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Magnetization dynamics and interparticle interactions in ferrofluids and nanostructures

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Magnetization dynamics and interparticle interactions in ferrofluids and nanostructures
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Morales, Marienette B
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Magnetic colloids
Nanoparticles
Transverse susceptibility
Relaxation phenomena
Superparamagnetism
Dissertations, Academic -- Physics -- Masters -- USF   ( lcsh )
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non-fiction   ( marcgt )

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ABSTRACT: Nanoparticle assemblies are of current interest as they are used in a wide variety of industrial and biomedical applications. This work presents two studies aimed at understanding the magnetization dynamics and interparticle interactions in nanoparticle assemblies and various types of ferrofluids. First, we studied the influence of varying strengths of dipolar interaction on the static and dynamic magnetic properties of surfactant-coated monodispersed manganese-zinc ferrite nanoparticles using reversible transverse susceptibility. We tracked the evolution of the anisotropy peaks with varying magnetic field, temperature, and interaction strength. The anisotropy peaks of weakly interacting particles appears as non-symmetric peaks and at lower fields in a unipolar transverse susceptibility scan. On the other hand, a strongly interacting particle system exhibits symmetric anisotropy peaks situated at higher field values.In the second study, we successfully synthesized stable ferrofluids out of high quality Fe₃O₄ and CoFe₂O₄ nanoparticles. Such ferrofluids are excellent systems for the investigation of physics of relaxation phenomena in magnetic nanoparticles. Motivated by the need to understand their peculiar magnetic response, a comparative study on Fe₃O₄- and CoFe₂O₄-based ferrofluids was performed. We investigated cases in which particle blocking and carrier fluid freezing temperatures were close and far apart from each other. Our experimental results reveal the true origin of the glass-like relaxation peaks that have been widely observed in ferrofluids by many groups but remained largely unexplained. Contrary to the speculation of previous literature, we argue that the formation of the magnetic anomaly is due not only to the particle blocking but also to its correlation with the the carrier fluid freezing effects.It is also shown that the nature of these peaks is strongly affected by varying particle size and carrier fluid medium. Quantitative fits of the frequency dependent AC susceptibility to the Vogel-Fulcher scaling law clearly indicate that the blocking of magnetic nanoparticles in the frozen state significantly affects the interparticle dipole-dipole interaction, causing characteristic spin-glass-like dynamics. A clear correlation between the blocking and freezing temperatures emerges from our studies for the first time.
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Thesis (M.S.)--University of South Florida, 2009.
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by Marienette B. Morales.
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MagnetizationDynamicsandInterparticleInteractionsinFerrouidsandNanostructuresbyMarienetteB.MoralesAthesissubmittedinpartialfulllmentoftherequirementsforthedegreeofMasterofScienceDepartmentofPhysicsCollegeofArtsandSciencesUniversityofSouthFloridaMajorProfessor:HariharanSrikanth,Ph.D.DaleJohnson,Ph.D.MartinMu~noz,Ph.D.GarrettMatthews,Ph.D.DateofApproval:June9,2009Keywords:magneticcolloids,nanoparticles,transversesusceptibility,relaxationphenomena,superparamagnetismcCopyright2009,MarienetteB.Morales

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DEDICATIONTomyparents,forgettingmestartedToIanandBabyJanna,forgivingmetheinspirationtonish

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ACKNOWLEDGMENTSFirstofall,Ioermysincerestgratitudetomysupervisor,Dr.HariharanSrikanth,forgivingmetheopportunitytoworkintheFunctionalMaterialsLab.Hehasalwaysbelievedinmycapabilitiesrightfromthebeginning.Hisunwaveringcondenceinmyabilitieshasledmetoextraordinaryexperiencesthroughoutthework.HiskindnessandgenerosityextendtomyfamilyaswellandforthisIamtrulygrateful.IamthankfultoDr.PritishMukherjee,departmentdirectorandheadoftheCenterofIntergatedFunctionalMaterialsCIFM,forthe2-yearresearchassistantship.Thankyoutomygraduatecommittee,Dr.DaleJohnson,Dr.GarrettMatthewsandDr.MartinMu~noz,forreviewingmythesis.Thankyouthatinthemidstofalltheiractivity,theyacceptedtobepanelmembersformydefense.Manythanksgotoourformerpostdocs,SrinathSanyadanamandPankajPoddar.SrinathtrainedmeontheinnerworkingsandoperationofthePPMS,whilePankaj,despitethebrieftimethatweworkedtogether,hasmadeatremendousimpactinthisthesis.Igratefullyacknowledgeourcurrentpostdoc,Dr.Manh-HuongPhanforhisguidanceandcrucialcontribution,whichmadehimabackboneofthisresearchandsotothisthesis.Ourotherpostdoc,Dr.SusmitaPal,whohasrevolutionizedthewaywedochemistryatFML,isworthyofaspecialtributeforhervaluablecontributiontotheprogressofmyresearch.Ihavebeenblessedwithacompanyoffriendlyandcheerfulco-workersandlabmates.SpecialthanksgoinparticulartoDr.NatalieFreyHuls,forteachingmeessentiallyeverythingIknowabouttransversesusceptibilitymeasurementandforbeingaverygoodfriendovertheyears.ThankyoutoDrewRebarandJamesGassforhelpingmegetontheroadtochemicalsynthesis.IacknowledgeMelodyMinerforthemostmeaningfuldiscussionsandforbeingagreatsailingpartner.Collectiveandindividualacknowledgmentsarealsoowedtomycurrentlabmates,whohavebecomemygoodfriends,AnuragChaturvedi,SayanChandra,NicholasBingham,andKristenStojak.Theywereveryreliableineverypossiblewayduringheluimrunsandwereabighelpbothpersonallyandprofessionally.Manythankstothemforcreatingsuchagreatatmosphereinthelab.

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Iam,asever,especiallyindebtedtomyparentsfortheiruninchingencouragementandlove,evenfromsomilesaway.Mostofall,thankyoutomylovinghusband,MichaelFrancisIanVega,forbeingthesourceofmystrengthandhappinessalltheseyearsandforbeingsuchagreatfathertoJanna.Finally,thankyoutobabyMalinJannaVega,whosemereexistencedelightsmetonoend,forbeinganinspirationtome,andforthatsweetsmilegreetingmeeveryday.iv

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TABLEOFCONTENTSLISTOFTABLESiiiLISTOFFIGURESivLISTOFABBREVIATIONSviABSTRACTviiiCHAPTER1INTRODUCTION11.1PropertiesofFerrouids31.2ApplicationsofFerrouids81.3MagneticPropertiesofNanoparticles10CHAPTER2SYNTHESISANDSTRUCTURALCHARACTERIZATIONOFNANOPARTICLESANDFERROFLUIDS152.1Chemicalco-precipitationofIronOxideFerrouids152.2SynthesisofCobaltFerriteCoFe2O4202.3StructuralCharacterization212.4SynthesisofManganeseZincFerriteMZFO22CHAPTER3MAGNETICMEASUREMENTTECHNIQUES253.1DCMagnetization253.2ACSusceptibility293.3RFSusceptibility31CHAPTER4STATICANDDYNAMICMAGNETIZATIONOFNANOPARTICLES364.1ExperimentalDetails374.2DCCharacterizations384.3TransverseSusceptibilityTSasaTooltoProbeInterparticleInteractions39CHAPTER5MAGNETICPROPERTIESOFFERROFLUIDS485.1FerrouidSamples505.2AnomalyinDCMagnetization505.3OriginofSpinGlass-likeRelaxationPeaks555.4FitstoNeel-ArrheniusandVogel-FulcherRelations575.5FrequencyDependenceoftheACSusceptibility66i

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CHAPTER6CONCLUSIONSANDFUTUREWORK68REFERENCES71APPENDICES77AppendixAListofPublications78AppendixBListofConferencePresentations80ii

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LISTOFTABLESTable2.1EectofamountofsurfactantorsolventvolumeonthesizeofFe3O4particles19Table4.1SummaryofresultsoftheDCmagnetizationforallMZFOsamples40Table5.1Besttvaluesofo,Ea=kandTofromVFrelationforFe3O4indodecaneFD60Table5.2Besttvaluesofo,Ea=kandTofromVFrelationforCoFe2O4ferrouids63Table5.3Besttvaluesofo,Ea=kandTofromNeel-Arrheniusrelationfor6nmFe3O4samples65Table5.4SummaryoftheZFCandACresultsforallferrouidsamples66iii

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LISTOFFIGURESFigure1.1Ferrouidunderamagneticeldofahand-heldmagnet2Figure1.2Coatedmagneticparticlesinaferrouid5Figure1.3Aferrouidinamagneticeldshowingnormal-eldinstability6Figure1.4Schematicdiagramofmagnetoviscouseect6Figure1.5Particleandspinmotioninaferrouidasdescribedbytheeggmodel7Figure2.1Three-neckedreactionvesselwithreuxcondenser17Figure2.2Schematicdiagramofchemicalco-precipitation18Figure2.3Co-precipitationofFe3O4usingScheme219Figure2.4Powderx-raydiractionproleof14nmFe3O4.21Figure2.5Powderx-raydiractionproleof6nmFe3O4.22Figure2.6Powderx-raydiractionproleof11nmCoFe2O4.23Figure2.7Transmissionelectronmicrographof11nmCoFe2O4.23Figure3.1PhysicalPropertiesMeasurementSystemPPMS26Figure3.2Magnetizationversuseldcurveofaferromagneticmaterial27Figure3.3ZeroeldcooledZFCandeldcooledFCmagnetizationversustemperaturecurvesforFe3O4nanoparticles28Figure3.4TheoreticaltransversesusceptibilityTandparallelsusceptibilityPcurves32Figure3.5SchematicdiagramoftheTDOcircuit34Figure4.1ZFC-FCcurvesofallMZFOsamples39Figure4.2MagnetizationvseldcurvesofallMZFOsamplesat300K41Figure4.3MagnetizationvseldcurvesofallMZFOsamplesat10K42iv

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Figure4.4TransversesusceptibilitycurvesforMZFOconcsample44Figure4.5TransversesusceptibilitycurvesforMZFOsample45Figure4.6ComparativeviewofTScurvesatvariousconcentrationsandtemperatures46Figure4.7PeakpositionHKversustemperaturecurvesinboththepositiveandnegativeeldforvariousconcentrations47Figure4.8Anisotropypeakheightdierenceversustemperaturecurvesatvariousconcentrations47Figure5.1MagnetizationversuseldcurveforFe3O4nanoparticlesat10Kand300K51Figure5.2MagnetizationversuseldcurveforCoFe2O4nanoparticlesat10Kand300K51Figure5.3ZFC-FCcurvesfor14nmFe3O4samples52Figure5.4ZFC-FCcurvesfor11nmCoFe2O4samples53Figure5.5ZFC-FCcurvesfor6nmFe3O4samples56Figure5.6ACsusceptibilitydatafor14nmFe3O4+hexanesampleFH57Figure5.7ACsusceptibilitydatafor14nmFe3O4+dodecanesampleFD58Figure5.8ACsusceptibilitydataforCoFe2O4+hexanesampleCH59Figure5.9ACsusceptibilitydataforCoFe2O4+dodecanesampleCD60Figure5.10ACsusceptibilitydatafornmCoFe2O4+dodecanesampleFD61Figure5.11TemperaturedependenceofZFCmagnetizationand00TofSampleFDFe3O4+dodecane62Figure5.12ThebesttsofTpourdatatotheVFmodelforSampleFD63Figure5.13ThebesttsofTpourdatatotheNeel-Arrheniusmodelfor6nmFe3O4.64Figure5.140TasafunctionoffrequencyofSamplesFHandCD67v

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LISTOFABBREVIATIONSACAlternatingCurrentCoFe2O4CobaltferriteDCDirectCurrentemuMagneticMomentUnitElectromagneticUnitFe3O4Magnetite/IronoxideHMagneticFieldHKAnisotropyFieldHCCoerciveFieldCoercivityJJouleKKelvinMDC-MagnetizationMRRemanentMagnetizationMSSaturationMagnetizationM-HMagnetizationvs.AppliedMagneticFieldMZFOManganesezincferriteNANeel-ArrheniusOeOerstedPPMSPhysicalPropertiesMeasurementSystemRFRadio-FrequencyssecondTTeslaTBBlockingtemperaturevi

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TFFreezingtemperatureTEMTransmissionElectronMicroscopeTSTransverseSusceptibilityVFVogel-FulcherXRDX-RayDiractionvii

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MAGNETIZATIONDYNAMICSANDINTERPARTICLEINTERACTIONSINFERROFLUIDSANDNANOSTRUCTURESMarienetteB.MoralesABSTRACTNanoparticleassembliesareofcurrentinterestastheyareusedinawidevarietyofin-dustrialandbiomedicalapplications.Thisworkpresentstwostudiesaimedatunderstand-ingthemagnetizationdynamicsandinterparticleinteractionsinnanoparticleassembliesandvarioustypesofferrouids.First,westudiedtheinuenceofvaryingstrengthsofdipolarinteractiononthestaticanddynamicmagneticpropertiesofsurfactant-coatedmonodispersedmanganese-zincfer-ritenanoparticlesusingreversibletransversesusceptibility.Wetrackedtheevolutionoftheanisotropypeakswithvaryingmagneticeld,temperature,andinteractionstrength.Theanisotropypeaksofweaklyinteractingparticlesappearsasnon-symmetricpeaksandatlowereldsinaunipolartransversesusceptibilityscan.Ontheotherhand,astronglyinteractingparticlesystemexhibitssymmetricanisotropypeakssituatedathighereldvalues.Inthesecondstudy,wesuccessfullysynthesizedstableferrouidsoutofhighqualityFe3O4andCoFe2O4nanoparticles.Suchferrouidsareexcellentsystemsfortheinvesti-gationofphysicsofrelaxationphenomenainmagneticnanoparticles.Motivatedbytheneedtounderstandtheirpeculiarmagneticresponse,acomparativestudyonFe3O4-andCoFe2O4-basedferrouidswasperformed.Weinvestigatedcasesinwhichparticleblock-ingandcarrieruidfreezingtemperatureswerecloseandfarapartfromeachother.Ourexperimentalresultsrevealthetrueoriginoftheglass-likerelaxationpeaksthathavebeenviii

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widelyobservedinferrouidsbymanygroupsbutremainedlargelyunexplained.Con-trarytothespeculationofpreviousliterature,wearguethattheformationofthemagneticanomalyisduenotonlytotheparticleblockingbutalsotoitscorrelationwiththethecarrieruidfreezingeects.Itisalsoshownthatthenatureofthesepeaksisstronglyaectedbyvaryingparticlesizeandcarrieruidmedium.Quantitativetsofthefre-quencydependentACsusceptibilitytotheVogel-Fulcherscalinglawclearlyindicatethattheblockingofmagneticnanoparticlesinthefrozenstatesignicantlyaectstheinter-particledipole-dipoleinteraction,causingcharacteristicspin-glass-likedynamics.Aclearcorrelationbetweentheblockingandfreezingtemperaturesemergesfromourstudiesforthersttime.ix

