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Ferrite-ferroelectric thin films with tunable electrical and magnetic properties

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Title:
Ferrite-ferroelectric thin films with tunable electrical and magnetic properties
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Book
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English
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Heindl, Ranko
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University of South Florida
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Tampa, Fla
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Subjects / Keywords:
BaFe12O19
Ba0.5Sr0.5TiO3
PLD
Sputtering
Bilayers
Multilayers
Composites
Coplanar waveguides
Microwave characterization
Dissertations, Academic -- Applied Physics -- Doctoral -- USF
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bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )

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Abstract:
ABSTRACT: A growing need for developing new multi-functional materials operating at microwave frequencies is demanding a better understanding of ferroelectric and ferrimagnetic materials and their combinations. Some of these materials exhibit tunable physical properties, giving an extra degree of freedom in the device design. New multifunctional ferroelectric and ferrimagnetic thin film structures are investigated in this dissertation research, in which dielectric and magnetic properties can separately be tuned over a certain frequency range. The materials of choice, Ba0.5Sr0.5TiO3 (BST) and BaFe12O19 (BaM), both well studied and used in many microwave applications, were prepared using rf magnetron sputtering and pulsed laser ablation. Thin-film bilayers, multilayers and composite thin films were grown on various substrates, and their underlying microstructure and crystallographic properties were analyzed and optimized. After identifying the most successful growth conditions,dielectric and magnetic properties were measured. Unusual features in magnetic hysteresis loops in both sputtered and laser ablated films grown under different conditions were observed. Microcircuits were fabricated using optical lithography and microwave properties and tunability were tested in the range 1-65 GHz.
Thesis:
Dissertation (Ph.D.)--University of South Florida, 2006.
Bibliography:
Includes bibliographical references.
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Mode of access: World Wide Web.
Statement of Responsibility:
by Ranko Heindl.
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Title from PDF of title page.
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Document formatted into pages; contains 126 pages.
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Includes vita.

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aleph - 001915616
oclc - 180703264
usfldc doi - E14-SFE0001782
usfldc handle - e14.1782
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Ferrite-ferroelectric thin films with tunable electrical and magnetic properties
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ABSTRACT: A growing need for developing new multi-functional materials operating at microwave frequencies is demanding a better understanding of ferroelectric and ferrimagnetic materials and their combinations. Some of these materials exhibit tunable physical properties, giving an extra degree of freedom in the device design. New multifunctional ferroelectric and ferrimagnetic thin film structures are investigated in this dissertation research, in which dielectric and magnetic properties can separately be tuned over a certain frequency range. The materials of choice, Ba0.5Sr0.5TiO3 (BST) and BaFe12O19 (BaM), both well studied and used in many microwave applications, were prepared using rf magnetron sputtering and pulsed laser ablation. Thin-film bilayers, multilayers and composite thin films were grown on various substrates, and their underlying microstructure and crystallographic properties were analyzed and optimized. After identifying the most successful growth conditions,dielectric and magnetic properties were measured. Unusual features in magnetic hysteresis loops in both sputtered and laser ablated films grown under different conditions were observed. Microcircuits were fabricated using optical lithography and microwave properties and tunability were tested in the range 1-65 GHz.
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BaFe12O19.
Ba0.5Sr0.5TiO3.
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Composites.
Coplanar waveguides.
Microwave characterization.
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Ferrite-FerroelectricThinFilmsWithTunableElectricalAndMagneticPropertiesbyRankoHeindlAdissertationsubmittedinpartialfulllmentoftherequirementsforthedegreeofDoctorofPhilosophyinPhysicsDepartmentofPhysicsCollegeofArtsandSciencesUniversityofSouthFloridaMajorProfessor:HariharanSrikanth,Ph.D.MyungK.Kim,Ph.D.PritishMukherjee,Ph.D.ThomasWeller,Ph.D.DateofApproval:November8,2006Keywords:BaFe12O19,Ba0:5Sr0:5TiO3,PLD,sputtering,bilayers,multilayers,composites,coplanarwaveguides,microwavecharacterizationcCopyright2006,RankoHeindl

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DEDICATIONTomyparentsDunja&Vlatko,mywifeKarol,andmywonderfulgrandmaEnisa-2005.

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ACKNOWLEDGMENTSIwouldliketothankmyadvisorDr.SrikanthHariharanforhisguidanceandyearlongsupport,Dr.TomWellerandhisstudentsSrinathBalachandranandSaravanaNatarajanfortheirassistancewithmicrolithographyandmicrowavemeasurements,USF-WAMILabforuseoftheirequipment,Dr.KevinCoey,Dr.NancyDudney,Dr.PritishMukherjee,andDr.SarathWitanachichifortheirhelpwiththethinlmpreparationandcharac-terization,andallowingmetousetheirfacilities,Dr.NagarajaHosakoppaandRobertHydeforassistancewithPLD,Dr.GarrettMatthewsandAugustHeimforassistancewithAFM,Dr.GopalanSrinivasanformicrowaveresonantmeasurements,Dr.DavidRabsonforsupportwithLATEX,myparentsDunjaandVlatkoforbelievinginmeandsupportingme,mywifeKarolforlovingmeandalwaysbeingthereforme,mydogLapiz,therestofmyfamily,andnallyallmyfriendsandcolleaguesfromUSFinparticularJeSanders,Dr.SrinathSanyadanamandNatalieFrey,aswellasalltheUSF-PhysicsandUSF-NNRCstamembers.Iamgratefulfora2-yearNSF-IGERTfellowshipfromtheUSFSKINSprogramthroughNSFIGERTGrantNo.DGE-0221681,andthenancialsupportfromAROGrantNo.DAAD19-03-1-0277andSURASummerResearchGrant.

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TABLEOFCONTENTSLISTOFTABLESiiiLISTOFFIGURESivLISTOFABBREVIATIONSviiiABSTRACTxiCHAPTER1INTRODUCTION11.1LiteratureReview41.2DissertationOutline7CHAPTER2REVIEWOFBASICPROPERTIESOFFERRITESANDFERRO-ELECTRICS82.1Ferrites82.1.1MagneticAnisotropy112.1.2FerrimagneticResonance112.1.3Barium-Hexaferrite132.2Ferroelectrics152.2.1Barium-Strontium-Titanate152.3MicrowavePropertiesofMagneticandDielectricOxides172.4CharacteristicsofMicrowaveDevices19CHAPTER3EXPERIMENTALMETHODS213.1ThinFilmDeposition213.1.1RFMagnetronSputtering213.1.2PulsedLaserDeposition223.2StructuralCharacterization233.2.1X-RayDiraction243.2.2ScanningElectronMicroscopy273.2.3AtomicForceMicroscopy283.3MagneticCharacterizationwithPPMS293.4MicrowaveCharacterizationofThinFilms303.4.1ExperimentalSet-Up303.4.2Micro-CircuitDesign333.4.3Micro-CircuitFabrication403.4.4NetworkAnalyzerMeasurements423.4.4.1ProbeCalibration43i

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3.4.5FerrimagneticResonanceMeasurements453.5Summary45CHAPTER4BAM-BSTCOMPOSITETHINFILMS474.1CompositeThinFilms474.2Cobalt-ImplantationofCompositeThinFilms504.3Summary54CHAPTER5SPUTTEREDBAM-BSTTHINFILMMULTILAYERS555.1ThinFilmMultilayersSputteredatORNL555.2ThinFilmMultilayersSputteredatUCF605.3Summary61CHAPTER6PULSED-LASERDEPOSITEDBAM-BSTBILAYERSANDCOM-POSITETHINFILMS636.1BaM-BSTbilayeronC-sapphire666.2BaM-BSTbilayeron100-MgO736.3BaM-BSTbilayeronA-sapphire786.4BaM-BSTcompositeonC-sapphire826.5BaM-BSTcompositeonA-sapphire866.6Summary89CHAPTER7MICROWAVEPROPERTIESOFBAM-BSTBILAYERSANDCOM-POSITETHINFILMS907.1CoplanarWaveguidePhaseShifters907.2ElectricalTunability927.3MagneticTunability1017.4SimulationsUsingCommercialSoftwarePackages1087.5MicrowaveMeasurementsatOaklandUniversity1097.6Summary110CHAPTER8CONCLUSIONANDFUTUREWORK111REFERENCES113APPENDICES123AppendixAListofPublications124AppendixBListofConferencePresentations126ABOUTTHEAUTHOREndPageii

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LISTOFTABLESTable2.1Summaryofferritecrystalstructuretypes9Table3.1XRDparameters24Table3.2Substratesparameters34Table3.3Tapersdimensionsfordierentsubstrates35Table3.4Maskdesignparameters37Table4.1DepositionparametersofthincompositelmssputteredatORNL.47Table4.2Cobalt-implantationprocessparameters.50Table4.3MagneticparametersoftheCobalt-implantedcompositethinlms.53Table5.1Multilayersputteringparameters.56Table5.2Coercivitiesofthemultilayerthinlms58Table5.3Dependenceofthinlmcoercivityonanisotropy58Table5.4UCFmagnetronsputteringdepositionparameters60Table5.5PartialresultsfromsputteringatUCF61Table6.1Pulsed-laserdepositionparameters63Table6.2MagneticcharacteristicsofBST/BaMbilayergrownonMgOsubstrate.76iii

