.[85]A.StancuandL.Spinu,Transversesusceptibilityforsinge-domainparticlewithcubicanisotropy,J.Magn.Magn.Mater.266,200{206.[86]E.C.StonerandE.P.Wohlfarth,Amechanismofmagnetichysteresisinheteroge-neousalloys,Phil.Trans.oftheRoyalSocietyofLondonA:Math.andPhys.Sci.240,599{642.[87]Y.Sun,M.B.Salamon,K.Garnier,andR.S.Averback,Memoryeectsinaninter-actingmagneticnanoparticlesystem,Phys.Rev.Lett.91,167206{1{167204{4.[88]A.K.Tagantsev,V.O.Sherman,K.F.Astaev,J.Venkatesh,andN.Setter,Fer-roelectricmaterialsformicrowavetunableapplications,J.Electroceramics11,5{66.[89]K.N.Trohidou,M.Vasilakaki,L.DelBianco,D.Fiorani,andA.M.Testa,Exchangebiasinamagneticordered/disorderednanoparticlesystem:AMonteCarlosimulationstudy,J.Magn.Magn.Mater.316,e82{e85.[90]E.Tronc,D.Fiorani,MNogues,A.M.Testa,F.Lucari,F.D'Orazio,J.M.Greneche,W.Wernsdorfer,N.Galvez,C.Chaneac,D.Mailly,andJ.P.Jolivet,Surfaceeectsinnoninteractingandinteracting-Fe2O3nanoparticles,J.Magn.Magn.Mater.262,6{14.[91]W.H.vonAulocked.,HandbookofMicrowaveFerriteMaterials,AcademicPress,1965.[92]H.Wang,T.Zhu,K.Zhao,W.N.Wang,C.S.Wang,Y.J.Wang,andW.S.Zhan,SurfacespinglassandexchangebiasinFe3O4nanoparticlescompactedunderhighpressure,Phys.Rev.B70,092409{1{092409{4.[93]H.Yu,M.Chen,P.M.Rice,S.X.Wang,R.L.White,andS.Sun,Dumbbell-likebifunctionalAu-Fe3O4nanoparticles,NanoLetters5,279{382.[94]R.K.Zheng,H.Gu,B.Xu,andX.X.Zhang,Memoryeectsinananoparticlesystem:Low-eldmagnetizationandacsusceptibilitymeasurements,Phys.Rev.B72,014416{1{0114416{7.149

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[95]R.K.Zheng,H.Liu,andX.X.Zhang,Cr2O3surfacelayerandexchangebiasinanacicularCrO2particle,Appl.Phys.Lett.84,702{704.[96]R.K.Zheng,G.H.Wen,K.K.Fung,andX.X.Zhang,Trainingeectofexchangebiasin-Fe2O3coatedFenanoparticles,Phys.Rev.B69,214431{1{214431{4.150

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APPENDICES151

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AppendixAListofPublicationsJournalArticlesC.Leighton,D.D.Stauer,Q.Huang,Y.Ren,B.Toby,M.A.Torija,S.El-Khatib,J.W.Lynn,J.Wu,L.Wang,N.A.Frey,H.Srikanth,J.E.Davies,K.Liu,andJ.F.Mitchell.Coupledstructural/magnetocrystallineanisotropytransitionsinthedopedperovskitecobaltitePr1)]TJ/F22 7.97 Tf 6.587 0 Td[(xSrxCoO3.Manuscriptinpreparation.M.H.Phan,J.Gass,N.A.Frey,H.Srikanth,M.Angst,B.C.Sales,andD.Mandrus.EnhancedrefrigerantcapacityingeometricallyfrustratedLuFe2O4withmultiplephasetransitions.Manuscriptinpreparation.P.Poddar,M.B.Morales,N.A.Frey,S.A.Morrison,E.E.Carpenter,andH.Srikanth.Transversesusceptibilitystudyoftheeectofvaryingdipolarinteractionsonanisotropypeaksina3Dassemblyofsoftferritenanoparticles.Manuscriptinpreparation.M.H.Phan,M.B.Morales,N.A.Frey,andH.Srikanth.Originofmagneticanomaliesinthefrozen,mixedandliquidstatesofferrouids.Submitted,Phys.Rev.Lett.S.Srinath,N.A.Frey,H.Srikanth,C.Wang,andS.Sun.MagnetismandsurfaceanisotropyincompositeAu-Fe3O4nanoparticles.Submitted,Phys.Rev.,B.N.A.Frey,S.Srinath,H.Srikanth,C.Wang,andS.Sun.StaticanddynamicmagneticpropertiesofcompositeAu-Fe3O4nanoparticles.IEEETrans.Mag.,43,3094-3096.N.A.Frey,S.Srinath,H.Srikanth,M.Varela,S.Pennycook,G.X.Miao,andA.Gupta.MagneticanisotropyinepitaxialCrO2andCrO2/Cr2O3bilayerthinlms.Phys.Rev.,B,74,024420-1{024420-8.152