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CHAPTER1INTRODUCTIONFerrouidsarestablecolloidaldispersionsofsingledomainmagneticnanoparticlesinaliquidmedium.Earlystudiesofferrouidsincludethosesuspensionsthatsettleoutveryslowly[1].However,atrueferrouiddoesnotsettleoutcompletely,althoughaslightconcentrationgradientmaybuildupafterbeingsubjectedtogravitationalormagneticeldoverlongperiodsoftime.AnaspectofthisbehaviorisillustratedinFigure1.1.Suchferrouidsarecomposedofsolid,magnetic,single-domainparticlesofabout3-15nmindiameter.Theparticlesareoftencoatedwithanorganiclayer,whichactsasadispersantinthecarrierliquid.ThermalagitationofthesolidparticleskeepthemsuspendedintheliquidthroughBrownianmotionandthemolecularcoatingpreventsthemfromagglomerating.Theearliestemergenceofferrouidswasintheformofmagneticuids",whichwereintroducedinthe1940s.Backthen,micron-sizedironparticlesweremixedwithclutchandbrakeoilforrheologicalinvestigations.Assuch,thesewereverydierentfrompresent-daycommercialferrouidsmadeofnanometer-sizedparticlesthatremainhomogeneouslysuspendedintheliquid.Intheearly60s,thestudyofferrouidssteadilyrose,fueledbythegrowingdemandforaliquidmaterialthatcanbestronglyinuencedbymoderatemagneticelds.Suchamaterial,enablingthecontrolofitsowandphysicalpropertiesoverawiderangebymeansofacontrollablemagneticforce,wasexpectedtogiverisetonumerousnewapplicationslikesealantsinrotatingshafts,voice-coildampersinloudspeakers,hyperthermicagentsinbiomedicine,etc.Amoredetaileddiscussionofapplicationsisprovidedbelow.Theprimarychallengeisthatmagneticliquidsdonotexistinnature.Allknownfer-romagnetshaveCurietemperaturesfarbelowtheirmeltingpoint.Hence,theylosetheir1

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Figure1.1.Ferrouidinatransparentvial,shapedandpositionedbyaneodymiummagnetheldoutsidethevialferromagneticpropertiesbeforeturningliquid.AnexceptiontothisistheundercooledmeltsofCo-Pdalloys.Butwhilethiswasfoundtoshowamagneticphasetransition[2],itisofnotechnicalimportanceintermsofmagnetic-eld-controlledowsandrelatedap-plications.Therefore,acompletelynewclassofmaterialshadtobedevelopedtomeetthenecessitiesoftheprojecteduseofamagneticuid.ThenalbreakthroughwasmadebyS.Pappell'ssuccessinproducingstablesuspensionsofmagneticnanoparticlesincarrierliquids[3].Thesesuspensionsshowedliquidbehavioraswellassuperparamagneticprop-erties.Thismeantthatminimalmagneticelds,whicharecomparabletogravitationalforces,canaecttheliquid.Thesesuspensionseventuallycametobecalledferrouids.Shortlyafter,theimprovedsynthesismethodsforferrouidsmotivatedthedevelopmentofuidsexhibitinglong-timecolloidalstabilityandreproducibleproperties.Astimewentby,concurrenttotheimprovementsintheliquidsthemselveswasthediscoveryofnumerousotherpracticalapplicationsforferrouids,someofwhichhavegainedhighcommercialimportance.Ferrouidsarenowwell-establishedasanimportantclassofmaterialsfor2

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industrialapplications.Andthisinitselfwarrantscloserscrutinyofitsphysicalproper-ties.Theprospectofbiomedicalapplicationsforferrouidshasfurtherintensiedthisneedtounderstandtheirphysicsatafundamentallevel.Theseconsiderationsconstitutetheunderlyingmotivationsofthepresentwork.1.1PropertiesofFerrouidsStabilityasauniformsuspensionisanimportantpropertyofferrouidsthatmostcom-mercialapplicationsrequire.Thisstabilityisdictatedmainlybythesizeoftheparticles,whichneedtobesucientlysmalltopreventprecipitationduetogravity.Chargeandsurfacechemistryalsocontributetostabilitybyprovidingcoulombicandstericrepulsion.Thecriticalsizetomaintainstabilityisestimatedbyconsideringtheenergiesinvolvedperparticle,namely,thermalenergy=kBT.1magneticenergy=oMHV.2gravitationalenergy=VgL.3dipole-dipolecontactenergy=oM2V.4wherekBisBoltzmann'sconstant,oisthepermeabilityoffreespace,volumeV=d3=6forasphericalparticleofdiameterd,isthedensitydierencebetweenparticlesandcarrierliquidandListheelevationinthegravitationaleldorthetypicalheightofacontainer.Theratiobetweenthethermalenergyandmagneticenergygivesthefavorablesizeforstabilityagainstsegregation:thermalenergy magneticenergy=gL oMH.5d6kT oMH1=3.63

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Interparticleinteractionispresentinacollectionofsmallmagneticnanoparticles.Throughthermalagitation,agglomerationamongparticlesispreventedaslongastheparticlesizeissucientlysmall.Thisbehaviorisgovernedbythemagneticenergytogravitationalenergyratio.ThenalrequirementoncolloidalstabilitythatneedstobeaddressedisthestabilityagainstvanderWaalsattraction.Theseparationofbaremagneticparticlesinacarrierliquidcannotbeguaranteed,sinceagglomerationduetovanderWaalsattractionwilloccurassoonasparticlescomeintocontact.Toavoidirreversibleagglomerationoftheparticles,theyhavetobepreventedfromcomingintocontact.Asmentionedearlier,thisisgenerallydonebymeansofasurfactantlayerconsistingoflongchainmoleculeswithapolarheadandanonpolartail[4].TheschematicdiagramofacoatednanoparticleisshowninFigure1.2.Thepolarheadisattachedtotheparticle'smagneticcore,whilethetailreachesintothecarrierliquid.Whenaparamagneticuidissubjectedtoasucientlystrongverticalmagneticeld,thesurfaceexhibitsanormal-eldinstability.Thiseectismademanifestinthespon-taneousformationofregularpatternsofcorrugations,asillustratedinFigure1.3.Theformationofthecorrugationsincreasesthesurfacefreeenergyandthegravitationalen-ergyoftheliquid,butreducesthemagneticenergy.Thecorrugationswillonlyformaboveacriticalmagneticeldstrength,whenthereductioninmagneticenergyoutweighstheincreaseinsurfaceandgravitationenergyterms.Ferrouidshaveanexceptionallyhighmagneticsusceptibilityandthecriticalmagneticeldfortheonsetofthecorrugationscanbereleasedbyasmallbarmagnet.Withaxedmagneticeld,theconcentrationoftheferrouidmaybevariedsuchthatthiseectbecomesvisible.Themostprominentpropertyofferrouidsisthechangeofviscosityduetoanappliedmagneticeld.TherstdiscoveryofsuchchangeswasmadebyRosensweigin1969[5]inconcentratedmagnetite-ferrouidsfollowedbytheindependentworkofMcTague[6]usinghighlydilutedCo-ferrouids.BothpapersdescribedanincreaseofviscosityofferrouidswithincreasingmagneticeldstrengthandMcTague'sexperimentsalsoshowedadependenceoftheeectontherelativeanglebetweenthemagneticelddirectionand4

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Figure1.2.Schematicrepresentationofthecoatedmagneticparticlesinaferrouid.thevorticityoftheow.ThersttheoreticalinvestigationofthephenomenawasdonebyShliomis[7]in1972.Itisassumedthatthemagneticmomentisxedintheparticle,i.e.themagneticrelaxationisgovernedbytheBrownianrelaxation.Ifamagneticeldisappliedtothesystem,themagneticmomentsoftheparticleswilltendtoalignwiththemagneticelddirection.Ifthemomentisxedinsidetheparticleasassumedabovetherotationcausedbytheviscousfrictionwillleadtoadisalignmentofmagneticmomentandmagneticeld.Ifthemagneticeldisperpendiculartothevorticityoftheow,thiswillcreateamagnetictorquethatwillcounteractthemechanicalorviscoustorqueseeFigure1.4[8].Thismagnetictorquehindersthefreerotationoftheparticles,andproducesanincreaseoftheuid'sviscosity.Thisincreaseisanisotropic,sinceitdependsonthe5

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Figure1.3.Aferrouidinamagneticeldshowingnormal-eldinstabilitycausedbyaneodymiummagnetbeneaththedishmutualorientationofvorticityandmagneticeld.Conversely,iftheuid'svorticityisalignedwiththeeld,therotationoftheparticlewillnotforceadisalignmentofmagneticmomentandelddirection,andthusnomagnetictorquewillcounteractthefreerotationoftheparticles.Thereforenoincreaseofviscositywillbeobserved. Figure1.4.Schematicdiagramillustratingtheoriginoftheelddependentincreaseofviscosityinaferrouid.Themovementofmagneticmomentandparticlesinaferrouidmaybedescribedbytheeggmodel[9].Themagneticnanoparticleisthewholeeggwhosemagneticmomentissituatedattheyolk.Abovethefreezingtemperature,theferrouidisintheliquid6

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Figure1.5.Themovementofmagneticmomentandparticlesinaferrouidmaybede-scribedbytheeggmodel.stateandtheparticlesareabletophysicallyrotateinadditiontotheinternalrotationofthespinsinsidethemagneticcore.ThegreenarrowinFigure1.5showstherotationoftheeggshellwiththesurroundinguidofviscositywithangularvelocity,!.ThisischaracterizedbytherotationaldiusionrelaxationtimeortheBrownianrelaxationtimegivenbyB=Dhyd 2kBT;.7whereDhydisthehydrodynamicvolumeoftheparticles,kBtheBoltzmannsconstantandTisthetemperature.Fromtheexpressionofrelaxationtime,itisseenthattheBrownianmechanismstronglydependsontheviscosityofthecarrierliquid.Inthefrozenstate,however,particlesarephysicallyimmobilized,whichwouldideallyeliminateallcontributionsfromBrownianrelaxation.Themagneticmomentsmaystillundergointernalrotationswithrespecttothecrystalaxes.Theyellowarrowrepresents7

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therotationofyolkinthewhitewithrespecttotheeggshellwithangularvelocity,!R.ThisrelaxesthroughtheNeelmechanismgivenbyEquation1.8.N=oexpKVmag kBT;.8where)]TJ/F19 7.97 Tf 6.587 0 Td[(1oistheLarmorfrequencyofthemagnetization.KistheeectiveanisotropyandVmagisthevolumeofthemagneticcore,whichcollectivelydenestheactivationenergyalsodenotedasEainthelatterchapters.TheNeelmechanismstronglydependsonthevolumeofthemagneticcore.1.2ApplicationsofFerrouidsWhenmagneticnanoparticlesaredispersedinacarrierliquid,itformsamagneticliquidorferrouidsthatareoutstandingforseveraltechnologicalapplications.Manyapplicationsarebasedonthespecialpropertyofferrouidsthattheyareattractedbymagneticeldgradientswhilepreservingtheirliquidcharacter.Ferrouidshavecapacitytowithstandrelativelyhighpressureandmagneticeldgradients,hence,theyarecommerciallybeingutilizedassealantforrotatingshaftsandasdampersinloudspeakers.Loudspeakercoils[10]consistofacylindricalvoicecoilttedtoacylindricalpermanentmagnetwithasmallgapthatallowsthecoiltomove.Theheatdevelopedinthevoicecoilcanbedissipatedrapidlywhentheairinthegapisreplacedbyaliquid.Anordinaryliquidisunstableandwoulddripout,butaferrouidisretainedbythemagneticeldalreadypresent.Ferrouidsarealsousedincomputerharddiskdrives,forpreventionagainstcontamination[11]andasmagneticinksforjetprinting[12],whicharesubjectedtomagneticeldgradientstodeectandpositiontheinkdropletsonthepaper.Theonlydisadvantageisthatittakeslongertimetodryonthepaper.Formagneticreadingoflettersandnumbers,e.g.oncheques,themagneticpropertiesofthedriedinkareofoutstandingimportance.Thepaperisinitiallysubjectedtoamagneticeld,beforeitisfedintothemagneticreader,sothedriedinkmusthaveasuitablyhighremanencefortheletterstoberead.Nowadays,8

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researchisperformedtoinuencethecolorofmagneticinksbybuildinguorescentdyesintotheparticles[11].Anotherattractiveaspectofferrouidresearcharisesfromthefactthattheyhavebeenshowntoexhibitinterestingcoupledmagneto-mechanicaleectswhensubjecttodierentowconditionsandmagneticelds.Thispropertyhasimplicationsfornovelenergyconversionandpowergeneration[13{17].Thebasisforpowergenerationarisesfromthewell-knownFaradayinductionprocesswhichstatesthatanemfisgeneratedbytimevaryingmagneticux.Inferrouids,onecanthinkofthesuspendednanoparticlesassuper-spinswithhighmagneticmoments.Timevariationofthesemomentscausedbyphysicalmotionoftheuidwiththeparticlescombinedwithcollectiveinteractionamongtheparticleswouldcauseaninducedemfinexternalpickupcoilsthusactingasapowersource[18{20].IntheworkofGazeauetal.[13],theysimultaneouslymeasuredtherotationalviscosityandtransversesusceptibilityanddependingontheratiooftheeldfrequencytotheuidvorticity,theparticlesbehaveasnano-motorsornano-generators.Inaddition,ferrouidsthataremadetoowthroughcapacitivemembranessuchasnanotubeswouldalsobeusefulforenergyconversion.Yangetal.[21]discussedtheeciencyofmicrochannelarraysforelectrokineticbatteryapplications.Withtheadventofnewsynthesistechniques,itisnowpossibletomakewater-basedmagneticsuspensionsthatareimportantforbiomedicalapplications.Theyareparticu-larlyusefulfordrugdelivery,hyperthermiaandcontrastenhancementinMRIimaging.Ferrouidshavebeenshowntoexhibitanincrediblyhighspecicabsorptionratiothatisexcellentforcancertherapythroughhyperthermia[22,23].Heatingbetween41Cand46CcanbeinducedbyanACmagneticeldatfrequenciesupto120kHz,whichistolerabletohumans.Ithasanadvantageoverheatingofdriednanoparticlessincetheliquidmediumfacilitateshomogeneousheatingofthetargettissue.Otherpotentialapplicationsofferrouidsaretheiruseasopticaldevices,suchasshut-ters,exploitingthechangeintransmittanceoflightuponalignmentoftheparticlesinamagneticeld.Here,smallmagneticparticlesareneededtodecreasetheresponsetimeof9