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LISTOFFIGURESFigure1.1Non-licensedbandsandtypicalapplications2Figure2.1MagnetizationCurve11Figure2.2Magneticanisotropyofahexagonalcrystal12Figure2.3FerrimagneticResonance13Figure2.4Ba-hexaferritecrystalstructure14Figure2.5Changeofpermittivitywithappliedvoltage15Figure2.6CrystalstructureofBarium-Strontium-Titanate.16Figure2.7Exampleofmagnetictunability18Figure2.8Relaxationmechanisms20Figure3.1Sputteringdepositionprocess22Figure3.2Pulsed-laserdepositionprocess23Figure3.3X-rayDiraction25Figure3.4ThinFilmOrientation26Figure3.5X-rayrockingcurve27Figure3.6ScanningElectronMicroscope28Figure3.7Atomicforcemicroscopeblockdiagram29Figure3.8MagneticMeasurements30Figure3.9High-frequencymeasurementset-up32Figure3.10ElectromagneticelddistributioninCPW33Figure3.11TransmissionLineCalculatorprogram.34Figure3.12Magneticeldvectors36iv

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Figure3.13CPWmaskdesigndetails38Figure3.14CPWmaskdesign39Figure3.15Microlithographyprocedurewithnegativephotoresist.41Figure3.16Problemswithfabrication42Figure3.17NetworkAnalyzer43Figure3.18SOLTcalibration44Figure3.19Resonantcavity45Figure4.1XRDofthecompositethinlm.48Figure4.2SEMofthecompositethinlm.48Figure4.3Magnetizationofthecompositethinlm.49Figure4.4SEMoftheCo-implantedcompositethinlm.51Figure4.5M-HloopsoftheCo-implantedcompositethinlms.52Figure5.1X-raydiractionofBaM-BSTmultilayers.56Figure5.2MagnetizationofBaM-BSTmultilayer.57Figure5.3Cross-sectionalSEMscanofBaM-BSTmultilayeronsapphire.61Figure6.1Crystalplaneorientations65Figure6.2PLDdepositedthinlmsonC-sapphire66Figure6.3-2scanandrockingcurvesofC-BaM-BSTbilayer.68Figure6.4CrystallographictextureanalysisofofC-BaM-BSTbilayer.69Figure6.5M-HloopsofthinlmsonC-sapphiresubstrate.70Figure6.6Bilayerthinlmgrowthduringdeposition71Figure6.7AFMofBaMonC-sapphire72Figure6.8Resultsfrom[105]72Figure6.9BST-BaMbilayerthinlmson100-MgO.73Figure6.10-2scanandrockingcurvesofMgO-BST-BaMbilayer.74Figure6.11CrystallographictextureanalysisofofMgO-BST-BaMbilayer.75v

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Figure6.12M-HloopofBaM-BSTbilayeronMgOsubstrate.77Figure6.13AFMofMgO/BST/BaMbilayer77Figure6.14PLDdepositedthinlmsonA-sapphire78Figure6.15-2scanandrockingcurvesofA-BaM-BSTbilayer78Figure6.16CrystallographictextureanalysisofofA-BaM-BSTbilayer.79Figure6.17M-HloopsofthinlmsonA-sapphiresubstrate.81Figure6.18AFMofBaMonA-sapphire81Figure6.19BaM-BSTcompositethinlmonC-sapphire.82Figure6.20-2scanandrockingcurvesofC-BaM-BSTcompositelm.83Figure6.21CrystallographictextureanalysisofofC-BaM-BSTcomposite.85Figure6.22M-HloopsofcompositethinlmsonC-sapphiresubstrate.85Figure6.23BaM-BSTcompositethinlmonA-sapphire86Figure6.24-2scanandrockingcurvesofA-BaM-BSTcompositelm.87Figure6.25CrystallographictextureanalysisofofA-BaM-BSTcomposite.88Figure6.26M-HloopsofcompositethinlmsonA-sapphiresubstrate.88Figure7.1BiasingofCPWphaseshifter.92Figure7.2Figuresofmeritofelectricaltunability.98Figure7.3Electricalphaseshift.99Figure7.4S-parametersofbilayerthinlmunderbiasE-elds.100Figure7.5Magnetictunability.101Figure7.6MagnetictunabilityoftheBaMonA-sapphire.102Figure7.7Resonantfrequencyvs.eldforA-BaMlm.103Figure7.8MicrowavepropertiesofBaMthinlmsonC-sapphire.104Figure7.9MicrowavepropertiesofBaM-BSTbilayeronA-sapphire.106Figure7.10MicrowavepropertiesofBST-BaMbilayeronMgO.107Figure7.11Magnetictunabilityofcompositelm.108vi

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Figure7.12MicrowaveabsorptionmeasurementsofA-BaM-BSTbilayer109vii

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LISTOFABBREVIATIONSADSAdvancedDesignSystemAFMAtomicForceMicroscopyACAlternatingCurrentAluminaPolycrystallineAlumina-Al2O3A-SapphireSapphiresubstratewithA-planeorientation"ATMAtmosphereAuGoldBaBariumBaMBarium-Iron-Oxide,Barium-Hexa-Ferrite,BaFe12O19BSTBarium-Strontium-TitanateBa0:5Sr0:5TiO3C,C0CapacitanceCoCobaltCPWCoplanarWaveguideCrChromeC-SapphireSapphiresubstratewithC-planeorientation"dBDecibelDCDirectCurrentDUTDeviceUnderTest0PermittivityofVacuumrRelativeDielectricConstant,RelativePermittivityemuMagneticMomentUnitElectromagneticUniteVElectron-Voltviii

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FMRFerrimagneticferromagneticResonanceFOMFigureofMeritFWHMFull-Width-Half-MaximumGCoplanarWaveguideGapBetweenSignalandGroundLinesGGroundConnectorGSGGround-Signal-GroundHMagneticFieldHaAnisotropyFieldHcCoerciveFieldCoercivityIDCIntedigitatedInterdigitalCapacitorKKelvin0PermeabilityofVacuumrRelativePermeabilityM,M-DCDC-MagnetizationMrRemanentMagnetizationMsSaturationMagnetizationMgOMagnesium-OxideM-HMagnetizationvs.AppliedMagneticFieldPLDPulsedLaserDepositionPPMSPhysicalPropertiesMeasurementSystemPPSPulsesPerSecondRFRadio-FrequencyRTRoomTemperatureSCCMStandardCubicCentimetersPerMinuteSEMScanningElectronMicroscopySiSiliconSubstrateSOLTShort-Open-Load-ThruS-parametersScatteringParametersix

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SrStrontiumTTeslatanLossTangentTEMTransverseElectromagneticTiTitaniumTRLThru-Reect-LineUVUltravioletVNAVectorNetworkAnalyzerWCoplanarWaveguideCentral-Conductor-WidthXRDX-RayDiractionZ0CharacteristicImpedancex

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Ferrite-FerroelectricThinFilmsWithTunableElectricalAndMagneticPropertiesRankoHeindlABSTRACTAgrowingneedfordevelopingnewmulti-functionalmaterialsoperatingatmicrowavefrequenciesisdemandingabetterunderstandingofferroelectricandferrimagneticmaterialsandtheircombinations.Someofthesematerialsexhibittunablephysicalproperties,givinganextradegreeoffreedominthedevicedesign.Newmultifunctionalferroelectricandferrimagneticthinlmstructuresareinvestigatedinthisdissertationresearch,inwhichdielectricandmagneticpropertiescanseparatelybetunedoveracertainfrequencyrange.Thematerialsofchoice,Ba0:5Sr0:5TiO3BSTandBaFe12O19BaM,bothwellstudiedandusedinmanymicrowaveapplications,werepreparedusingrfmagnetronsputteringandpulsedlaserablation.Thin-lmbilayers,multilayersandcompositethinlmsweregrownonvarioussubstrates,andtheirunderlyingmicrostructureandcrystallographicpropertieswereanalyzedandoptimized.Afteridentifyingthemostsuccessfulgrowthconditions,dielectricandmagneticpropertiesweremeasured.Unusualfeaturesinmagnetichysteresisloopsinbothsputteredandlaserablatedlmsgrownunderdierentconditionswereobserved.Microcircuitswerefabricatedusingopticallithographyandmicrowavepropertiesandtunabilityweretestedintherange1-65GHz.xi