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AppendixAContinuedN.A.Frey,R.Heindl,S.Srinath,S.Srikanth,andN.J.Dudney,MicrostructureandmagnetisminBariumStrontiumTitanateBSTO-BariumHexaferriteBaMmultilayers,Mat.Res.Bull.40,1286-1293.S.Srinath,N.A.Frey,R.Heindl,H.Srikanth,K.R.Coey,andN.J.Dudney,GrowthandcharacterizationofsputteredBSTO/BaMmultilayers,J.Appl.Phys.97,1-3.J.L.Wilson,P.Poddar,N.A.Frey,H.Srikanth,K.Mohomed,J.P.Harmon,S.Kotha,andJ.Wachsmuth.Synthesisandmagneticpropertiesofpolymernanocom-positeswithembeddedironnanoparticles.J.ofApp.Phys.,95,1439-1443.L.L.R.WilliamsandN.Frey.AngularDistributionofGamma-rayBurstsandWeakLensing.Astrophys.Jour.,583,594-605.ConferenceProceedingsN.A.Frey,M.B.Morales,H.Srikanth,andS.Srinath.Transversesusceptibilityasaprobeofinterfacemagnetisminfunctionalmultilayersandnanostructures.Ency-clopediaofAdvancedMaterials:ScienceandEngineeringPanStanfordPublishers,inpress2008.J.Gass,N.A.Frey,M.B.Morales,M.J.Miner,S.Srinath,andH.Srikanth.Mag-neticanisotropyandmagnetocaloriceectMCEinNiFe2O4nanoparticles.Mate-rialsResearchSocietySymposiumProceedings,962,0962-P05-03.S.Srinath,N.A.Frey,H.Srikanth,G.X.Miao,andA.Gupta.MagneticanisotropyandexchangecouplinginepitaxialCrO2andCrO2/Cr2O3bilayerthinlms.Pro-ceedingsofDAESolidStateSymposium2006.153

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AppendixAContinuedS.Srinath,N.A.Frey,H.Srikanth,G.X.Miao,andA.Gupta.ExchangeBiasinCrO2/Cr2O3bilayerthinlms.AdvancesinScienceandTechnology,45,2528.154

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AppendixBListofConferencePresentationsConferencePresentationsN.A.Frey,H.Srikanth,D.D.Stauer,andC.Leighton.MagneticanisotropyandswitchinginPr0:5Sr0:5CoO3usingRFtransversesusceptibility.SessionB23.10,AmericanPhysicalSocietyMarchMeeting,NewOrleans,LA.2008N.Frey-Huls,H.Srikanth,D.D.Stauer,andC.Leighton.UnusualmagneticanisotropyandswitchinginPr0:5Sr0:5CoO3probedbyRFtransversesusceptibility.SessionFR-01,52ndMagnetismandMagneticMaterialsConference,Tampa,FL.N.A.Frey,S.Srinath,H.Srikanth,C.Wang,andS.Sun.Staticanddynamicmag-neticpropertiesofdumbbell"andower"shapedAu-Fe3O4nanoparticles.SessionP14.12AmericanPhysicalSocietyMarchMeeting,Denver,CO.N.A.Frey,S.Srinath,H.Srikanth,C.Wang,andS.Sun.Staticanddynamicmag-neticpropertiesofdumbbell"andower"shapedAu-Fe3O4nanoparticles.SessionBB-05.10thJointMagnetismandMagneticMaterials/IntermagConference,Balti-more,MD.S.Srinath,N.A.Frey,H.Srikanth,G.X.Miao,andA.Gupta.MagneticanisotropyandexchangebiasinepitaxialCrO2/Cr2O3bilayerthinlms.SessionCP-1310thJointMagnetismandMagneticMaterials/IntermagConference,Baltimore,MD.N.A.Frey,S.Srinath,H.Srikanth,G.X.Miao,andA.Gupta.MagneticanisotropyinCrO2andCrO2/Cr2O3bilayerthinlms.SessionY22.10AmericanPhysicalSocietyMarchMeeting,Baltimore,MD.N.A.Frey,R.Heindl,S.Srinath,H.Srikanth,K.R.Coey,andN.J.Dudney.GrowthandCharacterizationoftunableBSTO/BaMmultilayersassubstratesfor155