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theshutter.Arelativelynewpotentialapplicationistheuseofalatticeofself-assembledmonodispersemagneticcolloidsincomputersfordatastorage[24,25].However,newmethodsforwritingandreadingofdatahavetobedevelopedrst,tobeabletousethisinpractice.Anotherveryinterestingapplicationwouldbetheuseofmagneticcolloidsattachedtocatalysts,whichcouldbeseparatedaftertheirusebymeansofmagneticeldgradients.Thisapplicationwouldthencombinetheadvantagesofhomogeneouscataly-sishigheciencyduetothelargeavailablesurfaceandheterogeneouscatalysiseasyrecyclingofthecatalyst.1.3MagneticPropertiesofNanoparticlesTheunderstandingofthemagneticphenomenaofvariousferrouidsrequiressomeknowledgeofthenanoparticlesthatcomprisethemagneticliquid.Magneticnanoparticlesshowremarkablepropertiessuchassuperparamagnetism,higheldirreversibility,highsaturationeld.Theseareduetonitesizeandsurfaceeects[26].Thetwomoststudiednitesizeeectsarethesingledomainlimitandthesuperparamagneticlimit.Itiswell-knownthatlargermagneticmaterialsarecomposedofuniformlymagnetizedregionscalleddomainsseparatedbydomainwalls.Domainwallformationisgovernedbythebalancebetweenthemagnetostaticenergy,whichisproportionaltothevolumeoftheparticleanddomain-wallenergy,whichisproportionaltotheinterfaceareasbetweendomains.However,aspredictedbyFrenkelandDorfman[27],forparticlesofaferromagnetbelowacriticaldiameter,usuallylessthan15nm,formationofdomainsisnotenergeticallyfavorable.Suchparticlesaresaidtobesingledomain.Asingle-domainparticleisuniformlymagnetizedwithallthespinsalignedinthesamedirectionandthemechanismforthisisbyspinrotationsincetherearenodomainwallstomove.Anotherimportantphenomenonknownassuperparamagnetismispossibleforthesesingle-domainparticles.Thisoccurswhenthethermalenergyisenoughtodemagnetizetheparticleintheabsenceofanappliedeld.Foranarrayofparticles,itmeansthatthenetmomentassociatedwitheachparticleeasilyalignswithanappliedeldbutisfree10

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torotateoncetheeldisremoved.Thissituationisanalogoustoaparamagnet,onlyinsteadofeachindividualspinaligningwiththeeldandthenrandomizingaftertheeldisremoved,thereareparticlescomposedofroughly105spinsthatcanalignwiththeeld.Thisresultsinamuchhighersusceptibilityandbettermagneticresponsethanatraditionalparamagnet,butthereisnocoercivityorremanentmagnetization.Tobetterunderstandthis,weneedtodeneanotherpropertycalledthemagneticanisotropyenergy.Themagneticanisotropyenergyperparticleistheenergyrequiredforholdingthemagneticmomentsalongacertaindirection.ThisisgivenbyE=KeffVsin2;.9whereVistheparticlevolume,Keffistheanisotropyconstantandistheanglebetweentheappliedeldandtheeasyaxis.Thecompetitionbetweenthethermalenergy,kBTandtheenergybarrier,KeffVdeterminesthestateofthespins.Asthesizeoftheparticlesdecreases,thethermalenergydominatesandthemagnetizationiseasilyipped.Thissystemiscalledasuperparamagnetsincethereisnowagiantspininsideeachoftheparticle.ThemagneticmomentrelaxesaccordingtotheNeel-BrownexpressioninEquation.10:=oexpKeffV kBT;.10Iftheparticlemagneticmomentipsatatimeshorterthanthemeasurementtimescales,thesystemisinthesuperparamagneticstate.Ingeneral,magneticnanoparticlescanexhibitsuperparamagnetismonlywhenthetemperatureishighenoughtocausede-magnetizationbyovercomingtheanisotropyenergyintheabsenceofaeld.Whenthetemperatureisloweredandthethermalenergyisnotenoughtodemagnetizetheparticles,theyagainbehaveassingledomainparticleswithmagnetichysteresis.Belowthiscriticaltemperature,themagneticmomentreversaltimeislongerthanthemeasurementtime,andthesystemissaidtobeintheblockedstate.Thetemperaturethatseparatesthetworegimesistheblockingtemperature,TBanditdependsontheparticle'sintrinsicproperties11

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andsize.AsimpleandrapidwaytoestimatetheTBisprovidedbytheDCmagnetizationmeasurement,inwhichazero-eldcooledandeld-cooledprocedureisemployed.ThisprocedureisdiscussedinSection3.1andisfurtherexplainedinSection5.2.Theconditionsgivenaboveforsuperparamagnetismallassumethattheparticlesde-magnetizeduetothermalenergysimultaneouslywiththeremovalofaeld,andonthetimescaleofmostDCmeasurementsthisappearstobetrue.However,thereisanitetimescalefortheparticlestodemagnetize,andwhenthemagneticpropertiesareprobedwithanACeld,theblockingtemperaturecanincrease.Thusmeasurementfrequencyisalsoafactorwhendescribingtheblockingbehaviorofasuperparamagneticsystem.ACsusceptibilitymeasurementsareespeciallyusefulforcharacterizingthetimescalesinvolvedinsuperparamagneticnanoparticles.AsoriginallytheorizedbyNeelandBrown[28],theparticlesareassumedtobenoninteractingandtheblockingtemperatureisgivenbyTB=E ln=okB.11whereEistheenergybarriertomagnetizationreversalinasingleparticle,isthemeasurementtimeandkBistheBoltzmannconstant.1/oiscalledtheattemptfrequencyanddescribeshowfastaparticlereversesitsmagnetization.Typically,thisvaluesfallintherange10)]TJ/F19 7.97 Tf 6.586 0 Td[(10)]TJ/F15 10.909 Tf 11.38 0 Td[(10)]TJ/F19 7.97 Tf 6.587 0 Td[(9s.RearrangingtheequationaboveyieldsthetypicalformoftheNeel-Arrheniusrelation:=oexpEa kT:.12Forweaklyinteractingparticles,thesystemsbehavesaccordingtoVogel-Fulcherlaw:=oexpEa k1 Tp)]TJ/F21 10.909 Tf 10.909 0 Td[(To;.13whereistherelaxationtime=1/f;fisthefrequency,oisthemicroscopicippingtimeoftheuctuatingspins,Eaisthethermalactivationenergy,Tisthetemperature,12

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andToisthecharacteristictemperaturewiththermalenergydominatingforT>ToandinteractionenergyforT
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works,areelucidated.Lastly,Chapter6concludesthethesisandproposesrecommenda-tionsforfuturework.Ourobservationsandresultsdocumentedindetailinthisthesishavealsobeenpresentedinthefollowingpublicationsoverthepastfewyears[29,30].14

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CHAPTER2SYNTHESISANDSTRUCTURALCHARACTERIZATIONOFNANOPARTICLESANDFERROFLUIDSThemethodofsynthesizingferrouidsconsistsofipreparationofmagneticnanoparti-clesandiithesubsequentstabilizationofthesurfactant-coatednanoparticlesindierentnonpolarsolvents.Awidevarietyofcompositionsandphasesofmagneticnanoparticles,includingironoxidessuchasFe3O4and-Fe2O3puremetalssuchasFeandCo,spinel-typeferritessuchasCoFe2O4,MnFe2O4,andMnxZn1)]TJ/F22 7.97 Tf 6.586 0 Td[(xFe2O4,havebeensynthesizedforseveraldecadesnow.Withtheadventofnewandecientsyntheticroutes,itisnowpossibletofabricateshape-controlled,highlystableandmonodispersemagneticnanopar-ticles.Someofthepopularmethodsincludingco-precipitation,microemulsionsynthesis,andhydrothermalsynthesiscanallbeemployedtoyieldhigh-qualitymagneticnanoparti-cles.SincethisworkislargelyfocusedontheFe3O4system,chemicalco-precipitationofFe3O4isdiscussedindetailinsection2.1.Othersynthesismethodsaredescribedbrieyinthefollowingsections.Forcompleteness,theprocedureusedbyourcollaboratorsforsynthesizingmanganesezincferriteisalsooutlinedinSection2.4.Uponproductionoftheferrouids,physicalandstructuralcharacterizationsareper-formedtoverifythecomposition,shapeandsizeofthenanoparticles.ResultsoftheXRDscansandTEMimagesarealsoreportedinthischapter.2.1Chemicalco-precipitationofIronOxideFerrouidsChemicalco-precipitationisafacileandconvenientrouteforsynthesizingFe3O4fromaqueousFe2+andFe3+saltsolutionsbyadditionofabaseatelevatedtemperature.Theentireprocedureisperformedunderinertgasenvironment.Therearetwomainmethods15

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toprecipitatepredominantlysphericalmagnetiteparticlesinsolution.TherstprocesswasdevelopedbyKhalafallaetal.[31]andisoutlinedbelow.Scheme1.Co-precipitationofironsalts.TheoverallstoichiometryofthismethodisgiveninEquation2.1.5NaOH+2FeCl3+FeCl2=FeOFe2O3+5NaCl+4H2O.1Inthisreaction,Fe3+andFe2+arein2:1molarratio.Sodiumhydroxidemaybereplacedbyotherbasicsolventsuchasammoniumhydroxide.TheprocedurewascarriedoutbydissolvinginareactionvesselFigure2.11.988gofFeCl24H20and5.406gofFeCl36H20in20mlwater.Asthemixturewasbeingsubjectedtovigorousmagneticstirringunderaowofnitrogen,itwasimmediatelyheatedto80C.Uponreaching80C,5mlofNH4OHwasaddeddropwise.Thisabruptlyturnsthesolutionblackincolor.Thesolutionwasallowedtoreuxfor2hoursandthencooleddown.Excessreactantsmaybediscardedtoextractthebarepowder.Theparticleswerefurthercleanedusingethanolfollowedbycentrifugationandmagneticdecantation.Atthispoint,theFe3O4nanoparticleshavealreadyformed.Thisrecipetypicallyyieldssphericalnanoparticleswithanaveragesizeof10to15nm.However,theas-synthesizedFe3O4atthisstageiseasilyoxidizedandbecomes-Fe2O3maghemiteuponexposuretoair.Inaddition,theresultingparticlesarenegativelychargedandtendtoagglomerateduetostrongdipole-dipoleinteraction.Forthepurposeofseveralapplications,formationoflargerclustersisunfavorableasthisdoesnotretaingooddispersioninhostmediaordesirablenanomagneticproperties.Itisnecessarytoperformanadditionalproceduretoprotectandstabilizemagneticnanoparticlesagainstagglomeration.Typically,anecientstrategytopassivatethesurfaceofthenanoparticlesistheuseofsurfactantorpolymercoatingandredispersioninorganicsolvents.Thisprocessiscalledpeptization.Peptizationresultsinstablecolloids.Theprocedureabovewasslightlymodiedsuchthat18goleicacidisaddedaftercoolingdownthesolution.Thecoatedparticleswerethenocculatedbyneutralizingthesolutionusinghydrochloricacid.Theblackproductsettlestothebottominaneutralenvironment.16

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Figure2.1.Three-neckedreactionvesselusedforferrouidssynthesis.Thelongreuxcondenserisconnectedthroughoneoftheinletstoconservethevolumeofthesolventduringreaction.Thisallowstheliquidtobeseparatedfromtheprecipitatethroughmagneticdecantation.Theparticleswerecleanedofexcesssurfactantsbyadditionofethanolandsubsequentcetrifugationat6000rpmfor5minutes.Therecoveredparticlesweredissolvedandstoredinhexane.AowchartoftheproceduredescribedaboveisillustratedinFigure2.2[32].Anadvantageofthisschemeisthatitisaneconomicalwaytoprepareironoxidenanoparticlesinreasonablylargequantities.However,thereislimitedcontroloverthesizeoftheparticlesandtheprocedureoftenyieldspolydispersenanoparticles.Scheme2.Seed-mediatedsynthesisofmonodispersedironoxidenanoparticles.Thesecondmethodismoreexibleintermsofsizetunability.Theoutcomeissmallerdiam-eternanoparticleswithnarrowsizedistribution.Incontrasttothepreviouslydescribedprocedure,thesurfactantisimmediatelyaddedtothemixtureandheatedathighertem-peraturesalongwiththeotherreagents.Thisstepencouragesmultipletinyclustersto17

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Figure2.2.Chemicalco-precipitationmethodforpreparingFe3O4ferrouidsnucleatesimultaneously.TherecipefollowscloselythemethoddescribedinRef.[33].Figure2.3showstheschematicrepresentationofScheme2co-precipitationofironoxidepreparation.Tosynthesize6nmsizeFe3O4,thefollowingreactantswerecombinedinthereactionvessel:FeIIIacetylacetonatemmoland1,2-hexadecanediol10mmolwereaddedto20mlbenzyletherwhilestirringvigorously.Theparticleswerecoatedwitholey-laminemmolandoleicacidmmol.Themixturewasheatedto2000Cfor2hoursunderargonenvironmentandthenallowedtoreuxfor1hourat3000C.Aftercoolingdowntoroomtemperature,theresultingblackmixturewaswashedbyadding40mlofethanol.Theblackprecipitatewasextractedaftercentrifugationandmagneticdecanta-tion.Thenanoparticleswerethendissolvedinhexanecontaining1dropeachofoleicacidandoleylaminetoensurethatnanoparticleswerefullycoated.Fe3O4nanoparticleswerestoredinhexaneliquid.18

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Topreparetheferrouids,measuredamountsofthedriedpowderobtainedbyevapo-rationofcarrierliquidwereredispersedinhexaneoranyotherorganicsolventofchoice. Figure2.3.Chemicalco-precipitationmethodforpreparingFe3O4ferrouidsTheparticlesizeisvariedbychangingtheamountsofsolventwhilekeepingalltheamountsofreagentsthesame.Ifthevolumeofbenzyletherisreducedto15ml,theresultingparticlesare8nminsize.Furtherreductioninsolventvolumeledto15nmFe3O4.Anotherfactorthataectsthesizeofparticlesduringsynthesisistheamountofsurfactantused.If4mmolofoleicacidand4mmolofoleylaminearemixedwith20mlbenzylethersolvent,theaverageparticlesizeis9nm.12mmololeicacidand12mmolofoleylaminein20mlbenzyletherledto5nm.Changingtheamountofbenzyletherto15mldoesnotseemtoaectthesizewhenonly4mmolofoleicacid/oleylaminewereused.Theeectofsolventreductionismoreevidentinthepresenceof6mmoland12mmololeicacid/oleylamine.TheseresultsaresummarizedinTable2.1. 4mmol6mmol12mmol 10mlNA15nm8nm15mlave.9nm8nm7nm20mlave.9nm6nm5nm Table2.1.RelationshipbetweenthesizeofFe3O4particlesinnmandamountofsurfac-tantinmmolorsolventvolumeinmlTheratioofmetaltosurfactantwaskeptat1:3asprescribedbytherecipe.Accordingtoliterature,thisparticularratiohasbeennotedtoyieldmonodispersenanoparticles.19