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CHAPTER1INTRODUCTIONOverthepasttenyears,therehasbeenasustainedeorttointegrateexistingferri-magneticandferroelectricmaterialsinmonolithicmicrowaveintegratedcircuitMMICtechnology,inwhichferriteandferroelectricelementstogetherwithotheractiveelementsareplacedonacommonsemiconductorsubstrate[10,115].Thebenetsofsuchintegrationcanincludesomeorallofthefollowing:smallerdevicesize,lowermanufacturingcosts,betterreliability,betterperformance,andlowerparasitics.However,therearenumerouschallengessuchasthinlmgrowthandprocessing,matchingthelmtothesubstrate,controllinglmstoichiometry,microstructure,stressesanddefects.Thisworkpresentsanextensiveresearcheortindevelopmentofferrimagnetic-ferroelectricthinlmstructuresthatexhibittunablepropertiesatmicrowavefrequencies.Ferritematerialsintheformofhighqualitythinlmshavethepotentialtoreplacebulkyexternalmagnetsincurrentmicrowavedevices[10].Theycanprovideuniquecircuitfunctionsthatcannotbereproducedwithanyothermaterials.Ferritematerialshavelowlossesatmicrowavefrequencies,highresistivityandstrongmagneticcoupling,makingthemirreplaceableconstituentsinmicrowavedevicetechnology.Ontheotherhand,ferroelectricshavebeenknownforover30years[43],yetonlysincetheimprovementsinmicrotechnologyandthin-lmdepositiontechniquesintheearly1990'shavetheycreatedrenewedinterestinthescienticcommunity[115].Incertainfer-roelectricmaterialssuchasBaTiO3,BaxSr1)]TJ/F19 7.97 Tf 6.586 0 Td[(xTiO3,SrTiO3andPbZr,TiO3,complexpermittivitycanbevariedusinganappliedelectriceld[76,99,114,115,117,128].Ferro-electricmaterialsareimportantforthenewgenerationoftunablephaseshifters,varactors,circulators,andantennas.TheadvantageofusingferroelectricthinlmsishighQatmi-1

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crowaveandmillimeterwavefrequencies,lowdcpowerconsumption,highrfpowerhandlingcapabilities,andlowmanufacturingcosts[76,99,114,117,128].Theproblemwithinte-gratingtunableferroelectricsintomicrowavedevicesisthatthepropertiesoffabricatedthinlmsaremuchpoorerthantheoreticallypredicted[115].Thereasonliesinthestrongdependenceofferroelectricpropertiesonthelmqualitymicrostructure,composition,andorientation.Mostofthestudiestodatehavebeendonetoaddresstheseissuesandimprovethetunability. Figure1.1.Adiagramofnon-licensedbandsandtheirtypicalapplications[71].In2001,theFederalCommunicationsCommissionFCC[4]allocatedacontinuousblockbetween57-64GHzfornon-licensedandcommercialwirelessapplicationsgure1.1.Themillimeter-wavepartoftheelectromagneticspectrumhasbeenmostlyunexploitedforcommercialwirelessapplications.Atthefrequenciesaround60GHz,theO2moleculehighlyabsorbstheelectromagneticradiation,whichenablesmoresignalstobeusedinthesamegeographicareawithoutinterference,aswellasincreasedsecurityduetotherapidattenuationofthesignal.Microwavedevicesneedtobedesigned,utilizingmaterialsthat2

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operateatthesefrequencies.Bothferritesandferroelectricsshowexcellentpropertiesinthismillimeter-waveportionoftheelectromagneticspectrum.Thisisjustoneexampleofthepossiblecommercialimpactofthesematerials.Thegoalofthisthesisistogrowferrite-ferroelectricmultilayerandcompositethinlmsincorporatingferrimagneticmaterialbarium-ferriteBaFe12O19,BaMandferroelectricmaterialbarium-strontium-titanateBa0:5Sr0:5TiO3,BSTonvarioussubstratesandstudytheirgrowthconditions,underlyingmicrostructure,electric,magneticandhighfrequencyproperties,aswellaselectricalandmagnetictunability.Characteristicimpedanceandphasevelocityofamaterialaredenedas:Zc/q andvp/1 p ,where,isitsmagneticpermeability,andisitselectricpermittivity[100].Inanonlinearmedium,applyinganelectriceldwillchangeits.Inasimilarway,aferrimagneticmaterialwillchangeitsinanappliedmagneticeld,althoughthepotentialformagnetictuning=issmallerthantheelectrictuning=[10,115].Thus,inamicrowavedevicecontainingbothferroelectricandferrimagneticmaterials,frequencyandphasecharacteristicscanbetunedintwoways:electricallyandmagnetically.ForcertainapplicationsthecharacteristicimpedanceZcandthephasevelocityvpshouldre-mainindependentofthetuning,whileforotherapplicationsimpedancetuningisdesirable[61,67,115].Bothcasescanbeachievedwithdualtuning.Themaingoalistoseeifbyusingferroelectric-ferrimagneticthinlmsitispossibletocreateadualtunablemicrowavematerialinwhichitsimpedanceandphasevelocitycanbetunedelectricallyandmagnet-ically.Thisresearchcouldprovidefurtherknowledgeaboutgrowthandcharacteristicsofferroelectricandferrimagneticmaterialsinthinlmform,andisimportantforthedevel-opmentoffuturemicrowavedevices.TheBaM/BSTsystemhasnotbeenstudiedbefore,andmoreoverthereisalackofsystematicresearchinthisarea,intermsofcorrelatingthestructure,magnetic,dielectricandfrequency-dependentproperties.3

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1.1LiteratureReviewTheeldofmagneto-dielectricmaterialshasseenincreaseinpublicationsintherecentyears,andsomeoftheresultsarepromisingforfuturetechnologies.Belowisashortsum-maryofthecurrentresearchontunableferroelectric-ferromagneticmaterialsthatwouldplacethisworkinperspectiveofcurrentstate-of-the-artworkintunablemicrowavethinlms:Themostcloselyrelatedpapertothisresearchis"ElectricallyandmagneticallytunablemicrowavedeviceusingBa,SrTiO3/Y3Fe15O12multilayer"[67].ABSTthinlmwasdepositedonaYIGsubstrate,followedbyaAu/Agcoplanarwaveguideonthetop.Adierentialphaseshiftwasmeasuredasafunctionoffrequency,electricandmagneticelds.Itwasreportedthata170/cmdierentialphaseshiftwasobtainedat10GHzwith21kV/cmofelectriceldand160Gaussofmagneticeld,whichissmallerthanresultsreportedinchapter7.Nofurtherresearchhasbeendoneonthissystem.Thepaper"Magneticallyandelectricallytunabledevicesusingferromagnetic-fer-roelectricceramics"[65]reportsonmagneticandelectrichysteresisloopsaswellaselectricaltunabilityofvariousbulkcompositesampleswithdierentratiosofgarnetferriteandferroelectricPbZr0:52Ti0:48O3PZT.Ahigherelectricaltunabilityforthecompositematerialwasobtainedcomparedtothepureferroelectricmaterial.AstudyofaheterostructureddeviceutilizingferroelectricBSTandferromagneticLSMOLa,SrMnO3thinlmswaspresentedinthepaper"HighfrequencytunableLCdeviceswithferroelectric-ferromagneticthinlmheterostructure"[132].C-Vcapacitance-voltageandL-Vinductance-voltagetunabilitywasmeasuredat1MHz,andhighertunabilitywasreportedbytuningbothLandC,comparedtotuningonlyLorC,whilemonitoringthechangeofthecharacteristicimpedance.4

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Inthepaper"Integrationofnonlineardielectricbariumstrontiumtitanatewithpoly-crystallineyttriumirongarnet"[61],biaxiallyorientedBSTlmsweregrownonpoly-crystallineYIGsubstrateswithanMgObuerlayertoimprovetheBSTcrystallinity.Goldcoplanarwaveguidesweredepositedontopofthemultilayeredstructureandcapacitanceanddielectriclossweremeasuredat100kHz.Bothandori-entedBSTlmswerereported,withtunabilitygreaterthen25%withanapplieddcvoltageof40V,anddecreaseindielectriclosswithanincreaseofthedcbiasvoltage.Howevernodataonmagnetictunabilitywasreported.Thepaper"Preparationofall-oxideferromagnetic-ferroelectric-superconductinghet-erostructuresforadvancedmicrowaveapplications"[53]presentsanelectricallyandmagneticallytunableLa0:70Sr0:30MnO3LSMO-PbZr0:52Ti0:48O3PZT-YBCOmul-tilayer.Ferroelectricandferromagneticpropertiesweremeasured,butnodataonhighfrequencypropertiesortunabilitywasgiven.Stackeddielectric-ferromagneticstructuresweredescribedinthepaper"Correlationbetweenmagneticpropertiesoflayeredferromagnetic-dielectricmaterialandtunablemicrowavedeviceapplications"[101],inwhichthickdielectriclayersofKaptonwereusedtocompensateforlossesinmetallicferromagneticCoNbZrthinlms.Usingapplieddcmagneticeld,theimpedanceofthelayeredstructurewastuned.Attheend,thestructurewasutilizedintoatunablebandstoplterandamagneticswitch.Thepaper"AdditionEectofBariumandStrontiumTitanateontheReectiv-ityofFerriandFerromagneticComposites"[98]describesmixingofnepowdersofCo2+-andTi4+-dopedbariumhexaferriteCo-TiBaMandbariumstrontiumti-tanateBST.Transmissionandreectionpropertiesatmicrowavefrequencieswereinvestigated.AdditionoftheBSTresultedinanincreaseinmicrowaveabsorption.Sometheoreticalandexperimentalworkonelectricallytunableferrite-ferroelectricphaseshifterwasreportedinthepaper"ElectricalTuningofDispersionCharacteris-ticsofSurfaceElectromagnetic-SpinWavesPropagatinginFerrite-FerroelectricLay-5