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AppendixBContinuedmagneticnanoparticles.SessionS42.6AmericanPhysicalSocietyMarchMeeting,LosAngeles,CA.N.A.Frey,H.Hajndl,P.Poddar,H.Srikanth,andN.J.Dudney.Growthandchar-acterizationofBSTO/BariumHexaferritemultilayeredlmsastunablesubstratesfornanoparticles.SessionN23.12AmericanPhysicalSocietyMarchMeetingMontreal,Quebec,Canada.ContributedConferencePresentationsM.B.Morales,P.Poddar,N.A.Frey,H.Srikanth,S.A.Morrison,andE.E.Carpenter.Probingtheeectofinterparticleinteractionsinferritenanoparticlesus-ingthereversibletransversesusceptibilitymethod.SessionD27.7AmericanPhysicalSocietyMarchMeeting,NewOrleans,LA.H.Srikanth,N.Frey-Huls,C.Vasani,andJ.Sanatamaria.MagneticanisotropyandvortexdynamicsinLCMO/YBCOheterostructures.SessionED-1052ndMagnetismandMagneticMaterialsConference,Tampa,FL.H.Srikanth,N.A.Frey,C.Visani,andJ.Santamaria.MagneticanisotropyandvortexdynamicsinLCMO/YBCOheterostructures.SessionL13.5AmericanPhysicalSocietyMarchMeeting,Denver,CO.2007J.Gass,M.B.Morales,N.A.Frey,M.J.Miner,S.Srinath,andH.Srikanth.Mag-netocaloriceectMCEinNickelFerritenanoparticles.SessionW14.8AmericanPhysicalSocietyMarchMeeting,Denver,CO.2007S.L.Morrow,N.A.Frey,S.Srinath,andH.Srikanth.SynthesisofBariumHex-aferritenanoparticlesforfunctionalmultilayers.SessionW22.6AmericanPhysicalSocietyMarchMeeting,Baltimore,MD.156

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AppendixBContinuedR.Hajndl,S.Srinath,N.A.Frey,H.Srikanth,S.Balachandran,andT.Weller.GrowthandCharacterizationofBSTO/Bariumhexaferritemultilayers.InternationalConferenceonFerroelectrics,SanFrancisco,CA.S.Srinath,N.A.Frey,R.Hajndl,H.Srikanth,K.R.Coey,andN.J.Dudney.GrowthofelectricallyandmagneticallytunableBSTO/BaFmultilayers.49thMag-netismandMagneticMaterialsConferenceJacksonville,FL.N.Frey,R.Hajndl,P.Poddar,andH.Srikanth.MicrostructureandmagnetisminBSTO/hexaferritecompositelms.2ndInternationalConferenceonMaterialsforAdvancedTechnologies&IUMRS,Singapore.157

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ABOUTTHEAUTHORNatalieFreywasbornandraisedinMinneapolis,Minnesota.SheattendedtheUni-versityofMinnesotaasanundergraduatewhereshedoublemajoredinphysicsandas-trophysics.ShejoinedtheUniversityofSouthFloridaPhysicsDepartmentinthefallof2002andtheFunctionalMaterialsLaboratorythefollowingspring.BesideshertimespentdoingresearchatUSFshehasspenttwosummersawayasavisitingstudent.In2003shewastherecipientofaSURAfellowshipwhichbroughthertoOakRidgeNationalLabora-toryinKnoxville,Tennessee.Inthesummerof2007,shewasaninternintheRecordingSystemsOperationsGroupatSeagateTechnologiesinherhometownofMinneapolis.Shehaspublishedseveralscienticpapersonawiderangeofmagneticsystemsfocusingmainlyontheuseoftransversesusceptibilitytomeasuremagneticanisotropy.In2006shewasawardedtheNSFIGERTfellowshipandhassincebroadenedherresearchtoincludetuningthemagneticpropertiesofnanoparticlesfordrugdeliveryandhyperthermia.ShecurrentlyresidesinTampawithherhusband,Deanandtheirthreecats.


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Surface and interface magnetism in nanostructures and thin films
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ABSTRACT: Nanostructured systems composed of two or more technologically important materials are useful for device applications and intriguing for the new fundamental physics they may display. Magnetism at the nanoscale is dominated by size and surface effects which combined with other media lead to new spin dynamics and interfacial coupling phenomena. These new properties may prove to be useful for optimizing sensors and devices, increasing storage density for magnetic media, as well as for biomedical applications such as drug delivery, MRI contrast enhancement, and hyperthermia treatment for cancer. In this project we have examined the surface and interface magnetism of composite nanoparticles and multilayer thin films by using conventional DC magnetization and AC susceptibility as well as transverse susceptibility, a method for directly probing the magnetic anisotropy of materials.Au and Fe3O4 synthesized together into three different nanoparticle configurations and ranging in size for 60 nm down to 9nm are used to study how the size, shape, and interfaces affect the most fundamental properties of magnetism in the Au-FeO system. The findings have revealed ways in which the magnetic properties can be enhanced by tuning these parameters. We have shown that by changing the configurations of the Au and FeO particles, exotic behavior can be observed such as a large increase in anisotropy field (H[subscript]K ranging from 435 Oe to 1650 Oe) and the presence of exchange bias. Multilayer thin films have been studied as well which combine the important classes of ferromagnetic and ferroelectric materials. In one case, barium hexaferrite/barium strontium titanate thin films, the anisotropic behavior of the ferromagnet is shown to change due to the introduction of the secondary material.In the other example, CrO/CrO bilayers, exchange coupling is observed as CrO is an antiferromagnet as well as a ferroelectric. This coupling is manifest as a uniaxial anisotropy rather than the unidirectional anisotropy associated with exchange biased bilayers. Not only will such multifunctional structures will be useful for technological applications, but the materials properties and configurations can be chosen and tuned to further enhance the desired functional properties.
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