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2.2SynthesisofCobaltFerriteCoFe2O4SynthesisoflargerdiameterCoFe2O4iscomposedofatwo-stepprocessinvolvingseed-mediatedgrowthasdescribedinRef.[34].ThegeneralstrategyisusingcoordinationcompoundsofironIIIandcobaltIIacetylacetonate,Feacac3andCoacac2,aspre-cursorstosynthesizesphericalCoFe2O4nanocrystalswithameandiameterof5nm.Theresulting5nmnanoparticleswillbeutilizedasseedstogrowlargersphericalparticles.Theprocedureiscarriedoutasfollows:2mmolofCoacac2isdissolved40mlofphenylether.20mmolof1,2-hexadecanediol,10mlofoleicacid,and10mlofoleylaminewasheatedto156CunderAr+gasenvironmentandwithvigorousmagneticstirring.At156C,4mmolFeacac3dissolvedin20mlofaphenyletherwasaddeddropwisetothesolution.Thetemperaturewasthenincreasedquicklyto260C,whichistheboilingpointofphenylether.Themixturewasallowedreuxfor30minbeforebeingcooleddowntoroomtemperature.Tocleantheparticles,ethanolisaddedfollowedbycentrifugingat9000rpmfor5minutesfollowedby6000rpmforanother5minutes.ThisyieldedsphericalCoFe2O4withadiameterof5nm.Toproduce9nmCoFe2O4,100mgof5nmseedswasmixedwith1mmolofCoacac2,2mmolofFeacac3,10mmolof1-octadecanol,5mlofoleicacid,and5mlofoleylamine.Then,thesolutionwasquicklyheatedto260Catarateof10-15C/minandkeptatreuxat260Cfor30min.Forthesampleusedinthestudy,someinstabilitiesinthetemperaturecontrolmayhaveaectedthegrowth.Thetemperatureusuallydoesnotsettletotheactualsetpointandoftenneedstobemaintainedmanually.Asaresult,mostofthereactionmayhavebeencarriedoutinalowertemperature,whichencouragesformationoflargerparticles.Severaltrialsofthesynthesisprocessweredoneandwehaveconsistentlyproducedsizeslargerthan9nm.CoFe2O4ferritesynthesisexhibitsmoresensitivitytotemperaturegradientsthanFe3O4sincethesynthesisofthelatterusingthesametemperaturecontrollerseemstobeunaectedbythissetback.Structuralcharacterizationoftheprecipitatednanoparticlesusingthisprocedureisreportedinthenextsection.20

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2.3StructuralCharacterizationX-raypowderdiractionXRDpatternsoftheparticleassemblieswerecollectedonaBrukerD8AdvancediractometerunderCuKradiation=1:5405A.SamplesfortransmissionelectronmicroscopyTEManalysiswerepreparedbyevaporationofaverydilutehexaneferrouidonamorphouscarbon-coatedcoppergrids.TheparticleswereimagedusingaMorgagni280kVTEM.TheXRDpatternoftheFe3O4nanopowderspreparedusingScheme1isshowninFigure2.4.Fromtheplot,wecanseethattheindexedpeaksareconsistentwiththecubicspinelstructureofFe3O4.FromahistogramanalysisoftheTEMimageintheinsetofFigure2.4,theaverageparticlesizewasestimatedas143nm.TheTEMmicrographsshowsthatthenanoparticlesaresphericalinshapeandpolydispersed.Figure2.5showsthestructuralcharacterizationofFe3O4preparedusing Figure2.4.Powderx-raydiractionproleofthedriedferrouidindexedwiththehklreectionsofthecubicFe3O4phase.Upperleftpanel:selectedareafromTEMimagesofFe3O4nanoparticles.Upperrightpanel:histogramoftheparticlesizedpopulationsasobservedfromTEMimages.Scheme2.TheXRDpatternexhibitthediractionpeaksofthecubicspinelFe3O4asin21

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Figure2.5.Powderx-raydiractionproleofthedriedferrouidindexedwiththehklreectionsofthecubicFe3O4phase.Upperleftpanel:selectedareafromTEMimagesofFe3O4nanoparticles.Upperrightpanel:histogramoftheparticlesizedpopulationsasobservedfromTEMimages.Figure2.4.ThesizeoftheparticlesobtainedfromtheTEMimageinsetisfoundtobe6nm2nm.Thehistogramisalsopresentedinthesamegure.Thebroaddistributionisconsistentwiththepolydispersequalityofthesample.TheXRDpatternfortheCoFe2O4nanoparticlesisshowninFigure2.6.ThepeaksobservedareconsistentwiththespinelstructureofCoFe2O4.Sphericallyshapednanopar-ticleswereprecipitatedfromtheseed-mediatedgrowthdescribedintheprevioussectionasseenfromtheTEMmicrographinFigure2.7.Themeandiameteris11nm3nm.2.4SynthesisofManganeseZincFerriteMZFObyReverseMicelleMethodTheothernanoparticlesystempresentedinthisresearchweresynthesizedbycollabo-rators.ManganesezincferriteMZFO,wasreceivedfromS.MorrisonandE.Carpenter22

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Figure2.6.Powderx-raydiractionproleofthedriedferrouidindexedwiththehklreectionsofthespinelCoFe2O4phase. Figure2.7.Transmissionelectronmicrographof11nmsphericalCoFe2O4nanoparticles.fromtheVirginiaCommonwealthUniversity.MonodisperseMZFOnanoparticleswithanaveragesizeof15nmweresynthesizedusingaversatilereverse-micelletechnique.Tosynthesizebyreversemicellemethod,tworeversemicellarsolutionswereprepared,therstisthestocksolutionsof0.5MsodiumdioctylsulfosuccinateAOTpreparedin2,2,4-23

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trimethylpentaneisooctaneandthesecondwasanaqueousmetalsolutionpreparedbycombining0.045MFeCl2,0.0176MMnCl2,and0.0176MZnCl2.Thetwosolutionsarecombinedunderconstantstirring.Thereactionwasallowedtoproceedfor2h.Par-ticleocculationwasinducedbyadditionofexcessmethanol.Tocollecttheparticles,methanolwasaddedtothesolutionfollowedbycentrifugation.Thisstepremovesexcesssurfactant.Theparticleswerefurthercleanedbyamethanol:watersolutiontoremoveanyadditionalunreactedions.Afternalcentrifugation,thematerialwasdriedovernightunderadynamicvacuum,andsubsequentlyredat525Cfor5hunderowingnitrogen.ThestoichiometryofeachsamplewasdeterminedtobeMn0:68Zn0:25Fe2:07O3.Powderx-raydiractionconrmedthesinglephaseandcrystallinityofthematerial.ThefullsynthesisdetailsandcharacterizationresultsarefoundinRef.[35].24

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CHAPTER3MAGNETICMEASUREMENTTECHNIQUESAllmagnetizationmeasurementsinthisworkwereperformedinourlaboratoryusingaPhysicalPropertiesMeasurementSystemPPMSfromQuantumDesign.ThePPMSshowninFigure3.1consistsofaliquidheliumdewarequippedwithalongitudinalsu-perconductingmagnet,whichcangiveaeldupto7Tesla.Thetemperaturecanbevariedintherange2Kto450K.MagnetizationversuseldM-Hcurves,magnetizationversustemperatureM-TandACsusceptibilitymeasurementswereallperformedusingabuilt-inprobeofthePPMS,theAC/DCMagnetometrySystemACMS.ThetransversesusceptibilitymeasurementswereperformedusingtunneldiodeoscillatorTDO,whichisaresonantcircuitintegratedwithacustom-builtprobethattsintothePPMSS.Thedierentmagneticcharacterizations,suchasDCmagnetization,ACsusceptibilityandRFsusceptibilityarediscussedinthefollowingsections.3.1DCMagnetizationThemagneticpropertiesofamaterialarecommonlyprobedbytheapplicationofanexternalDCeld.OneofthemostimportantDCmagneticmeasurementsisthemagneti-zationversuseldorthehysteresisloop.Anumberofquantitiescanbedeterminedfromthehysteresisloop,namelythesaturationmagnetizationMS,remanentmagnetizationMR,andcoerciveeldHC.EachofthesequantitiesareindicatedinasamplegraphshowninFigure3.2andaredenedasfollows.Onestartswithamaterialinanunmagnetizedstate,M=0.Thisisimmersedinamagneticeld,H,thatisgraduallyincreasedfromzeroeldinsomearbitrarypositivedirection.Themagnetization,M,increasesfromzerotoMS.Aftersaturation,themagneticeldisreducedandtheMSdecreasestoMR,whichisthe25

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remanentorresidualmagnetization.Thereversedeldrequiredtodecreasethemagneti-zationtozeroiscalledthecoercivity,HC.Theeldisthenbroughttonegativesaturationandthenbacktothepositivemaximumeldtoclosetheloop.Mostmaterials,exceptthosethatexhibitexchangebias,havethesamemagnitudeofHCbothinthepositiveandnegativesaturation.MaterialsthathavehighHCandhighMR,whichresultsinanopenloop,aresaidtobehardmagnets.Superparamagneticnanoparticles,ontheotherhand,shownocoercivityabovetheblockingtemperatureanddependingonthenanoparticlesystem,lowtemperaturecoercivityisintherangeof500Oeto10000Oe.Theshapeofthehysteresisloopimpliesthesuitabilityofthematerialtoaparticularapplication.Forexample,asquare-shapedloopwithlargeremanenceandcoercivityaresuitablefordatastoragesinceithastwostablemagneticstates.Thesestatescanrepresentthe1sand0sinbinarylogic.Ontheotherhand,M-Hloopswithverysmallcoercivity Figure3.1.PhysicalPropertiesMeasurementSystemPPMS.26

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Figure3.2.Magnetizationversuseldcurveofaferromagneticmaterial.arecharacteristicofsoftmagneticmaterialsandareusefulintransformercoresandotherACandRFapplications.ByobservingtheM-Hloop,softandhardmagnetsmaybedistinguishedfromoneanother.Softmagnetsarecharacterizedbyanarrowloopwithsmallcoercivity.Iftheloopisnarrow,itissuitablefortransformercoreapplicationsincethepolarizationiseasilychangedwithasmalleld.M-Hloopscanalsohaveshapesdeterminedbythegeometryandintrinsicmagneticanisotropy.Wealsocharacterizeoursamplesbymeasuringthetemperaturedependenceofthemagnetizationintwodierentconditions:rstthematerialiscooledintheabsenceofaeldtoobtainthezero-eldcooledZFCcurveandsecondinthepresenceofanexternalDCeldtoobtaintheeld-cooledcurveFC.Thistypeofmeasurementisespeciallyusefulfordeterminingtheaverageblockingtemperature,TB,ofnanoparticles.AsampleZFC-FCcurveofananoparticlesystemcomposedofFe3O4powderisshowninFigure3.3.TomeasureZFCcurve,thesampleisinitiallycooledtoatemperatureaslowas5KwithH=0.Whenlowtemperatureisachieved,asmallconstanteldofabout100Oeisappliedandthemagnetization,MZFCistakenduringthewarmingcycle.Atroomtemperature,thenanopartclesarerandomlyorientedandtheeectivemagneticmomentis27

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Figure3.3.ZeroeldcooledZFCandeldcooledFCmagnetizationversustemperaturecurvesforFe3O4nanoparticles.zero.Whenthenanoparticlescooldown,theybecomefrozeninarandomstate.ThentheeldisappliedbutnoneofthemomentsareabletoaligninthefrozenstatesoMZFCispracticallyzero.Asthetemperatureincreases,somemomentsunfreezeandalignwiththeexternaleldandMZFCincreases.Itwillcontinuetoincreaseuntilthermaluctuationsofthenanoparticlesarestrongenoughtodemagnetizethesample.ThetemperatureatwhichthenanoparticleschangesfromtheferromagneticphasetosuperparamagneticphaseistheblockingtemperatureanditappearsasapeakintheZFCcurve.Ontheotherhand,theFCmeasurementisobtainedbycoolingthesampleinthepresenceofanexternaleld.Themomentsofthenanoparticlesbeingeasilymagnetized,instantlyalignwiththeeld.Asthetemperaturedecreasestofreezingtemperature,thenanoparticlemomentsbecomefrozenrandomlywiththemomentsonaveragealignedintheelddirection.TheDCeldremainsasthemeasurementistakeninthewarmingcycle.MFCstartsatanitevaluehavingallmomentsalignedinthedirectionoftheeld.Itbeginstofalloasthetemperatureincreasesandthemomentsphysicallyuc-28

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tuateinthesuperparamagneticstate.Similartosuperparamagneticnanoparticles,theZFC-FCmagnetizationmeasurementisastandardmethodtostudyspin-glasssystems,whichareprimarilydisorderedmagnets.ApeakintheMZFCcurveistypicalofbothsuperparamagneticnanoparticlesandspin-glasses.However,aminimumisobservedinMFCatlowtemperatureonlyforspinglass.ThepeakofMZFCagreesqualitativelywiththefreezingtemperature.ThiscanbeobtainedquantitativelyfromtheACsusceptibilitymeasurements,whichwillbediscussedinthenextsection.3.2ACSusceptibilityInACmagneticmeasurements,asmalloscillatingmagneticeldisappliedcausingatime-dependentmomentinthesample.Theeldofthetime-dependentmomentinducesacurrentinthepick-upcoils,allowingmeasurementwithoutsamplemotion.AtlowfrequencieswherethemeasurementiscomparabletoaDCmeasurement,themagneticmomentofthesamplefollowstheM-HcurvethatwouldbemeasuredinaDCexperiment.AslongastheACeldissmall,theinducedACmomentisgivenbyMAC=HACsin!t.1whereHACistheamplitudeofthedrivingeld,and!isthedrivingfrequency.=dM/dHisthesusceptibilityaswellastheslopeoftheMHcurveatverysmalleldswherethemagnetizationisstillreversible.HACisusuallyaround10Oewherethesusceptibilityisstillinthelinearregime.OneadvantageofACsusceptibilityisthatthemeasurementisverysensitivetosmallchangesinmagnetization.SincetheACmeasurementissensitivetotheslopeofMHandnottotheabsolutevalue,smallmagneticshiftscanbedetectedevenwhentheeectivemomentislarge.Athigherfrequencies,theACmomentofthesampledoesnotfollowalongthereversiblepartoftheDCmagnetizationcurveduetodynamiceectsinthesample.Essentially,therotationofthemagneticmomentcannotkeepupwiththealternatingmagneticeld.In29