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eredStructures"[29].Ferromagneticyttrium-iron-garnetlmsgrownongadolinium-gallium-garnetsubstratesandferroelectricBa0:65Sr0:35TiO3thinlmswereused.Dispersioncharacteristicsandphaseshiftofsurfacespinwavescontrolledbyadcvoltageweremeasuredandcalculated.Signicantimprovementsinelectricaltun-abilitywasreported.Recently,tworeportsonelectricallyandmagneticallytunablemicrowaveresonatorsincorporatingferrimagneticYIGandferroelectricmaterialsBSTandPZTwerepublished[36,103].Theoperatingfrequencywasaround5GHz.Theferroelectricmaterialwasfabricatedinametal-insulator-metalcongurationandlargevoltagesneededtobeappliedforelectricaltuningupto1000V.Thestructureswerenotmultilayerlms,butthinbulkpiecesbondedtogether.Bothpaperscommentonadvantagesofusingdualtunabledevices.Thisshortreviewofthepublicationsrelevanttoelectricallyandmagneticallytunablematerialsshowsanincreaseininterestoverthepastfewyears,andnumerousresearchpos-sibilities.However,theinformationavailableisfairlyincomplete,limitedtolowfrequenciesbelow10GHzandlackssystematicresearch.Inaddition,manyoftheinvestigatedsam-plesareinbulkformoratleastonecomponentisbulksubstrate.Inordertomakeitattractiveforpotentialapplications,thematerialsneedtobedepositedinthinlmformoncommonlyusedmicrowavesubstrates.Thepurposeofthisresearchistoinvestigatethecombinationsofferriteandferroelectricthinlmsformicrowaveapplications.6

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1.2DissertationOutlineChaptertworeviewsthemainpropertiesofferrimagneticmaterialsferritesandfer-roelectrics,aswellastheirpropertiesatmicrowavefrequencies.Chapterthreegivesanoverviewofallexperimentaltechniquesandmeasurementproceduresusedinthisdisserta-tion.Chaptersfourandvearesummariesofauthor'sworkonsputteredthinlms,whichresultedinhismaster'sthesisaswellasfourpublications.Chapterssixandsevenarethemainpartofthisdissertation.Chaptersixdescribesthefabricationandresultsofcrys-tallographic,microstructuralandmagneticanalysisofpulsed-laserdepositedlms,whilechaptersevencontainstheresultsofmicrowavecharacterizationoftheselms.Chaptereightgivestheconclusionandfurtherprospectsofthiswork.7

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CHAPTER2REVIEWOFBASICPROPERTIESOFFERRITESANDFERROELECTRICSThischapterisintendedtoprovideareviewofbasicphysicalpropertiesofferritesandferroelectricmaterialsandtheirbehaviorathighfrequencies.Duetotheirfavorableproperties,bothmaterialshaveveryimportanttechnologicalapplications.2.1FerritesFerritesareaclassofelectricallyinsulatingferrimagneticcompoundsofvariousmix-turesofironoxideFe2O3.ExamplesincludecubicspinelferritesMOFe2O3,garnets3M2O35Fe2O3,andhexagonalferritesmagnetoplumbitesMO6Fe2O3.McanbeadivalentionsuchasMn,Ni,Zn,Co,Mginthecaseofcubicferrites;inhexagonalferritesMisusuallyBaorSr,andingarnets,Misyttriumorarareearthelement.Ferrimagneticmaterialsaresimilartoferromagnetics,havingaspontaneousmagnetizationbelowtheCurietemperature.Inferrimagnets,twodierentsublatticeshaveoppositebutdierentmagneticmoments,thuscreatinganon-zeronetmagneticmoment.Theirlowelectricalconductivitymakesthemusefulforapplicationswhereferromagneticmaterialsthatareusuallymetallicwouldbedetrimental.Athighfrequencies,forexample,eddycurrentsarenotgeneratedininsulatingmaterials,andelectromagneticwavescanpropagatethroughandinteractwiththematerial.Thispropagationandinteractioncanbecontrolledusingexternalmagneticeld,makingferritesirreplaceablematerialsinmanyrfandmicrowavedevices.Sometypicalmagneticpropertiesofferritesareillustratedintable2.1.Itcanbeseenthatgarnetsandspinelsareusedinfrequencyrangeupto12GHzX-band,whilethehexaferritesareusedusuallyuparound55GHz.Itcanalsobenoticedthatgarnets8

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Type Ferrite MO Commonferrite Typicalmagnetic structure composition materials properties:4McG, HaOe,HOe Garnet 5Fe2O3: Rareearth Y3Fe5O12 1750,82, cubic 3M2O3 metaloxide 0.5X-band Spinel 1Fe2O3: Transitionmetal Ni,ZnFe2O4 3800,300, cubic 1MO oxide Mn,ZnFe2O4 77X-band Magneto6Fe2O3: Divalentmetal BaFe12O19 4700,16500, plumbite 1MO oxidefromgroupIIA SrFe12O19 26GHz hexagonal -BaO,CaO,SrO Table2.1.Summaryofferritecrystalstructuretypes[10].haveverysmallferrimagneticresonancelinewidthHwhichisrelatedtolosses,andthathexagonalferriteshaveverylargeanisotropyeldsHa,usefulformakingself-biasedmagneticdevices.Thenon-linearrelationbetweentheappliedmagneticeldHandmagnetizationMresultsinmagnetichysteresis,anditcanbeobservedinbothferromagneticandferrimag-neticmaterials.Atypicalmagnetichysteresiscurveisshowningure2.1,andisusuallyreferredtoasM-HcurvemagnetizationMvs.appliedeldH.Thedottedlinerepresentavirgin"magnetizationcurvethatoccurswhenanunmagnetizedmaterialisexposedtoincreasingmagneticeld.Threeimportantpointsinthehysteresiscurvearemarked-sat-urationmagnetizationvalueofmagnetizationwhensampleisfullymagnetized,remanentmagnetizationvalueofmagnetizationafterthesampleisfullymagnetizedandtheeldisreturnedtozero,andcoerciveeldorcoercivityvalueofmagneticeldneededtobringthesamplebacktounmagnetizedstate.Themagneticstructureinmacroscopicsam-plesisdividedintomagneticdomainsinwhichallmagneticmomentsareorientedinthesamedirection.Sincetheorientationofthedomainsisingeneralrandom,aferrimagneticsampledoesnotshowanynetmagnetizationwithoutexternalmagneticeld.Duringthemagnetizationprocess,thedomainsmove,changesizeandrotatetominimizetheoverallenergy.ThisprocessgovernsthevalueofthecoerciveeldandtheshapeoftheM-Hcurve.Crystalimperfectionsandimpuritiescanhinderthemovementofmagneticdomainwallsduringmagnetizationanddemagnetizationprocessandthusincreasethecoercivity.9

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Dependingonthevaluesofsaturationandremanentmagnetizationandcoercivity,mag-neticmaterialscanbecategorizedintwogroups-softandhardmagnets.Magneticallysoftmaterialshavesmallcoercivity,andsmallareaofthehysteresisloop.Theyareeasytomagnetizeanddemagnetize,andareusedinapplicationsweremagnetizationhastobereversedmanytimesasecondwithlowdissipation,suchastransformers,generatorsandmotors.CommerciallyimportantmaterialslikePermalloyhavealsolargepermeabilityandsquarenessMr/Msclosetoone.Hardmagneticmaterials,ontheotherhand,aremorediculttomagnetizeanddemagnetize.Theyhavelargehysteresisandhighcoercivity.AclassicalexampleofahardmagnetisAlnico,analloyofaluminum,nickelandcobalt.Propertiesofferritesathighfrequenciesareinuencedbyfollowingeects[11]:Faradayrotation:rotationoftheplaneofpolarizationofaTEMwaveasitpropagatesthroughaferritesample.Ferrimagneticresonance:resonantabsorptionofelectromagneticradiationmoreinsection2.1.2.Fielddisplacement:displacementofelectromagneticelddistributiontransversetothedirectionofpropagation.Nonlineareects:eectsoccuringathighpowerlevels.Spinwaves:short-wavelengthwavesofmagnetizationthatpropagateinthesample.Ferritematerialshavelowlossesatmicrowavefrequencies,highresistivityandstrongmagneticcoupling,makingthemirreplaceableconstituentsinmicrowavedevicetechnol-ogy.Intheformofhighqualitythinlms,theyhaveapotentialtoreplacebulkyexternalmagnetsincurrentmicrowavedevices[10].Commonferritemicrowavedevicesincludemicrowavephaseshifters,isolatorsandcirculators.Thesedevicesusereciprocalandnon-reciprocalfeaturesoftheferritematerials,andprovideuniquecircuitfunctionsthatcannotbereproducedwithanyothermaterials.10