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thishigherfrequencycase,themagnetizationofthesamplemaylagbehindthedriveeld,aneectthatisdetectedbythePPMSusingmutualinductancecircuitry.Thus,theACmagneticsusceptibilityoftenknownasthedynamicsusceptibilitymeasurementyieldstwoquantities:themagnitudeofthesusceptibility,,andthephaseshift,,relativetothedrivesignal.Alternatively,onecanthinkofthesusceptibilityashavinganin-phase,orreal,component0andanout-of-phase,orimaginary,component00.Thetwotermsarerelatedby0=cos.200=sin.3=p 02+002.4InthelimitoflowfrequencywheretheACmeasurementisnotdierentfromtheDCmeasurement,therealcomponent0asdiscussedabove.Theimaginarycomponent,00,indicatesdissipativeprocessesinthesample.Inferromagnets,anonzeroimaginarysusceptibilitycanindicateirreversibledomainwallmovementorabsorptionduetoaper-manentmoment.Also,both0and00areverysensitivetothermodynamicphasechanges,andareoftenusedtomeasuretransitiontemperatures.ACsusceptibilitymeasurementsareespeciallyusefulforcharacterizingsuperparam-agneticnanoparticles.AsoriginallytheorizedbyNeelandBrown[28],theparticlesareassumedtobenoninteractingandtheblockingtemperatureisgivenbyTB=E ln=okB.5whereEistheenergybarriertomagnetizationreversalinasingleparticle,isthemeasurementtimeandkBistheBoltzmannconstant.1/oiscalledtheattemptfrequencyanddescribeshowfastaparticlereversesitsmagnetization.Typically,thisvaluesfallin30

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therange10)]TJ/F19 7.97 Tf 6.587 0 Td[(10)]TJ/F15 10.909 Tf 11.38 0 Td[(10)]TJ/F19 7.97 Tf 6.587 0 Td[(9s.RearrangingtheequationaboveyieldsthetypicalformoftheNeel-Arrheniusrelation.=oexpEa kT.6Thepeaksobservedfromgraphsofboth0and00correspondtotheblockingtemper-atureatthetransitionfromferromagnetismtosuperparamagnetism.SinceTBdependsonthemeasurementfrequency,thepeakin00vs.Toccursatdierenttemperaturesfordierentfrequencies.Fromsuchmeasurement,onecancheckthattheparticlesaretrulynoninteractingbyverifyingthedependenceofTBonmeasurementtimeasgivenbytheNeel-Browntheory.Departuresfromthistheoryindicateinterparticleinteractions,forex-ampledipole-dipoleorinterparticleexchangeinteractions.Thefrequencydependentpeakin00vs.Texhibitedbysuperparamagneticparticlesisacharacteristicsimilartospinglasses.However,spinglassesshowacooperativephasetransitionwhilesuperparamagnetsshowagradualblockingofparticles.TakenotethattheNeel-Arrheniusrelationislimitedtothemagnetizationreversalofanon-interactingsingledomainparticleoverananisotropybarrier,Ea.InChapter5,wediscusstheoriginoftheglass-likepeaksobservedintheACsusceptibilitymeasurementofferrouids.3.3RFSusceptibilityTransversesusceptibilityTismeasuredbytakingthemagneticsusceptibilityinonedirectionwhileanexternalmagneticeldisorientedperpendiculartothedirectionofmea-surement.MeasurementofTisusefulinadirectdeterminationofmagneticanisotropy.TheanisotropyeldofamaterialistheeldneededtosaturatethemagnetizationofamaterialintheharddirectionandisrelatedtotheeectiveanisotropyKethroughtheequation:HK=2Keff=MS.731

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Directlymeasuringtheanisotropyeldofamaterialbytransversesusceptibilityisveryusefulincomparisonwithothermethods.Theunderlyingprincipleoftransversesusceptibilityisdescribedinaclassictheoreticalpaperby[36],Aharonietal.in1957.TheycalculatedTasafunctionofHDCforso-calledStoner-Wohlfarthparticles[37],whichareellipsoid,singledomainferromagneticparticleswithuniaxialanisotropy.AccordingtoAharoni'stheory,measurementofTwithrespecttoHDCappliedperpendiculartotheeasyaxisofmagnetizationshouldyieldthreecuspscorrespondingtothepositiveandnegativeanisotropyelds,HKandtheswitchingeld,HS.ThetheoreticalplotinFigure3.4extractedfromtheoriginalpapershowsboththetransverseandparallelsusceptibilityastheeldischangedfrompositivetonegativesaturation. Figure3.4.TheoreticaltransversesusceptibilityTandparallelsusceptibilityPcurvesasafunctionofreducedeldhh=HK/HDCascalculatedbyAharonietal.Figureadaptedfromreference[36].Inordertomeasuretransversesusceptibilityweuseaself-resonantRFfrequencytech-niquebasedonatunneldiodeoscillatorTDOinsteadofastandardsusceptometer.ThistechniquewasdevelopedbySrikanth,etal.[38]andwasrsttestedonamanganeseper-32

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ovskitesample.ItiscomposedofanLCtankcircuitpoweredbyatunneldiodebiasedinitsnegativeresistanceregionandisresonantat12to15MHz.TheeldgeneratedbythecoilmaybeorientedparallelorperpendiculartothevariableexternalDCeldprovidedbythesolenoidofPPMSmagnetandwecanindependentlyresolvetheparallelandtransversecomponentsofsusceptibility.ThiscircuitisincorporatedinamodiedmultifunctionprobethatcouldbeinsertedinthesamplechamberofthePPMS.TheLCtankcircuitoperatesatafrequencygivenby!=1 p LC.8Whenasampleisinsertedintotheinductivecoil,thereisasmallchangeinthecoilinductanceL.IfL=L<<1,onecandierentiateEquation3.8andobtaintheexpression! !)]TJ/F15 10.909 Tf 21.195 7.38 Td[(L 2L.9Anychangeininductanceisrelatedtothechangeinthesusceptibilityofthematerial.Theinductancecoilinthisexperimentalsetupservesasthesamplespaceinwhichagelcapcontainingthesamplecant.ThisentirecoilisinsertedintothesamplechamberofourPPMSusingacustomizedradiofrequencyRFco-axialprobe.TheschematicdiagramoftheTDOprobeisshowninFigure3.5.TheDCmagneticeldHDCisvariedusingthePPMS.TheoscillatingRFeldHRFproducedbytheRFcurrentowinginthecoil,isorientedperpendiculartoHDCandthisarrangementsetsupthetransversegeometrydescribedintheprevioussection.WhenHRFisperpendiculartothevaryingHDC,thechangeininductanceisactuallydeterminedbythechangeintransversepermeabilityTofthesample.Thus,wecanderiveanabsolutevalueforthetransversesusceptibilityT=T)]TJ/F15 10.909 Tf 11.767 0 Td[(1.ThepercentchangeintransversesusceptibilitythencanbedenedasT T%=jTH)]TJ/F21 10.909 Tf 10.909 0 Td[(satTj satT100.1033

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Figure3.5.SchematicdiagramoftheTDOcircuitandsamplespaceleftandCADdrawingoftheinductancecoilwhichservesasthesampleholderright.wheresatTisthetransversesusceptibilityatthesaturatingeldHsat.DataacquisitioniscontrolledbyaLabviewprogrambyspecifyingthemagnitudeofthesaturatingeldinOeunitsandthesamplingrateinOe/sec.Theprogramprotocolinitiallysaturatesthesampletothemaximumpositiveeldthenreducestheeldtozero.Theeldcrossesoverzerotonegativesaturationeld.Fromthiseld,thescanretracesitspathtozeroeldandnallyterminatesasthepositivesaturationeld.Theoutcomeofeachrunisabipolarscanofthesusceptibilityasafunctionoftheeldthathasallthefeaturesnecessaryforextractionofanisotropyandswitchingelds.Overtheyears,thistechniquehasbeenprovenbyourgrouptobeanexcellentprobeofmagneticanisotropyofbulk,thinlmsandnanoparticles.Sincethisisaresonantmethod,weareabletousethehighdegreeofsensitivity10Hzinaresonanceof10Mhztomeasuresamplesevenwithveryweakmomentsthatarediculttobedirectlypickedupbyconventionalmagnetometry.TransversesusceptibilitymeasurementsusingTDOisusefulinavarietyofapplications.Inthiswork,weutilizetransversesusceptibilitymeasurements34

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indirectlyprobingthedipolarinteractionsinnanoparticleassemblies.TheresultsofthisstudyarediscussedinChapter4.35

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CHAPTER4STATICANDDYNAMICMAGNETIZATIONOFNANOPARTICLESInstudyingthemagneticpropertiesofnanoparticles,standardmeasurementssuchasDCmagnetizationandACsusceptibilityaretypicallydone.Inthischapter,anewtech-niqueofcharacterizingnanoparticlesisdiscussed.Thisnonconventionalwayofmeasuringthemagneticproperties,particularlymagneticanisotropy,hasbeenusedbyourgroupinstudyingmagneticbulkandthinlmsystemsandisfoundtobeusefulevenfornanoparti-clesinathreedimensionalassembly.TheresultsofthestandardDCandACmeasurementsarealsopresented.Oneofthemostimportantpropertiesofmagneticmaterialsisthemagnetocrystallineormagneticanisotropy.Magneticanisotropyistheintrinsicenergeticpreferenceofferromag-neticmomentstoalignalonganeasydirectionofmagnetization.Themagneticanisotropyenergyistheenergyinvolvedinrotatingthemagneticmomentfromaneasyaxistoanotherdirectionhardaxis.Dierentdegreesofmagneticanisotropyisdesiredindierentap-plications,suchaspermanentmagnets,informationstoragedevices,recording/readheadsandbiomedicine.Therefore,itisimportanttostudytheeectofvariousfactorsonama-terial'sanisotropy.Infact,recentresearcheortsinmagneticallyanisotropicmaterialsformediastorageathigherdensitieshavedemandedtherequirementforexperimentaltech-niquesthatcandirectlyprobethemagneticanisotropyanditstemperaturedependenceinaprecisemanner.Thereexistseveralapproachestomeasurethemagneticanisotropyinnemagneticparticlesystems.Examplesincludethesaturationapproach,thetorsionalpendulum,rota-tionalhysteresis,remanent-torquemagnetometry,complexpermeabilityspectra,etc.but36

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mostofthesetechniquesareindirectandoftenunwieldyforextractinginformationabouttheanisotropy[39{48].Forexample,inthesaturationapproach,togettheanisotropyconstant,onehastodoM-Hmeasurementsalongthehardaxisdirectionandextrapolatetheeectiveanisotropyfromtheslopechangeineld-dependentmagnetization.However,thisisdicultinaggregatesofmagneticnanoparticlesduetotheuniaxialsymmetryforsphericalparticlesanddistributioninpreferredaxisdirections[44,45].Recently,magneticforcemicroscopyhasbeenshowntoyieldinformationabouttheanisotropyofindividualhemisphericalnanoparticlesgrownontoasubstrate[49].Thismethod,whileexcellentfordetermininganisotropydistributionandorientation,islimitedtoroomtemperature.TransversesusceptibilityTS,whichisasingularpointdetectiontechnique,hasbeenrea-sonablysuccessfulinassembliesofpolycrystallinesingle-domainparticlesinprobingtheanisotropyeldinthepresenceofparticlesizedistributionandtexturingorientationofeasyaxes[39,40].Despiteawealthofliteraturebeingavailableonmeasurementsandmodelingofmag-neticanisotropyusingthetransversesusceptibilityTStechnique,onlyveryfewstudiespresentsystematicmeasurementsoftheeectofinterparticledipolarorexchangeinterac-tionsontheeectiveanisotropypeaks.PortionsofthischaptercanbefoundinRef.[29]wherewehavedescribedhowtransversesusceptibilitymeasurementcanbeaverypowerfultoolindirectlyprobingtheeectofinterparticleinteractionsinnanoparticles.Thisworkrepresentstherstcorrelationofanisotropypeakbehaviorwithinterparticleinteraction.4.1ExperimentalDetailsThehighqualitymanganesezincferriteMZFOnanoparticlesusedarehighlymonodis-persedandcoatedwithsurfactant.Tostudytheeectofvaryingstrengthofdipolarinter-actionsonTSpeaksinareasonablycontrolledfashion,wevariedtheconcentrationofthenanoparticlesembeddedinwax.Weusedparanwaxsinceitismagneticallybenignanddoesnotinterfereinanymagneticmeasurement.Thecoatedparticleswererstdispersedinpropanol.ThreesampleslabeledasMZFO,MZFO,MZFOwereprepared37

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byadding50,100,500lofsuspension,respectivelyinsameamountofmoltenparanwax.Thepresenceofexcesssurfactantpreventedagglomerationoftheparticlesandweletthewaxsolidifyundersonicationtoensurethattheparticlesarerandomlydistributedallthroughoutthewaxmatrix.Andthenafourthsample,MZFO-concentratedMZFO-concwaspreparedbysimplydryingoutthesolventwithoutanywax.ThevariationincoercivityandblockingcharacteristicsasobservedthroughdcsusceptibilityexperimentsweremeasuredandcomparedwithTSexperimentsdescribedinthepreviouschapter.4.2DCCharacterizationsDCcharacterizationresultsinFigure4.1showtheM-HloopcomparingMZFO,MZFO,MZFOandMZFOconcsamples.Theblockingtemperaturecanbeidentiedbythesharptransitioninthecurves.Asseenfromthegraph,theblockingtemperatureincreasesforincreasingconcentration.Thesharpnessofthepeakindicatesthenarrowdistributionoftheparticlesize,otherwise,abroadtransitionisobserved.Thiswouldalsomeanthatthewaxdoesnotfacilitateagglomerationoftheparticlesincontrasttomediasuchaspolymersusedinseveralstudies.Ithasbeenshownthatthenormaltendencyofparticlesinpolymermatricesistoagglomeratebecauseofthestrongstericforcesandtheireectonthemagneticpropertiescouldbeeasilyobserved[50].Inapreviousstudy,wehaveshownhowthisagglomerationofthenanoparticlesinpolymerscanbeovercomeusingcarefulchoiceofsolventandsurfactantmixturesbutthesecomplicationsarenotpresentinparanwaxusedinthisstudysowedonotneedspecialprocedures.Figures4.2and4.3showsthecomparisonoftheM-Hloopsofallthesamplesat300Kand10K,respectively.Thecurvesat300Kconrmsthesuperparamagneticstateofthenanoparticlesshowingreversiblebehaviorandzerocoercivity.At10K,asystematicincreaseinthecoercivitywithincreasingpackingfractionfrom144to192Oecanbeseenwhichclearlyshowsaneectofinterparticleinteractionstrength.Thisincreaseincoercivityisconsistentwithincreaseintheblockingtemperature.ThevaluesareoftheblockingtemperatureandcoercivityforallthesamplesaresummarizedinTable4.1.38