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a b Figure2.1.Typicalmagnetizationvs.appliedmagneticeldM-Hcurve:aMainchar-acteristicsoftheM-Hloopareindicated-coercivityHc,saturationmagnetizationMsandremanentmagnetizationMr,bM-Hcurveforasoftandahardmagneticmaterial.2.1.1MagneticAnisotropyMagneticanisotropyisanimportantfactorinuencingthemagneticpropertiesoffer-ritethinlms.Themagnetizationofferrimagneticandferromagneticmaterialsusuallyprefersaspecicenergeticallyfavorabledirection,calledeasyaxisofmagnetization.Lessfavorabledirectioniscalledahardaxisofmagnetization.Themaincontributionstomagneticanisotropyinthecaseofhexaferritethinlmscomefromthecrystalmagne-tocrystallineandshapeanisotropies.Theoriginofthecrystalanisotropyisthespin-orbitcouplingandisrelatedtothematerial'scrystallographicstructure.Theout-of-planecrys-talanisotropyofahexagonalcrystalismuchlargerthanthein-planeanisotropyseegure2.2.Thesourceoftheshapeanisotropyisthedemagnetizationenergyassociatedwiththeshapeofthesample.Inthinlms,theeasyaxisliesusuallyinthelmplane.Theeasyaxisdescribesadirectionofmagnetizationinwhichthetotalenergysumofallanisotropycontributionsisminimized.2.1.2FerrimagneticResonanceFerrimagneticferromagneticresonanceFMRistheresonantabsorptionofelectro-magneticradiationbythespinsystemofaferrimagneticferromagneticmaterial.Ina11

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Figure2.2.Magneticanisotropyofahexagonalcrystal:ain-plane,bout-of-plane.standardFMRexperiment,thesampleunderinvestigationissetinadcmagneticeldandirradiatedbyanelectromagneticeldinthemicrowaverange.FMRismostlyusedformeasurementsofstaticanddynamicpropertiesofmagneticmaterialssuchaseectiveandsaturationmagnetization,magneticanisotropy,dampingandelectrong-factor.Classicalequationofmotionofmagnetizationwithoutdamping,givenbyLandauandLifshitzis:d~M dt=)]TJ/F18 10.909 Tf 8.485 0 Td[(~M~Heff;.1whereM:magnetization,~HeffHex,Hd,Ha,Ho:totaleectiveeld,:gyromagneticratio,Hex:exchangeeld,Hd:demagnetizingeld,Ha:anisotropyeld,H0:appliedeld.SolutionoftheLandau-Lifshitzequationrelatestheresonantfrequency!tothetotaleectivemagneticeld:!=~Heff:.2Fromhereweseethattheresonancefrequencyoftheferritedependsonitsmagneticanisotropy.ForapplicationsinGHzregion,highanisotropymaterialsarerequired.12

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Figure2.3.Ferrimagneticresonancefrequencyvs.appliedmagneticeldparallelandper-pendiculartotheeasyaxisofmagnetization[122].Figure2.3illustratesatypicalrelationofferrimagneticresonancetotheappliedeldforanisotropicsamples,whenthemagneticeldisappliedparallelandperpendiculartotheeasyaxisofmagnetizationofthesample.Itcanbeseenthatfortheeldparalleltotheeasyaxis,thereisalinearrelationbetweentheFMRfrequencyandtheappliedeld.Fortheeldsappliedperpendiculartotheeasyaxis,therelationislinearonlyateldsmuchlargerthantheanisotropyeld.Thiswillbeobservedinchapter7.3.2.1.3Barium-HexaferriteThecompoundBaFe12O19Barium-ferrite,Barium-hexaferrite,Barium-M-hexaferrite,BaM,Ferroxdure,BaO6Fe2O3isthemostimportantofhexagonalferrites,andisusedinavarietyofmagneticrecordingandhigh-frequencyapplications.ItbelongstotheM-typeclassofhexagonalferiteshexaferrites.Thistypeofhexagonalferriteshasamagnetoplumbitecrystalstructure,showningure2.4,whichconsistsoffourinterchangingspinelSandSandrhombohedralRandRblocks.Theferrimagneticpropertiescomeentirelyfromthe24Fe3+ions,eachhavingamagneticmomentof5BB:Bohrmagnetron,andthetotalmagneticmomentequalsto40B[87].ThelatticeparametersoftheunitcellofBaMarea5.89Aandc23.19A[123].ThemostoutstandingpropertyofBaMisitslargemagneticanisotropy17kOeatroom13

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Figure2.4.Ba-hexaferritecrystalstructure:right-projectionoftheunitcellshowingthefourblocksSRSR,characteristicofM-typehexaferrites,lefttop-hexagonalcrystalstructureofBaM,leftbottom-magneticmomentdistributionforhalfunitcell[28,108].temperature[106].Theeasydirectionofmagnetizationisalongthehexagonalc-axis,andtheharddirectionisalongthehexagonala-axis,seealsogure2.2.Thisbuiltin"biasingeldisusedforconstructionofmanyresonanceisolators,phaseshiftersandcirculators[10].ThenaturalferrimagneticresonancefrequencyofBaFe12O19isaround50GHz,butcanbevariedbetween40to130GHzbysubstitutionofCo,Al,TiorZnforsomeoftheBaatoms[87].Fromtheequation2.2itcanbeseenthatthehighvaluesofFMRcomefromthehighmagneticanisotropyofBaM.TherelativepermittivityoftheBaMisaround15,whilethevaluesofthepermeabilityare1farbelowtheferrimagneticresonancefrequency[120],makingitagoodcandidateforlosslessmicrowaveapplications.14

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2.2FerroelectricsFerroelectricmaterialsbelongtotheimportantclassofdielectricmaterials[23].Theirpropertiesincludehighresistancetoelectriccurrentandanonlinearresponseofpolariza-tiontoelectriceld.BelowtheCurietemperature,ferroelectricmaterialshavelongrangedipole-dipoleinteractionsmanifestedinspontaneouspolarizationanddisplayhysteresiseectsofpolarizationvs.electriceld,justlikeferri-andferromagneticmaterials,whichistheoriginoftheprexferro"theydonotcontainanyiron,though.AbovetheCurietemperature,spontaneouspolarizationdisappearsandthepolarizationresponsebecomeslinear-thisstateiscalledaparaelectricstate.Ferroelectricmaterialsalsohavedomainsinwhichthepolarizationishomogeneous.Thepermittivityofferroelectricmaterialsisafunctionoftemperature,withrealparthavingasharpmaximumattheCurietemperature.Thepermittivityalsochangeswithappliedelectriceldelectricaltunability.Figure2.5illustratesatypicalbehaviorofatunableferroelectricmaterial. Figure2.5.Atypicalchangeofpermittivitywithappliedvoltageforaferroelectricsample.2.2.1Barium-Strontium-TitanateBarium-strontium-titanateBSTisthemostwidelyusedtunableferroelectric,havingtheformulaBa1)]TJ/F19 7.97 Tf 6.587 0 Td[(xSrxTiO3.Thecompoundwithx=0.5isinaparaelectricstate,withCurietemperaturewellbelowtheroomtemperature,reasonabletunabilityandlowlosses[126].15

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CurietemperaturecanbeadjustedbychangingthevalueofxBaTiO3isferroelectricandSrTiO3isparaelectricatroomtemperature. Figure2.6.CrystalstructureofBarium-Strontium-Titanate.ThespontaneouspolarizationcomesfromtherelativedisplacementofBaSrandTiatomstotheOatoms.ThecrystalstructureofBSTisshowningure2.6,withlatticeparameteraround4A.AttheCuriepoint,thelatticeundergoesaphasetransitionfromcubicparaelectrictotetragonalferroelectric.SomeofthemainfeaturesofBSTmakingitanattractivematerialformicrowaveapplicationsinclude:largepermittivity,hightunability,fastresponsetotheelectricalelds,highbreakdownelds,lowdielectricleakagecurrents,symmetricnonlinearityandsimplefabrication[12,76,95,99,114,115,117].16

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2.3MicrowavePropertiesofMagneticandDielectricOxidesMacroscopicbehaviorofferrimagneticsandferroelectricscanbedescribedbytheircomplexpermeabilityandpermittivity.Thesequantitiesdescribetheinteractionofthematerialwithappliedmagneticandelectricelds,respectively.Theseinteractionsaremanifestedbyenergystoragelosslesspartofenergythatisexchangedbytheeldandthematerialandenergydissipationpartofenergyabsorbedbythematerial.Thisleadstopermeabilityandpermittivitybecomplexquantities,andthesecanbealsotuned"inappliedelds.Electricaltunabilitywasdescribedintheprevioussection.Formicrowaveapplications,ferritescanbedividedintwogroups-ferritesthataremagneticallytunedtoferrimagneticresonanceFMRandferritesthataretunedawayfromtheFMR[121].Theideabehindthemagnetictunabilityisthatinferrimagneticmaterials,whicharecharacterizedbyahysteresisloop,thepermeabilityisafunctionofappliedmagneticeld,andisingeneralatensorquantity[100,121]:~B=[]~H;whereisatensor.The~Hisdenedasthederivativeofmagneticuxdensity~Bwithrespecttoappliedmagneticeld~H.Typicalvaluesoftheinitialpermeability~H=0rangefrom10to104.Foralosslessferritematerialinanappliedmagneticeldalongthez-direction,thetensorpermeabilityisgivenby[100]:[]=266664j0)]TJ/F18 10.909 Tf 8.485 0 Td[(j0000377775;where=1+!0!m !0)]TJ/F18 10.909 Tf 10.909 0 Td[(!m;=0!!m !20)]TJ/F18 10.909 Tf 10.91 0 Td[(!2;17