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Figure4.1.Zeroeldcooledandeldcooledmeasurementcurvesperformedat100Oeforvarioussamples.Straightlineisguidetoeye.Theinsetshowszoomviewofthecurvesinthemainpanel4.3TransverseSusceptibilityTSasaTooltoProbeInterparticleInteractionsWediscussheretheresultsofthetransversesusceptibilitymeasurementstakenatdierenttemperatures.Thesampleswererstzero-eldcooledandeachcurvewasobtainedwhilewarmingup.Ateachtemperature,wedidabipolarscanfrompositivesaturationtonegativesaturationandthenbacktopositivesaturation.Wecouldidentifythetwoanisotropypeaks,HK1positiveandHK2negative.Andfromtheseplotswecanseehowtheanisotropypeaksevolvewithincreasingtemperature.WehaveshownrepresentativeTScurvesforsamplesMZFO100andMZFOconcinFigures4.5and4.4.Thefamilyofcurvescomposedofbipolarscanstracestheevolutionofthesystemasthetemperatureisincreased.Ascanbenoticedinthesegures,whenthetemperatureislowered,thesinglepeakintheTSatzeroeldsplitsintotwopeaks,whichbecomemoreprominentwithdecreasingtemperatureandshifttohighereldvalues.ItisalsoclearfromFigures4.539

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SampleDescriptionTBKHCOeat10K MZFO50lofnanoparticles28144dispersedinwaxMZFO100lofnanoparticlesdispersedinwax29163dispersedinwaxMZFO500lofnanoparticlesdispersedinwax30178dispersedinwaxMZFOconcNanoparticleinsolutiondried32192insideagelcaptoformathicklayer Table4.1.SummaryofresultsoftheDCmagnetizationcharacterizationforallMZFOsamplesand4.4thattheoverallshapeoftheTScurvesincludingthepeakpositionsisremarkablydierentforthetwoextremecasesofMZFOconcandMZFO.TofollowthesedierencesinfeaturesintheTSclosely,wehavecomparedinFigure4.6theunipolarscansfrompositivesaturationtonegativesaturationfordierentsamplesatfourxedtemperatures.InFigure4.7,wehaveplottedthepeakpositionversustemperaturecurvesforallthefoursamplesinboththepositiveandnegativeeldoftheunipolarscan.Thepeakposi-tionsarealmostsymmetrictoeachotherandstronglydependentupontemperature.AgainfromthecurvesinFigure4.6,itcanbeobservedthatat15K,thepeakscorrespondingtoMZFOconcaresituatedatlargereldvaluesincomparisontheothersamples[MZFOconc>MZFO>MZFO100>MZFO50].Thisshowsthatthevaluesofeectiveanisotropypeakpositions,asobtainedfromTScurves,arestronglyaectedbyinterpar-ticledipolarinteractionsandincreaseswiththestrengthoftheinterparticleinteractionstrength.Ourexperimentalresultsarethusincontrasttoearliertheoreticalstudiesthatpredictedadecreaseintheanisotropyeldswithenhancedinteractions[51{53].AnotherimportantfeatureofTScurvesistheheightandshapeoftheTSpeakslocatedatHK.AsobservedfromFigure4.6,intheTSexperiments,whengoingfrompositivesaturationtothenegativesaturationeldvalues,HK1isalwayslargerthanHK2.Thisisafactthatwehaveconsistentlyobservedinourpreviousstudiesaswell[50,54{56].However,40

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Figure4.2.MagnetizationvseldcurvesofallMZFOsamplesmeasuredat300K.Theinsetshowsthezoomviewofthecurvesitwasunclearwhatcausesthedierenceintheheightofthepeaksandthesystematicstudieshereonsamplesofvaryinginteractionstrengthsallowsustoexploretheoriginofthepeakheightsymmetry.Tobetterexpresstheissueofpeakheightquantitatively,inFigure4.8,wehaveplottedthepeakheightdierence,Peakheightdierence=PeakheightHK1)]TJ/F15 10.909 Tf 10.909 0 Td[(PeakheightHK2 PeakheightHK1%.1asafunctionofsampletemperature.Itisremarkabletoseethattheconcentratedsample,thepeakheightdierenceishighlyreducedincomparisontotheweaklyinteract-ingparticlesystem.OurresultsindicatethatthedierenceinpeakheightslocatedatHK,ortheasymmetry,isalsoextremelysensitivetoachangeinthedipolarinteractionstrengthandincreaseswithdecreasinginteraction.InarecenttheoreticalinvestigationbyMatarranzetal.[53]itwasclaimedthatforatexturedassemblyofuniaxialsingle-domaininteractingparticles,thedispersionintheanisotropyeldparticlesize,shapecausesthe41

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Figure4.3.MagnetizationvseldcurvesofallMZFOsamplesmeasuredat10K.Theinsetshowsthezoomviewofthecurvespeakheightasymmetryandbroadening.Theycalculatedthatthatforamonodispersedassemblyofparticles,theTSpeakswouldbeverysharpandsimilarinheight.However,ourexperimentalstudyisinsharpcontrasttothisandthetrendistheopposite.Notethatinourcasewehavetakenparticlesfromthesamesynthesisbatchandallotherex-perimentalconditionsremainthesameforallthesamplesexceptfortheconcentrationoftheparticlesinwaxthatdeterminestheinterparticleinteractions.Therefore,weinferthattheanisotropyelddispersionremainsconstantforallthesamples.Wealsoconcludethatthechangeinthepeakheightdierenceisnotduetothedistributionoftheanisotropyeldaloneasindicatedbyprevioustheoreticalstudiesbuttheinterparticleinteractionalsoplaysamajorrole.Overall,wedemonstratethatTSissensitiveenoughtopickupsmallchangesintheinteractionstrength.Wenowpresentargumentsthatwillhelpqualitativelyunderstandtheevolutionofthepeakheightasymmetryfromlargetosmallasthenanopar-ticlesystemgoesfromweaklytostronglyinteractingcases.Toexplainthis,recallthatthesamplewasrstsaturatedatapositiveeldandHK1alwaysoccursaftersaturation,42

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whentheZeemanenergyishighest.Sincetheparticlesarelargelyaectedbythechangingeld,therotationofthemomentsismorecoherentcausingasharperpeakwithahighermagnitudeatHK1.Whentheeldcrosseszero,themomentshaveessentiallyrandomized.Atthispoint,theinter-particleinteractionsaredominatingthemagneticresponse.Hence,forweaklyinteractingparticles,theresponseintherandomizedstatewillbemuchsmallereveniftheappliedeldisincreased.Asaresult,weobserveHK2tobeofsmallerheight.Conversely,stronglyinteractingparticlesshouldresulttomoresymmetricpeakssincetheeldeachparticleexperiencesfromitsneighborsshouldhaveasimilarresponsetoanap-pliedeld.AdecreaseinZeemanenergyshouldnothaveaslargeaneectonthecollectiveresponseofthesystem.Thus,ingeneral,whentheeldisreversedinpolarity,theheightandwidthofthesecondpeakonthenegativesidearedierentforthetwocasesi.e.,smallandbroadforthecaseofnoninteractingparticlesversuslargerandsharperforthecaseofinteractingparticlesanddependsonthedierentenergylandscapesseenbythenon-interactingandinteractingparticlesatzeroelddemagnetizedstate.ThisexplainstheTSpeaksbeingsymmetricinheightforsampleswithstronginteractionsanddisplayingalargeasymmetryfornoninteracting/weaklyinteractingparticles.OurexplanationisalsoveryconsistentwithTSdataonferromagneticthinlmswhichalwaysdisplaysharppeakssymmetricinheight[57,58].43

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Figure4.4.RepresentativetransversesusceptibilitycurvesforMZFOconcsample.44

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Figure4.5.RepresentativetransversesusceptibilitycurvesforMZFOsample.45

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Figure4.6.Comparativeviewoftransversesusceptibilitycurvesatvariousconcentrationsandtemperatures.46

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Figure4.7.PeakpositionHKversustemperaturecurvesinboththequadrantsforvariousconcentrations.TheeldpositionsoftheanisotropypeaksinpresentedasIforthepositiveeldandIIforthenegativeeldasafunctionoftemperature Figure4.8.Anisotropypeakheightdierencearbitraryscaleversustemperaturecurvesatvariousconcentrations.47

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CHAPTER5MAGNETICPROPERTIESOFFERROFLUIDSInthepreviouschapter,wehaveseenthemagneticpropertiesofnanoparticlesem-beddedinasolidwaxmatrix.Wenowinvestigatethemagneticresponsewhenmagneticnanoparticlesaredispersedinacarrierliquid.Ferrouids,whichhaveextensivepotentialintechnologicalapplications,asdiscussedinChapter1,areexcellentmodelsystemsforstudyingthemagneticcharacteristicsandrelaxationphenomenainmagneticnanoparticles[8,59{66].Whenthemagnetizationofaferrouidismeasuredoverarangeoftemperature,unlikeinawaxmatrix,weareabletoexaminenanoparticleswhiletheyareinaliquidaswellasinasolidenvironment.Thedynamicsthatoccuratthetransitionfromliquidtosolidastheferrouidiscooleddownandentersafrozenstatearelikewiseaccessible.Ithasbeenshownthatthemagneticpropertiesofferrouidsaredominatedbythedipole-dipoleinteractionbetweenthesuspendedparticles[59].Particlesizedistribution[60],concentration[61,62],andsurfactantcoating[63]aswellassolventusedinthesuspension[64]allaectthedipole-dipoleinteraction[59],whichinturngiverisetotherangeinmagneticbehaviorobserved[65,66].Despiteanumberofpreviousstudies[67{69],however,theunderstandingofthephysicaloriginofthemagneticanomalies[66]andrelaxationphenomena[65]inferrouidsremainsunclearinpartduetothecomplexnatureofthesystemandalsothefactthatmanyoftheseearlierstudiesfocusedononetypeofferrouidsysteminwhicheitherthenanoparticlesizedistributionorsolventremainedthesameandtypicallytheparticlevolumefractionwasvariedtoexploredipolarinteractioneects.Inparticular,Luoetal.[65]reportedtheirobservationofthetwocharacteristicpeaksinthetemperature-dependentcomplexsusceptibility=0)]TJ/F21 10.909 Tf 22.123 0 Td[(i00inFe3O4-basedferrouids,buttheoriginofthesepeaksremainedunknown.Theydemonstratedvia48

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dierentialscanningcalorimetryDSCmeasurementsthatthereexistthreecharacteristicstatese.g.liquid,mixedandfrozenstatesinaferrouidorasolvent.FortemperaturesabovethepourpointTpouraliquidphaseexistsasthesystemowslikeauid,whereasasolidphaseorafrozenstateispresentatlowtemperaturesbelowTs)]TJ/F22 7.97 Tf 6.586 0 Td[(m,andamixedphaseexistsinthetemperaturerangebetweenTs)]TJ/F22 7.97 Tf 6.587 0 Td[(mandTpour,whereTs)]TJ/F22 7.97 Tf 6.586 0 Td[(misthetransitiontemperaturefromthesolidphasetothemixedphase.Importantly,therstpeakin00athightemperaturehasbeenfoundtobelongtothetemperaturerangeofTs)]TJ/F22 7.97 Tf 6.586 0 Td[(m
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5.1FerrouidSamplesFe3O4andCoFe2O4nanoparticlesweresynthesizedusingthechemicalsynthesistech-niquedescribedinChapter2.StructuralcharacterizationswerealsodiscussedinSection2.3.Themeanparticlesizeswere14nm3nmand62nmforFe3O4and11nm3.Forthedispersingliquids,hexaneanddodecanewerechosensincetheyhavethesamecompositionbutdierinroomtemperatureviscosityandmeltingpoints.Dodecane,havingaalongerhydrocarbonchainthanhexane,hasaviscosityof1.35cPcentipoiseandfreezesat264K.Hexane,ontheotherhandhasaviscosityof0.294cPandafreezingtemperatureof177K.Thevolumeconcentrationswerekeptalmostthesameforallthesamplesinvestigated2vol%.Forclarity,wewillusethefollowinglabelsintherestofthechaptertorefertospecicsamples:SampleFPFe3O4indrypowderform,Sam-pleFHFe3O4inhexane,SampleFDFe3O4indodecane,SampleCPCoFe2O4indrypowderform,SampleCHCoFe2O4inhexaneandSampleCDCoFe2O4indodecane.5.2AnomalyinDCMagnetizationWemeasuredmagnetizationversuseldcurveforthedrypowdersofFe3O4andCoFe2O4at10Kand300K.FromtheM-HcurvesinFigures5.1and5.2,wendthatat300K,thereisnohysteresiswhereastheloopsat10Kexhibitaclearhysteresiswithacoercivityofaround150to200OeforFe3O4.CoFe2O4hasalowtemperaturecoercivityof1900Oe.Thisischaracteristicofthesamplebeingsuperparamagneticatroomtemper-atureandenteringablockedstateatlowtemperaturewhichresultsinopeningupofthehysteresisloop.Thegeneralobservationsareconsistentwiththatreportedinapaperbyus[70]andotherauthors[66,71]onFe3O4andCoFe2O4nanoparticles.TheZFCandFCmagnetizationcurvestakenat100Oefor14nmFe3O4and11nmCoFe2O4areshowninFigures5.3and5.4.ThetoppanelsinbothgurescorrespondtothepowdersamplesFPandCP,themiddlepanelsareforhexaneferrouids,FHandCHandthebottompanelsarefordodecaneferrouids,FDandCD.Itcanbeobservedin50

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Figure5.1.MagnetizationversuseldcurveforFe3O4nanoparticlestakenat10Kand300K. Figure5.2.MagnetizationversuseldcurveforCoFe2O4nanoparticlestakenat10Kand300K.51

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Figures5.3aand5.4a,thattheZFCcurveexhibitthetypicalblockingprocessofanassemblyofsuperparmagneticFe3O4CoFe2O4particleswithadistributionofblockingtemperaturesatTB200K212K.RecallthattodetermineTBintheZFCcurve,thesampleisrstcooleddowninzeroeldfromahightemperatureKtoalowestmeasuredtemperatureTo,atwhichthetotalmagnetizationofthesampleiszerosincethemagneticmomentsofparticlesarerandomlyoriented.Onheatingthesample,theapplicationofamagneticeld,H,inducesanetmagneticmomentalongthemagneticelddirection,whichwillincreasewithincreasingtemperatureintherangeToTB,thenetmomentoftheparticlesthatarealreadysuperparamagneticdecreasesandfollowsaCurielawasafunctionoftemperature[71].Asaresult,apeakappearstooccurintheZFCcurve,andthispeaktemperatureistheaverageTBforthewholesample. Figure5.3.ZFC-FCcurvesfor14nmFe3O4samplesaFe3O4powderFPbFe3O4+hexaneFH,cFe3O4+dodecaneFD.52