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Figure2.7.Exampleofmagnetictunability:spectrumofa10-layercompositeof1.9mCoFeamorphouslmona12mmylarsubstrate:left-biasH=10Oe,right-biasH=110Oe[124].!0=0H0;!m=0Hs;andthegyromagneticratioisaround[31]:=2:8GHz kOe:Inordertoaccountforlosses,adampingfactorisintroduced:!!!0+j!:Applyingthistotheaboveexpressionforthepermeabilitygivesthecomplexperme-ability:=0)]TJ/F18 10.909 Tf 10.909 0 Td[(j00:Fromthisexpression,itcanbeseenthatthecomplexpermeabilitywillchangedepend-ingontheappliedmagneticeld.Thisisreferedtoasmagnetictunability.Anexampleofmagnetictunabilityisshowningures7.3,wherethemagneticcomplexpermeabilityofaferrimagneticmaterialischangedbyapplyingasmallmagneticeld.Itcanbeseenthatthevaluesof'and"arechangedinthelowfrequencyregionawayfromtheFMR,andalsothepositionoftheFMRfrequencyischanged,astheybothdependontheappliedmagneticeld.Thesetwophenomenacanbeusedfortailoringtunablemagneticdevicesusingferritematerials.18

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Thepermittivityofamaterial,whichdescribesmaterial'sabilitytotransmitanelectriceld,isalsoatensorquantityingeneral,andrelatestheelectricdisplacementeld~Dtotheelectriceld~E:~D=[]~E=0~E+~P;where0isthepermittivityoffreespaceand~Piselectricpolarization.ThedielectricconstantisdenedasarelativepermittivityK=r==0.Thevaluesofdielectricconstantrangefromlessthan10formostdielectricstoseveralthousandsforsomeferroelectrics.Aschematicviewofthefrequencypropertiesofmagneticanddielectricmaterialsisshowningure2.8.Themainpolarizationmechanismsindielectricmaterialsincludeionicconduction,dipolarrelaxation,atomicpolarization,andelectronicpolarization.Thepermittivityisalsoinuencedbythemovementsandresonancesofferroelectricdomainwalls.Thepermeabilityofmagneticmaterialsisrelatedtothemovementsandresonanceofmagneticdomainwalls,andgyromagneticresonanceferro-andferrimagneticresonance.2.4CharacteristicsofMicrowaveDevicesWhendesigningdevicescontainingferrimagneticandferroelectricmaterials,onehastoconsiderbothintrinsicaswellasextrinsicfactorsinuencingthedeviceperformance.Dependingonthesizeofthedevicecomparedtotheoperatingwavelength,theyaredividedintoelectricallysmall"andelectricallylarge"devices[126].Theelectricallysmalldevicescanbedescribedwithdiscretecircuitcomponentslumped-elementmodel,whileintheelectricallylargedevicesfrequencydependenceneedstobetakenintoaccount,andtheyaredescribedbydistributednetworksofcommoncircuitelementsinductorsandcapacitors.Mostcommonexampleofelectricallylargedeviceisatransmissionline.ThetransmissionlineischaracterizedbyitscharacteristicimpedanceZc/q ,whichdependsonthematerial'spermittivityandpermeability,butalsoonitssizeandshape.Optimumvalueofcharacteristicimpedanceinmostsystemsis50.Devicesbasedontransmissionlines19

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a b Figure2.8.Relaxationmechanismsofhypotheticaladielectric[69]andbmagneticma-terials[30].shouldbedesignedastoavoidchangesinthevaluesofthecharacteristicimpedance,sincethatcancauseimpedancematchingproblemspartoftheelectromagneticsignalwillbereectedattheplaceswheretheZcchanges.Presenceofferroelectricandferrimagneticmaterialsinatransmissionlinecancauseseriousimpedancematchingproblems,andcareshouldbetakenduringthedesign.20

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CHAPTER3EXPERIMENTALMETHODSThischapterdescribestheexperimentalset-upsandmeasurementtechniques.MostoftheworkwasperformedattheUSFUniversityofSouthFlorida,butsomeworkwasdoneduringresearchvisitstotheORNLOakRidgeNationalLaboratoryandUCF-AMPACUniversityofCentralFlorida-AdvancedMaterialsProcessingandAnalysisCenterfacilities.Thinlmswerefabricatedusingrfmagnetronsputteringandlaserablation.ThestructuralcharacterizationwasdoneusingX-raydiractometer,atomicforceandscanningelectronmicroscopes.MagneticmeasurementswereperformedbythePhysicalPropertiesMeasurementSystem.Specialattentionisgiventomicrolithographyandmicrowavecharacterization,whichcontainspeciallydevelopedinstrumentationandtechniquespresentedinthiswork.FMRmeasurementsweredonethroughcollaborationwithProfessorG.SrinivasanatOaklandUniversity.3.1ThinFilmDepositionThisworkcontainstwomethodsofsynthesizingthinlms:rfsputterdepositionandpulsedlaserdeposition.ThesetechniquesarepartofthePhysicalVaporDeposition"PVDthinlmpreparationprocess,inwhichatargetisbombardedandtheremovedmaterialisdepositedontoasubstrate.BothsputteringandPLDhavebeenusedextensivelytoproducehighqualityferriteandferroelectricthinlms.3.1.1RFMagnetronSputteringInanRFmagnetronsputteringprocess,ahighfrequencyplasmadischargeiscreatedbycapacitivelycouplinga13.6MHzfrequencytothecathode,asseeningure3.1[125].21

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Usingasmallbiasvoltagebetweenanodeandcathode,theionizedgasatomsintheplasmaareacceleratedtowardthetargetatthethecathodeandsputterothedepositswhichtraveltowardthesubstratelocatedattheanode.TypicalsputteringconditionsforBaMandBSTthinlmdepositionincludeargongasormixtureofargonandoxygengases,rfpowerdensitiesofseveralW/cm2,andsubstratetemperaturesofseveralhundred0C.Postannealingoftheas-depositedlmsisusuallyperformedtoimprovelmcrystallinity.SputteringhasbeensuccessfullyusedtodepositBaM[18,57,58,75,93]andBST[13,95,116,129]thinlms. Figure3.1.Sputteringdepositionprocess-leftgureshowsaschematicofarfsputteringsystem,rightgureillustratesthesputteringprocess[125].3.1.2PulsedLaserDepositionPulsedlaserdepositionalsoreferredtoaspulsedlaserablationisadepositiontech-niqueusedtoproducethinlmswithhighstoichiometry.Theprocessoflaserdepositionconsistsofbombardingthetargetwithshort,high-energylaserpulses,creatingaplasmafromthevaporizedtargetsurface-seegure3.2.Thisplasmacontainsatoms,electrons,ions,molecules,clustersandparticulateserodedfromthetarget,whichgetdepositedonthesubstrateplacedparalleltotheexpansiondirectionoftheplasma.Thesubstrateisusuallyheatedtoseveralhundred0Ctopromotethenucleationandgrowthofthelm.22

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Thelasertargetinteractionisacomplexprocessinvolvingevaporation,ablation,plasmaformationandexfoliation[25].OneofthemainadvantagesofPLDisthattheplasmaarrivingatthesubstrateishighlyenergetic,providingsucientionmobilityforgrowthofepitaxiallms.AnotheradvantageisthatPLDcanbeperformedbothinhighvacuumaswellasinthepresenceofabackgroundgas,e.g.oxygen.AdisadvantageofPLDisthatsmalldropletsparticulatesofthetargetmaterialareoftendepositedonthesub-strates,whichisreferedtoassplashing"[25,125].Also,theareaofdepositionisrelativelysmallcomparedtosputteringandotherdepositiontechniques.Thisisassociatedwiththehighlydirectionalnatureoftheplume.HighestqualityBaM[25,84,85,105,107]andBST[19,20,25,54,62]thinlmshavebeenreportedusingthisdepositiontechnique. Figure3.2.Pulsed-laserdepositionprocess:atypicalexperimentalset-up,bschematicoftheevolutionoftheplasmaplume[125].3.2StructuralCharacterizationStructuralinvestigationisanimportantstepinthinlmcharacterization.Ithelpsinunderstandingwhatdepositionconditionsprovideoptimumlmqualityaswellasinrelatingmicrostructuralpropertiestootherthinlmpropertiessuchasmagnetization,po-larization,conductivity,high-frequencyresponse,etc,whicharerelevantfortechnological23

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applications.ThetechniquesusedinthisstudyincludeX-raydiractionXRD,scanningelectronmicroscopySEMandatomicforcemicroscopyAFM.3.2.1X-RayDiractionX-raydiractioncanprovideinformationaboutthinlmcrystallinity,chemicalmakeup,phaseidenticationandplanarorientation.Inthinlms,certaincrystallographicdirectionsareusuallypreferredleadingtoanisotropicpropertiesofcrystalorientation.Thisphenom-enoniscalledtextureorpreferredorientation[16].Insuchlms,certainBraggreectionsin/2diractionscansaremorepronouncedthantheothers.ThetechniquesusedheretostudythinlmtextureincludedeterminationofBraggpeaksby/2-diraction,rock-ingcurves!-scans,azimuthalscans-scansandpoleguremeasurementsofselectedBraggpeaks.XRDdatawascollectedusingPhilipsX'PertatUSFNanomaterialsandNanomanu-facturingResearchCenter-NNRCandBrukerAXSatUSFPhysicsDepartmentdirac-tometersequippedwiththin-lmdiractometerandpole-guregoniometer-gure3.3a.Somemeasurementparametersareshownintable3.1.QualitativeanalysiswasdoneusingX'PertHighScoresoftwareaswellasbycomparisonwiththepublishedcrystallographicdatabase[1,2,3]. XRDDiractionParameters AnodeMaterial Copper Monochromator PW3123/10forCu IncidentSlit Soller0.04rad DivergenceSlit Fixed,1/20 IncidentMask Fixed,5and10mm GeneratorSettings 45kV,40mA ScanType /2,!-tiltand-tiltfollowedby-azimuthalrotation Table3.1.X-Raydiractionparametersfor/2andpole-gurescans.ThebasicprincipleofdeterminingthecrystalstructureusingX-raysisgivenbytheBragg'slaw-seegure3.3bandc.AbeamofparallelX-raysimpingesthecrystalsurfaceatanangle,andisreectedfromtheparallelplanesofatomsformedbythe24

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crystallatticeofthematerial.Twoconsecutivereectedbeamshaveaphasedierencebecausetheytraveladierentpath.Constructiveinterferenceofthereectedraysoccursonlywhentheirpathdierenceisequaltoamultipleofthewavelength[69]:2dsin=n;n=1;2;:::.1a Figure3.3.PrinciplesofX-raydiraction:aX-raydiractometershowingtheaxisofrotation,bselectionprincipleforexclusivemeasurementofsurface-parallellatticeplanesina/2scan[16],cdiractionofX-raysfromthelatticeplanes.Inaconventional/2scanalsocalledapowderdiractionmethod,thesampleunderinvestigationisbombardedbyanX-raybeamdirectedtothesamplesurfaceattheconstantangle,whileontheothersideadetectormeasurestheintensityofthescatteredradiationatthesameangle[16].Duringthescan,theangleofincidenceandtheangleofdetectionarecontinuouslyvariedbutkeptequaltoeachother.AcharacteristicdiractionpatternemergesandisusuallyplottedasafunctionofItype.Themeasuredpatternisthencomparedwithaknowndatabaseofreferencepatterns.Thinlmsdepositedonsinglecrystalsubstratesareusuallytexturedpolycrystallineoranepitaxialsinglecrystal.Texturedpolycrystallinelmsoftengrowinapreferredorientationasshowningure3.4andveryrarelyhaverandomcrystallitemorphologies.25

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a b c Figure3.4.Schematicillustrationofpreferredorientationofpolycrystallinethinlmsar-rowsshowachosenhkldirection:arandomorientation,bweakorientation,cstrongorientation.Thedegreeoftexturecanbeprobedbyseveralx-raydiractiontechniquesmentionedearlier.Theconventional/2diractogramshowsonlytheBraggreectionsofthelatticeplanesthatareparallelornearlyparalleltothesurfaceofthethinlm-gure3.3b.Inordertodeterminetheorientationdistribution,theintensityoftheBraggreectionsofallcrystallitesismeasuredforvariousvaluesofazimuthangleandtiltangle,seegure3.3a.TheintensityI;isusuallyplottedinacircularplane,andsuchplotiscalledanintensitypolegure.Typicalpoleguresfordierenttypesofcrystalliteorientationareshowningure3.4.Ifthecrystalliteorientationswereperfectlyrandom,thepolewouldbeuniformlydistributedonapolegure.Inatexturedmaterial,thepolesgrouptogetherincertainplaces.Thestrongerthetexture,thetighterthegroupingofthepoles,asillustratedingure3.4bandc.Rockingcurveanalysis!-scanisusedtoquantitativelymeasureplanarorientationormosaicnessofthinlms.Itismostlyusefulforhighlyorientedorepitaxiallms.Thismeasurementisperformedbyxingthedetectoronthe20positionoftheBraggpeakunderinvestigation,andthentiltingrocking"byangle!inthevicinityoftheBraggangle0.ThefullwidthathalfmaximumFWHMobtainedfromtherockingcurverevealsthedegreeoflmorientation.Anarrowpeakisindicationofagreatertextural26

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Figure3.5.X-rayrockingcurvemeasurement!-scan:aschematicdiagram,b!-scanspectradependenceonlatticeplaneorientations.coherenceofthecrystallites.Figure3.5illustratesthedependenceoftherockingcurvesontheorientationofthinlmlatticeplanes.Foraperfectsinglecrystal,thepeakisafunction.Twoperfectcrystalsgivetwo-peaks.Therockingcurveofahighlyorientedpolycrystallinethinlmwouldshowanitepeakwidth.Azimuthal-or-scanisahigh-accuracyslice"ofthepolegure,usedtoprovidequantitativedataforthinlmtextureandepitaxy.Itisperformedbytiltingtheangletoadesiredvalueandvaryingtheazimuthangle.TheFWHMofthemeasuredpeaksinthe-scanalsoservesasameasureofthequalityofthethinlms.3.2.2ScanningElectronMicroscopyScanningelectronmicroscopyisusedtoimageasamplesurfacewithahighresolution,bydetectingsecondaryelectronsemittedfromthesurfacewhenexcitedbytheprimaryelectronbeam.TheelectronbeamwithenergiesrangingfromafewhundredeVtotensofkeVisemittedfromatungstenorlanthanumhexaborideLaB6cathode,focusedandrasteredacrossthesamplebymeansofcondenseranddeectingcoils.Asillustratedin27

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gure3.6,severalprocessesoccurandcanbedetectedwhentheelectronbeamhitsthesamplesurface:elasticandinelasticbackscattering,emissionofx-rays,Augerelectronsandsecondaryelectrons[8,125]. Figure3.6.ScanningElectronMicroscope:ablockdigram,boccuringprocessesduringanSEMscan[125].ThetopologyofthesamplesurfacehillsandvalleysdeterminestheresolutionoftheSEM.Thesteepsurfacesandedgesemitmoresecondaryelectronsthanaatsurface.Thisproducesadierenceinthebrightnessontheprojectionscreencreatingathree-dimensionalappearanceofthesurface.TheadvantageoftheSEMisitsabilitytoimagelargesampleareaandalsobulksamples,whileoneofthemaindisadvantagesisrelativelylowspatialresolutionwhencomparedtootherimagingtechniquessuchastransmissionelectronmicroscopyandatomicforcemicroscopy.3.2.3AtomicForceMicroscopyAtomicforcemicroscopyAFMisaveryhighresolutionscanningprobetechnique,usedforimagingatthenanoscale.Atypicalatomicforcemicroscopeconsistsofami-croscalecantileverwithanetippositionedclosetothesample.Thedistancetothesamplesurfaceiscontrolledbypiezoelectricelements.Duringthescan,theprobingtipisbroughtclosetothesamplesurface,andisbeingaectedbyvanderWaalsforcesbetweenthetipandthesurface.Thedeectionofthecantileverispickedupbyalaserspotreectedfromthetopofthecantilever-seegure3.7.28

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Figure3.7.Atomicforcemicroscopeblockdiagram[7].Theheightofthedeectionisthenplottedasafunctionofthelateralposition,revealingthetopographyofthesurface[7,125].AFMiscapableofimagingsurfaceswithatomicresolution.Atomicforceimagesofsamplesinchapter6wereperformedincollaborationwithProfessorG.MatthewsandhisgraduatestudentA.Heimattheirlaboratory.3.3MagneticCharacterizationwithPPMSPhysicalPropertiesMeasurementSystemPPMSfromQuantumDesignwasusedtoperformmagnetizationmeasurementsonallsamples.ThePPMSconsistsofaliquidHeliumdewarwitha7Tlongitudinalsuperconductingmagnetandatemperaturecontrollerintherange1.9to400K-seegure3.8a.TheorientationofthesamplewithrespecttothemagneticeldinsidethePPMScanbevaried.Inthisstudy,thesamplesurfaceispositionedeitherparallelorperpendiculartotheappliedmagneticeld.Figure3.8bshowsanidealizedmagnetizationversusappliedmagneticeldM-Hcurveofathinlmhavingeasymagnetizationaxisparalleltothelmplane.Whenthesampleispositionedwithitssurfaceparalleltotheappliedeldthesampleiseasily29

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a b Figure3.8.Magneticmeasurements:aPhysicalPropertiesMeasurementSystemPPMS,bidealizedM-H-curvesofathinlmwitheasymagnetizationaxisparalleltothelmplane.magnetizedtosaturation.Whenthesamplelieswithitssurfaceperpendiculartothemagneticeld,muchlargereldisnecessarytofullymagnetizethesample.3.4MicrowaveCharacterizationofThinFilmsThenalstageofthethinlmcharacterizationinvolvesthestudyofhighfrequencyresponseinthepresenceofelectricalandmagneticelds.Forthispurposeaspeciallydesignedexperimentalset-upwasassembledinamagneticeldenvironmentgeneratedbyanelectromagnet,andcustomizedcircuitsweredesignedandfabricatedusingmicrolitho-graphy.Thedatawastakenusingavectornetworkanalyzeraftercarefulcalibrations.FerrimagneticresonancemeasurementsweredonethroughacollaborationwithOaklandUniversity,theexperimentalset-upisbrieydescribed.Thissectionispresentedingreaterdetail,asunliketheotheranalyticalinstrumentsthatwerecommercial,themicrowavemeasurementswereset-upentirelyduringthecourseofthisresearch.3.4.1ExperimentalSet-UpAspeciallydesignedexperimentalstagewasusedformeasurementsofhigh-frequencyresponseandtheelectricandmagnetictunabilityofthinlms-seegure3.9.Theset-upconsistsofacustombuiltmicrowaveprobestationinsideawater-cooledelectromagnet30