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Figure5.4.ZFC-FCcurvesfor11nmCoFe2O4samplesaCoFe2O4powderCP,bCoFe2O4+hexaneCH,cCoFe2O4+dodecaneCD.Thebehaviordescribedaboveisverydierentfromthatofferrouids.Figure5.3bandcshowtheZFCandFCcurvesforsampleFHandFD,respectively.TheZFCcurvesexhibitasharppeakinsamplesFHandCH,whileasmallpeakfollowedbythemagneticanomalyi.e.,thesharpdropintheZFCmagnetizationatlowtemperatureareobservedforsamplesFDandCD.Wenote,herein,thattheTBoftheFe3O4andCoFe2O4magneticnanoparticles,200Kand212K,respectivelyareaboveandclosetothefreezingtemperatureTFofhexaneK,whereastheblockingtemperaturesofbothmagneticnanoparticlesismuchlowerthanthefreezingtemperatureTFofthesolventdodecaneK.ThiscleardistinctioninthesamplesallowsustoattributetheobservedpeakandmagneticanomalyinFigure5.3cand5.4ctothefreezingeectofthesolventandtheblockingeectofmagneticnanoparticles,respectively.Importantly,wendthat53

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themagneticanomalyisonlyobservedinferrouidshavingTBTF,theZFC-FCmagnetizationintheliquidstateT>TFisnotaectedbytheparticleblockingbutfortheferrouidshavingTBTF,thepeaksat186Kand189Kareascribedtothecombinationofboththeblockingandfreezingeects,whereasonlythefreezingeectcontributestothepeaksat270KforsampleFDand272KforsampleCDfortheferrouidshavingTB
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Theideaistotunetheblockingtemperaturewithrespecttothefreezingtemperatureofthesolventandthiscanbeachievedintwoways.Oneoptionistokeeptheparticlesizexedforagivennanoparticlesystemandchangethedispersingliquid.Thesecondmethodisbyvaryingsizeoftheparticlesforagivensolventandnanoparticlesystem.Thelattergivesabetterexibilityintermsoftherangeofblockingtemperatures.Toexaminetheeectofparticlesize,similarexperimentsweredoneon6nmFe3O4nanoparticles.ThegraphonFigure5.5showtheZFC-FCcurvesforthesamplescontaining6nmnanoparticles,denotedas6FP,6FH,6FD.TheblockingtemperatureoftheFe3O4powderisat35K,whichisfarbelowfreezingofbothhexaneanddodecane.Sincethenanoparticlesaresmallandwelldispersed,thefreezingpeakisnotvisibleandonlyasinglepeakduetotheparticleblockingeectisseen.Theseresultsclearlyindicatethattheblockingandfreezingeectsareparticle-sizedependent.TheanalysisoftheACsusceptibilitydatainthenextsectionwillclearlyshowthatthechangeinparticlesizeaectstheinterparticleinteraction,andinturn,categorizesthetwosizestodierentinteractionregimes.5.3OriginofSpinGlass-likeRelaxationPeaksWenowattempttocorrelatethephysicaloriginofthetwopeaksintheACcomplexsusceptibilityobserved[65]tothepeakandmagneticanomalyintheZFCDCmagnetiza-tion[66]thatremainedunexplainedbyothergroups.Figures5.6,5.7,5.8and5.9showtherealpart,0,andimaginarypart,00,oftheACsusceptibilityasafunctionoftemperaturetakenatdierentfrequencies.Asthefrequencyincreases,theblockingtemperaturepeaksinboth0and00shifttohighertemperature.Toanalyzethisfurther,wefocusedonthepeaksinthe00versusTplotsastheyaresharperandmorepronounced.Thecollectionofthegraphsfor00isshowninFigure5.10.Asseeninthegure,the00TcurvesshowasinglepeakfortheferrouidshavingTB>TFsamplesFHandCH,whiletwodistinctpeaksareobservedfortheferrouidshavingTB
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Figure5.5.ZFC-FCcurvesfor6nmFe3O4samples.singleplotasinFigure5.11.Here,onecaninferthatthesecondpeakin00Tisascribedtotheblockingofmagneticnanoparticles,whiletherstpeakin00Tisassociatedwiththefreezingofthesolvent.Asonecanalsoseeclearlyinthecombinedplotthattherstpeakreectsthemagneticbehaviorinthemixedstate,whilethesecondpeakrepresentsthemagneticbehaviorinthefrozenstate.Ournewndingsallowustocorrectlyclassifythetwopeaksin00Tcorrespondingtotheglass-likepeakandmagneticanomalyasduetofreezingandblockingfortheFe3O4ferrouidswithkerosenesolventreportedpreviouslybyLuoetal.[65]andforFe3O4ferrouidswithhexanesolventreportedbyZhangetal.[66].Notethatitisonlyourcomparativeanalysisbetweenferrouidswithvaryingblock-ingandfreezingtemperaturesthatleadstothisclaricationoftheoriginofthepeaksandtheconclusionwouldnotbeapparentjustbylookingatthedataononetypeofferrouidaloneasinthecaseofthepaststudies.56

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Figure5.6.ACsusceptibilitydatafornmFe3O4+hexanesampleFH.5.4FitstoNeel-ArrheniusandVogel-FulcherRelationsToprobethemagnetizationdynamicsinthesestates,weexaminethepeaksinthe00Tplots.Thepeaksobservedfromgraphsofboth0and00correspondtotheblockingtemperatureatthetransitionfromfrozenstatetosuperparamagnetism.SinceTBdependsonthemeasurementfrequency,thepeakin00vs.Toccursatdierenttemperaturesfordierentfrequencies.Fromsuchmeasurement,onecancheckthattheparticlesaretrulynoninteractingbyverifyingthedependenceofTBonmeasurementtimeasgivenbytheNeel-Browntheory.Departuresfromthistheoryindicateinterparticleinteractions,forexampledipole-dipoleorinterparticleexchangeinteractions.Thefrequencydependentpeakin00vs.Texhibitedbysuperparamagneticparticlesisacharacteristicsimilartospinglasses.However,spinglassesshowacooperativephasetransitionofmomentswhilesuperparamagnetsshowagradualblockingofparticles.TakenotethattheNeel-Arrheniusrelationislimitedtothemagnetizationreversalofanon-interactingsingledomainparticleoverananisotropybarrier,Ea.57

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Figure5.7.ACsusceptibilitydatafornmFe3O4+dodecanesampleFD.ACsusceptibilitypeaksobservedatdierenttemperaturesACsusceptibilitypeaksobservedatdierenttemperaturecanbettedtodierenttheoreticalmodelstoextractparametersinherenttothesample.Well-knownmodelssuchasNeel-ArrheniusandVogel-Fulcherrelationsarewidelyusedforttingdata.TheNeel-Arrheniusisvalidfornon-interactingparticlescanbettedbytwoparameters.Ontheotherhand,Vogel-Fulchermodeldescribesasystemofweaklyinteractingparticlesandrequiresthreeparameters.TheVogel-FulcherrelationisgivenbyEquation.13,whereistherelaxationtime,Eaistheactivationenergy,kistheBoltzmannconstant,Tparethepeaksfromthegraphof00andToistheinteractionenergy.Forttingthedata,itismoreconvenienttoexpresstherelationinEquation.13intermsofthefrequencyasinEquation.1where=1=fwasused.Here,foistheattemptfrequency.lnf=lnfo)]TJ/F21 10.909 Tf 12.105 7.38 Td[(Ea k1 Tp)]TJ/F21 10.909 Tf 10.909 0 Td[(To.158

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Figure5.8.ACsusceptibilitydatafornmCoFe2O4+hexanesampleCH.BymakingaplotofTpversuslnfandperforminganonlinearcurvetting,,Ea=kandToweredetermined.Theresultofdipolarinteractionsistoslowdowntheattemptfrequencybecausetheparticlesmustalsoovercomethelocalenergypresentfromneigh-boringparticles.However,asmentionedbefore,thisrelationisonlyvalidforinteractingnanoparticlesystems,andattimeswouldyieldunphysicalvalues.Inthatcase,thethirdparameter,ToissettozerotottheNeel-ArrheniusmodelgivenbyEquation3.6.Fortheferrouidsinvestigated,thepeakin00TshiftstohighertemperatureasthemeasurementfrequencyisincreasedRefertoFigure5.10indicativeofglassybehavior[76].Theglassynatureinferrouidsisreasonablycomplexascontributiontotheglassystatesinthemixedandfrozenregionsresultsfromthecombinedblockingandfreezingeects[65,73{75].Toclarifytheblockingandfreezingeectsontheglassybehaviorsinthemixedandfrozensates,weanalyzethefrequencydependenceofthepeaksof00TFigure5.10,byttingthedatausingbothNeel-ArrheniusNAmodelinEquation3.6andVolgel-FulcherVFscalinglawinEquation1.13.59

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Figure5.9.ACsusceptibilitydatafor11nmCoFe2O4+dodecanesampleCDOurresultsrevealthatthe00TdatacanbetusingEquation3.6,butthettingparametersobtainedareunphysical,similartothecasereportedinearlierreferences.ThissuggeststhattheseferrouidsbelongtotheclassofinteractingparticlesystemsforwhichtheNeel-Arrheniusmodelisinvalid[77].Forourferrouids,theVFmodelhasbeenfoundtotwellthe00TdatawithacceptabletparametersandtheresultsareshowninFigure5.12.ThebesttsforsampleFDyieldacceptablevaluesforo,Ea=kandTo.TheactualvaluesaresummarizedinTable5.1.NotethatpeaksinTP1areinthemixedstateandTP2areinthefrozenstate.ThedierenceinoandEa=kforthecasesofTP1 TP1TP2 o0.3x10)]TJ/F19 7.97 Tf 6.586 0 Td[(7s1.80.4x10)]TJ/F19 7.97 Tf 6.587 0 Td[(6sEa/k.40.5x102K.50.4x102KTo232K149K Table5.1.Besttvaluesofo,Ea=kandTofromVFrelationforFe3O4indodecaneFDandTP2indicatesthatfortheferrouidhavingTB
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Figure5.10.Temperaturedependenceofimaginarypartofsusceptibility,00,ofaSampleFH,bSampleFD,cSampleCHanddSampleCDat10Hz-10kHzmeasuringfrequencyrange.ThemagnitudeofanappliedacmagneticeldisHAC=10Oe.innaturebetweenthemixedandfrozenstates.Thisisunderstandableasthemagneticparticlesareunblockedintheformercase,whereastheyarealreadyblockedinthelattercase.ThelargervaluesofoandEa=kforthecaseofTP2indicatethattheblockingeectofmagneticnanoparticlesontheglassybehaviorinthefrozenstateissimplytocauseslowingdownofthedynamicsofthesystem.Inaddition,wendthatTo=232KforthecaseofTP1whichcoincideswiththetemperatureatwhichtheferrouidentersintothefrozenstatefromthemixedstateandthistransitionistheoriginofthedivergenceintheviscosityoftheferrouid,whereasthedivergenceoftherelaxationtimeatTo=149KforthecaseofTP2suggeststhatthesystementersaglassystateatthistemperature.Inviewoftheseresults,weproposethattheblockingofmagneticnanoparticlesinthefrozen61

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Figure5.11.TemperaturedependenceofZFCmagnetizationandimaginarypartofmag-neticsusceptibility,00,ofSampleFDFe3O4+dodecane.LS:LiquidSate;MS:MixedState;FS:FrozenState.Toisthetemperatureatwhichthesystementersaglassystate.TP1andTP2representthe00peaksinthemixedandfrozenstates,respectively.statesignicantlyaectstheinterparticledipole-dipoleinteraction,causingcharacteristicspin-glass-likedynamics.Thisisinagreementwiththepreviousstudiesontheinuenceofdipolar-dipolarinteractiononthelow-temperaturemagneticallyglassybehaviorandspindynamicsinfrozenferrouids,whichindicatethatblockedmomentsareobservedfortheisolatedparticlesinadilutedferrouidowingtoanitemeasurementtimewhereasthemagneticmomentsoftheinteractingparticlesinaconcentratedferrouidfreezecooperativelyasseeninaspinglasssystem[74,75].Thisalsoallowsonetoreconciletheobservationsofspin-glass-likestates,magneticrelaxationandagingeectreportedintheliteratureforferrouids[59,65{67,72,75].Wenowturntoclarifythedierenceinmagneticallyglassynaturebetweenthemixedandfrozenstatesinferrouids,withparticlemomentsintheblockedstateinbothregimes,throughstudyingthemagneticallyglassycharacteristicsofsampleCHandsampleCD.Wenotethatthepeakin00TforCHreectsthemagneticbehaviorinthemixedstateTP1,62

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Figure5.12.ThebesttsofTpourfdatatotheVFmodelextractedfromACsusceptibilityofSampleFDforthecasesofTP1andTP2,SampleCHandSampleCD.whilethesecondpeakin00TforCDreectsthemagneticbehaviorinthefrozenstateTP2.RefertoFigures5.10candd.ThettingresultsarealsoshowninFigure5.12.ThebesttsforCHandCDyieldvaluesforo,Ea=kandTotypicalforCoFe2O4systems.ThevaluesaresummarizedinTable5.2 CHTP1CDTP2 o.30.4x10)]TJ/F19 7.97 Tf 6.587 0 Td[(6s1x10)]TJ/F19 7.97 Tf 6.587 0 Td[(6sEa/k.60.2x102K.60.5x102KTo151K173K Table5.2.Besttvaluesofo,Ea=kandTofromVFrelationforsamplesCHandCD63

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Figure5.13.ThebesttsofTpourdatatotheNeel-Arrheniusmodelfor6nmFe3O4.TheslightdierenceinoandEa=kforthesetwosamplesindicatesthattheglassynaturebetweenthemixedandfrozenstatesarealmostthesameifthemagneticnanopar-ticlesarealreadyblockedinthesestates.Thediscrepancyherecanarisefromthedierenceintheviscosityofthesolventsused,thatishexaneordodecane.Arecentpaperonthegeneraldynamicsofnanoparticlesinsupercooledliquidssuggestscomplexorganizationoftheparticlesduetohyperdiusivityinthevicinityoftheglasstransitionofthecarrieruid[78].Itisthelargervalueoftheviscosityofdodecane.35cPinsampleCDthatresultsinthelargervaluesofoandEa=kforthissamplewhencomparedtosampleCHhexane:0.294cP.Thisindicatesthattheviscosityofthesolventalsoplaysaroleinthemagneticallyglassybehaviorintheferrouids.Thisndingisimportantfromthetech-nologicalapplicationperspectivesincetheapplicationofamagneticeldtoaferrouidisshowntosignicantlychangeitsviscositythatinturnmodiesthemagneticbehavioroftheferrouid[8,10].Apartfromthis,wenotethatthedierenceinobetweenCHandCD,o=.80.4x10)]TJ/F19 7.97 Tf 6.587 0 Td[(6srefertoTable5.2ando=1x10)]TJ/F19 7.97 Tf 6.587 0 Td[(6,respectively,arisesfromthedierenceinparticlesize14nmforFe3O4and11nmforCoFe2O464

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andtheirmagneticproperties.ThehighervalueofToofCDKcomparedtothatofCHKsuggestsastrongerinterparticledipole-dipoleinteractioninthissample[77].Thisagreeswiththetrendthatthedipole-dipoleinteractionincreasesthecharacteristicrelaxationtimeoofaparticlesystem[79].OurcomprehensivestudiesonDCmagne-tization,ACsusceptibilityandpreviousworks[74,75,78,80,81]onmagneticrelaxationandagingphenomenaprovideevidenceoftheexistenceofalowtemperaturespin-glass-likephaseinferrouids,butweshoulddistinguishthisphasefromthatobservedintraditionalspinglasses[76].Inferrouidsthemagneticmomentofamagneticnanoparticleisseveralorderslargerthanatomicmomentsinspin-glassmaterials.Thetimeneededforspinip-pingofmagneticnanoparticleso10)]TJ/F19 7.97 Tf 6.586 0 Td[(6isthusmuchlongercomparedwiththatofanatomicspino10)]TJ/F19 7.97 Tf 6.587 0 Td[(13,anditdependsexponentiallyontheparticlesize[79,82].Asaresult,inferrouidsevenasmalldistributionoftheparticlesizecangiveabroadrangeofrelaxationtimes[78,80,81],contributingpartiallytotheobservedglassyphenomena.Thesamplescontainingsmallernanoparticleswereanalyzedinasimilarfashion.Forthe6nmFe3O4samples,thedatawerefoundtobestttheNeel-ArrheniusrelationandnottheVogel-Fulchermodel.Thebesttcurvesforsamples6FP,6FH,6FD,areshowninFigure5.13.ValuesoftheparametersextractedfromthebesttsaresummarizedinTable5.3.Fromthesevalues,itisclearthattherelaxationtimeisfasterforferrouidsinvolvinghighviscosityliquidssuchasdodecane. 6FP6FH6FD o.690.4x10)]TJ/F19 7.97 Tf 6.587 0 Td[(10s.070.5x10)]TJ/F19 7.97 Tf 6.587 0 Td[(11s.250.6x10)]TJ/F19 7.97 Tf 6.587 0 Td[(11sEa/k.110.6x102K.340.2x102K.220.1x102K Table5.3.Besttvaluesofo,Ea/kandTofromNeel-Arrheniusrelationfor6nmFe3O4samplesTable5.4summarizestheresultsoftheZFCandACstudiesdiscussedindicatingwhetherornotthemagneticanomalyispresentineachofthesamples.Wecansee65

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SampleTBZFCACAnomalyModel 14FHTB>TFsharppeaksinglepeakabsentNA14FDTBTFsharppeaksinglepeakabsentVF11CDTBTF,adoubleslopefeaturein0fisobservedinthemixedstatewhichevidentlyindicatescoexistenceofbothBrownianandNeelrelaxationinthis66

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state,while0fshowsonlyalinearlymonotonicdecreaseinthefrozenstate,signifyingthepresenceofonlyNeelrelaxation.FortheferrouidshavingTB
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CHAPTER6CONCLUSIONSANDFUTUREWORKTheachievementsofthisresearchmaybesummarizedasfollows:Wehaveinvestigatedtheinuenceofincreasingdipolarinteractionsontheblockingtemperature,coercivity,andthemagneticanisotropyinmonodispersedmanganesezincferritenanoparticles.Transversesusceptibilityhasbeenusedtodirectlyprobetheeectofinter-particleinteractionsontheanisotropyanditisobservedthattheanisotropyeldsincreasewithincreasinginteractions.TheasymmetryinTSpeaksisalsoassociatedwiththestrengthofinteractionswithweaklyinteractingparticlesexhibitingstrongasymmetryintheTSanisotropypeakswhereastheyaresymmetricforstronglyinteractingparticles.WehaveestablishedforthersttimeacorrelationbetweentheblockingtemperatureofmagneticnanoparticlesTBandthefreezingtemperatureofthesolventTFinferrouids.Thiscorrelationallowsonetoexplainthephysicaloriginsofrelaxationpeaksinthecomplexsusceptibilityandthespin-glass-likecuspandmagneticanomalyinthezero-eld-cooledmagnetizationinferrouids.ItisdemonstratedthatthemagneticanomalyisobservedforferrouidshavingTBTF.Theblockingeectofmagneticnanoparticlesplaysasignicantroleinestablishingthemagneticbehaviorinthemixedandfrozenstates,butnotintheliquidstate.Inthefrozenstatetheblockingeectofmagneticnanoparticlessignicantlyaectstheinterparticledipole-dipoleinteraction,causingspin-glass-likeslowdynamics.Theviscosityofthesolventalsoplaysaroleinthemagneticallyglassybehaviorintheferrouids.InthefrozenstatebothNeelandBrownrelaxationscoexistforferrouidshavingTBTFonlyNeelrelaxationispresent.68

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Wehavedemonstratedthattheparticleblockingandcarrierliquidfreezingeectsareimportanttotheformationoftheglass-likerelaxationpeaksinferrouids.Theseeectsareparticle-sizedependent.Thefreezingeectofthecarrierliquidonthemagnetizationdynamicsofferrouidscontaininglargermagneticnanoparticlesismorepronouncedthanthatcontainingsmallermagneticnanoparticles.Eventhoughferrouidshavebeenstud-iedfordecadesandtheyareusedinseveraltechnologicalapplications,ourresultsinthisresearchindicatesubtleinterplaybetweentheparticlesizeeectsandcarrieruidproper-ties.Thusthesetwopropertiescanbesuitablytunedtoproduceengineeredferrouidsforspecicapplications.Extremecasesofnon-interactingandinteractingparticlesystemshavebeenreportedinthisworkandthepropertiesofferrouidscontainingnanoparticlesofintermediatesize10nmarecurrentlybeingexplored.Moreover,theeectofvaryingthenanoparticleconcentrationinferrouidsandperformingtransversesusceptibilitysusceptibilitymeasure-mentsarebeingconsideredforfuturework.FerrouidsthatexhibitaverysharppeakintheMvs.Tcurvearepotentiallyusefulinmagneticrefrigerationapplications.Thebasisofthisisthatthesharptransitionindicatesalargeentropychange,whichisacommonfeatureofmagnetocaloricmaterials.Theongoingworkonmagnetocaloricmaterialsinourlaboratorywouldenableustoconsiderexplorationofthesephenomenainferrouids.WehavealsorecentlyaddedaheatcapacityprobetoourPPMS.Thiswouldopenupnewresearchavenuesforfuturestudentsinthelaboratoryintermsofstudyingthethermomagneticpropertiesofferrouids.FutureworkonferrouidsmaybeextendedtobiologicalsamplestoimprovemagneticresonanceimagingMRIcontrastandhyperthermiafortumortherapy.Preliminaryworkonhyperthermiastudiesthathasbeendoneincludesynthesisofwater-basedferrouidsandsettingupofaroomtemperatureexperimentformeasuringdissipativeheating.Thesyn-thesistechniquesdescribedinthisthesisweremodiedtomakebiocompatibleferrouids.IronoxidenanoparticleswerecoatedusingeithercitricacidorpolyethyleneglycolPEGtoenablethemtosuspendinwater.Anotherschemethatwasemployedwastoconvert69

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oil-basedferrouidstowater-basedones.Thishasbeencarriedoutbytheauthorusingcontrolledamountsofblockcopolymer,particularlyPluronic157,dissolvedinwaterandmixedwiththeas-synthesizednanoparticlesstoredinavolatileorganicsolvent.Fromthisprocedure,verystablesuspensionswereproduced.Shelvedforalmostayearandoccula-tionintheseferrouidshasnotbeenobserved.Suchferrouidsareexcellentforbiomedicalapplicationssuchashyperthermiacancertreatment.Aroomtemperatureexperimenthasbeensetuptomeasuretheenergydissipatedthroughrelaxationofthesuperparamagneticnanoparticles.Itisasimplesetupthatmakesuseofhand-woundcoilsandtheonlymajorcomponentofthesetupisthesourceoftheexternalACmagneticeldcoupledwithanamplierthatoperatesupto120kHz.Themainchallengeistoachieveheatingupto46CusinganACeldof120kHzorlessasoperatingfrequencyandminimalamplitude.Itisalsoimportanttodeterminetheoptimalsizeofthenanoparticlesthanwillgeneratesuchenergy.70

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APPENDICES77

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AppendixAListofPublicationsJournalArticlesM.B.Morales,M.H.PhanandH.Srikanth.EectofcarrierliquidsonmagneticrelaxationinFe3O4-basedferrouids,InProgressM.B.Morales,M.H.Phan,N.A.Frey,andH.Srikanth.ParticleblockingandcarrieruidfreezingeectsonthemagneticpropertiesofFe3O4-basedferrouids.J.Appl.Phys.105,07B511.M.H.Phan,M.B.Morales,C.N.Chinnasamy,B.Latha,V.G.HarrisandH.Srikanth,MagnetocaloriceectinnanostructuredGd3Fe5O12garnetnanoparticles,J.Phys.D:Appl.Phys.42M.H.Phan,M.B.Morales,H.Srikanth,C.L.ZhangandS.W.Cheong.PhasecoexistenceandgiantmagnetocaloriceectinLa,Pr,CaMnO3,Submitted,Phys.Rev.B,2009S.Pal,M.B.Morales,P.MukherjeeandH.Srikanth.Synthesisandmagneticpropertiesofgoldcoatedironoxidenanoparticles,J.Appl.Phys.105,07B504P.Poddar,M.B.Morales,N.A.Frey,S.A.Morrison,E.E.CarpenterandH.Srikanth.Transversesusceptibilitystudyoftheeectofvaryingdipolarinteractionsonanisotropypeaksina3Dassemblyofsoftferritenanoparticles,J.Appl.Phys.104,063901ConferenceProceedingsN.A.Frey,M.B.Morales,H.Srikanth,andS.Srinath.Transversesusceptibilityasaprobeofinterfacemagnetisminfunctionalmultilayersandnanostructures.Ency-clopediaofAdvancedMaterials:ScienceandEngineeringPanStanfordPublishers,inpress2008.78

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AppendixAContinuedJ.Gass,N.A.Frey,M.B.Morales,M.J.Miner,S.SrinathandH.Srikanth.Mag-neticanisotropyandmagnetocaloriceectMCEinNiFe2O4nanoparticles.Mate-rialsResearchSocietySymposiumProceedings,962,0962-P05-03.79

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AppendixBListofConferencePresentationsConferencePresentationsM.B.Morales,J.GassandH.Srikanth.MagneticpropertiesofFe3O4andCoFe2O4ferrouids,SessionW14.10,AmericanPhysicalSocietyMarchMeeting,Denver,Colorado.M.B.Morales,J.GassandH.Srikanth.Acomparativestudyofmagneticproper-tiesofhexaneanddodecanebasedferrouidswithFe3O4andCoFe2O4nanoparticledispersions,SessionEP-02,52ndMagnetismandMagneticMaterialsConference,Tampa,Florida.M.B.Morales,P.Poddar,N.A.Frey,H.Srikanth,S.A.Morrison,E.E.Car-penter.Probingtheeectofinterparticleinteractionsinferritenanoparticlesusingtransversesusceptibility,SessionD27.7,AmericanPhysicalSocietyMarchMeeting,NewOrleans,Louisiana.M.B.Morales,P.Poddar,N.A.Frey,H.Srikanth,S.A.Morrison,E.E.Car-penter.Probingtheeectofinterparticleinteractionsinferritenanoparticlesus-ingtransversesusceptibility,1stIEEEMagneticsSocietySummerSchool,ColoradoSprings,Colorado.M.B.Morales,M.H.Phan,N.A.Frey,S.PalandH.Srikanth.OriginofGlass-likePeaksandMagneticRelaxationinFerrouids,SessionDR-05,53ndMagnetismandMagneticMaterialsConference,Austin,Texas.M.B.Morales,M.H.Phan,N.A.Frey,S.PalandH.Srikanth.Originofmag-neticanomaliesandrelaxationmechanismsinferrouids,SessionZ31.3,AmericanPhysicalSocietyMarchMeeting,Pittsburgh,Pennsylvania.80


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Magnetization dynamics and interparticle interactions in ferrofluids and nanostructures
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ABSTRACT: Nanoparticle assemblies are of current interest as they are used in a wide variety of industrial and biomedical applications. This work presents two studies aimed at understanding the magnetization dynamics and interparticle interactions in nanoparticle assemblies and various types of ferrofluids. First, we studied the influence of varying strengths of dipolar interaction on the static and dynamic magnetic properties of surfactant-coated monodispersed manganese-zinc ferrite nanoparticles using reversible transverse susceptibility. We tracked the evolution of the anisotropy peaks with varying magnetic field, temperature, and interaction strength. The anisotropy peaks of weakly interacting particles appears as non-symmetric peaks and at lower fields in a unipolar transverse susceptibility scan. On the other hand, a strongly interacting particle system exhibits symmetric anisotropy peaks situated at higher field values.In the second study, we successfully synthesized stable ferrofluids out of high quality FeO and CoFeO nanoparticles. Such ferrofluids are excellent systems for the investigation of physics of relaxation phenomena in magnetic nanoparticles. Motivated by the need to understand their peculiar magnetic response, a comparative study on FeO- and CoFeO-based ferrofluids was performed. We investigated cases in which particle blocking and carrier fluid freezing temperatures were close and far apart from each other. Our experimental results reveal the true origin of the glass-like relaxation peaks that have been widely observed in ferrofluids by many groups but remained largely unexplained. Contrary to the speculation of previous literature, we argue that the formation of the magnetic anomaly is due not only to the particle blocking but also to its correlation with the the carrier fluid freezing effects.It is also shown that the nature of these peaks is strongly affected by varying particle size and carrier fluid medium. Quantitative fits of the frequency dependent AC susceptibility to the Vogel-Fulcher scaling law clearly indicate that the blocking of magnetic nanoparticles in the frozen state significantly affects the interparticle dipole-dipole interaction, causing characteristic spin-glass-like dynamics. A clear correlation between the blocking and freezing temperatures emerges from our studies for the first time.
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