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-9000Gauss.TheelectromagnetwaspoweredbyaKepcopowersupplyATEW,andthemagneticeldwasmeasuredbyaLakeshoreGaussmetermodel410.TheprobestationisequippedwithamicroscopewithCCDcameraandLCDdisplay,usedforviewingandmagnifyingthemicrocircuits.Keithleypowersupplyincombinationwithbias-teeswasusedfortheelectriceldbiasing.Themicrocircuitswereprobedbyground-signal-groundmicroprobesfromGGBCompanywith150mpitchseparation.Theprobeswereconnectedtothenetworkanalyzervianon-magneticsemi-rigidcoaxialcables.ScatteringparametersofthemicrowavecircuitsweremeasuredbyAnritsu37397Cvectornetworkanalyzeroperatinginthefrequencyrangefrom40MHzto65GHzthiswillbeexplainedinmoredetailinthesection3.4.4..SomeoftheelectricaltuningexperimentswerealsoperformedattheUSFCenterforWirelessandMicrowaveInformationSystemsWAMI,equippedwithaprobestationandVNA,butwithoutthemagneticeld.Thefundamentalconceptbehindtheexperimentisthatthehighfrequencyresponseinthematerialismeasuredusingthenetworkanalyzer,whilethesampleisinthemagneticeldand/orunderabiasdcelectriceld.Thepreparationforthemicrowaveexperimentincludesdepositionofmicrocircuitscoplanarwaveguidesandinterdigitatedcapacitorsonthethinlm,calibrationandprobingthecircuitswiththemicroprobes.31

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Figure3.9.Experimentalset-upforhighfrequencymeasurementsofthinlmproperties.32

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3.4.2Micro-CircuitDesignInordertomeasurefrequencydependentdielectricandmagneticproperties,coplanarwaveguidesCPW'sarefabricatedonthetopofthesamplesunderinvestigation.Acopla-narwaveguideisatypeofguidedplanartransmissionlineusedinmicrowaveintegratedcircuits,inwhichallconductorsgroundandsignallieonthetopsurfaceofthesubstrate-seegure3.10.ThethreeparallelconductorsbuildaGround-Signal-GroundGSGcon-gurationseealsogure3.13a,inwhichtheoutertwoconductorscarrytheground"signalandthemiddleconductorcarriestheactualhigh-frequencysignal.Thefundamentalmodeofpropagationisquasi-TEMtransverseelectromagneticmode,onlyatveryhighfrequenciesitbecomesnon-TEMduetoalongitudinalcomponentofmagneticeld[45].Theelectricandmagneticelddistributionisnearlysymmetricalaboveandinsidethesubstrate.ThemainadvantageofaCPWisthatallthreeconductorsareonthesamesideofthesubstrate,whichisaneasiergeometrytofabricate,andthattheimpedancedoesnotdependonthethicknessofthesubstrate,butonlyonthedielectricconstantandthewidthofthegapandthecentralconductor. Figure3.10.Electromagneticeldsdistributioninacoplanarwaveguide.Thesolidlinerepresentselectriceldandthedashedlinerepresentsmagneticeld[23].Threedierentsubstrateswereusedinthisstudy:sapphirewithA-andC-planeorientation,and-orientedMgO.Thepropertiesofthesesubstratesarelistedintable3.2.Inorderforthecircuitstobeprobedwiththe150mpitchprobes,specialtapers"weredesignedthathavelargergapandcentralconductorsize-seegure3.13a.33

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C-sapphire A-sapphire -MgO Dielectricconstant 11.58 9.4 9.8 Losstangent10)]TJ/F16 7.97 Tf 6.586 0 Td[(5 5 2 1 Substratethicknessmicrons 500 500 1000 Table3.2.SubstratesparametersThedimensionsofCPWsandtaperswerecalculatedusingafreelydistributedtrans-missionlinecalculatorTXLineVersion1.1"fromthecompanyAppliedWaveResearch,Inc.-gure3.11[6].Theanalysisofcoplanarwaveguideswasdoneaccordingtothereference[52],andthereportedaccuracyiswithin5%. Figure3.11.TransmissionLineCalculatorprogram.Fourdierentgapsizeswerechosen-5,10,15and20m.Allworkwasdonewiththecircuitswith5mgapsize,butitwasdesirabletodesignoptionalgapsizesincasethateitherfabricationormeasurementof5mgapdidnotproducedesirableresults.De-pendenceofthecircuitperformanceonthegapsizewillbealsodiscussedinchapter7.Fordierentsubstratesanddierentgapsizes,thecentralconductorwidthwasadjustedtobringthecharacteristicimpedanceascloseto50aspossible.Conductivityofgoldconductorswas=4:1107S=mforallcircuits.Thelengthofthelineswasadjusted34

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fordierentfrequencies,sothattheelectricallengthvalueisaround900.FortheTRLcalibration,theoptimumelectricallengthofadelaylineatparticularfrequencyischosentobe900orquarterwavelength,butitisacceptableforallfrequenciesthathavetheelec-tricallengthbetween200and1600.Dierentlengthsofdelaylinesweredesigned,givingapossibilityofaTRLThru-Reect-Linecalibrationinthefrequencyrangefrom1to80GHz.MoredetailsregardingthedesignoftheCPWswillbeaddressedinsection3.4.4.1whendiscussingthecalibrationprocedures.ThedimensionsofallCPWsandtapersforthethreesubstratesaregivenintables3.3and3.4.Afterthecircuitdimensionswerecalcu-latedusingtheTransmissionLineCalculator,thecompletemaskwasdrawninAutoCADasseeningures3.14a,b.Inordertoaccountforsmallfeaturesizes5mCPWgap,aspecialchrome-on-glassmaskwasfabricatedbyAdvanceReproductionsCorporation.Thepatternfromachromemaskwasthentransferedtoanironoxidemaskforeasierhandlingofthemaskandinordertokeeptheoriginalmaskprotected. C-sapphire A-sapphire -MgO Gapmicrons 60 41 45 Centerlinewidthmicrons 100 100 100 Lengthmicrons 200 200 200 Table3.3.TapersdimensionsfordierentsubstratesInadditiontotheCPW,coplanarwaveguideinterdigitatedcapacitorsIDCwerealsofabricated,inordertostudythechangeofcapacitancewithappliedelectriceld.Interdig-itatedinterdigitalcapacitorisamultingerperiodicstructurethatusesthecapacitancethatoccursacrossagapinthinlmconductors[88].ThedesignanddimensionsoftheIDCsareshowningure3.13b.Thecapacitanceisvariedbychangingthenumberanddimensionsofthengersorbychangingthedielectricpropertiesofthethinlmbelow,whichisexactlytheintentionofthisexperiment.ApproximateanalyticalmethodsexisttoextractthecapacitanceofthethinlmfrommeasuredS-parameters.Twotypesofmasksweredesigned,onewithCPWsandIDCs-gure3.14a,andonewithbent"CPWs,asshownin3.14b.TheCPWsconsistof'Thru','Reect'and'Delay35

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Line'structures,whichwillbeexplainedlater.The'Taper-to-Taper'structureisusedtodeembedtheeectoftapersfromthemeasurement.The'bent'linesarespeciallydesignedformeasurementsinappliedmagneticeld.Figure3.12illustratestherelativeorientationoftheacanddcmagneticeldsintheexperimentseealsogure3.10formagneticelddistributioninCPWstructure.Forthebent"CPWstheapplieddcmagneticeldisperpendiculartothepropagatingacmagneticeld.Thisset-upisessentialformicrowavemeasurementsofmagneticsamples,asintheFMRexperiment,theperturbingaceldisperpendiculartothelargedceld. Figure3.12.Relativeorientationofacanddcmagneticeldvectorsinthehigh-frequencymeasurementexperiment.Left:straight"CPWs,right:bent"CPWs.36

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C-sapphire C-sapphire C-sapphire C-sapphire C-sapphire straightline straightline straightline straightline bentline G=5m G=10m G=15m G=20m G=5m W=8:5m W=17m W=25m W=34m W=8:5m Linesm: Linesm: Linesm: Linesm: Linesm: 8000,7000,5900 8000,7000,5900 8000,7000,5900 8000,7000,5900 8000,7000,5900 3000,2000,1500 3000,2000,1500 3000,2000,1500 3000,2000,1500 3000,2000,1500 615,400 615,400 615,400 615,400 615,400 C-sapphire C-sapphire C-sapphire A-sapphire A-sapphire bentline bentline bentline straightline straightline G=10m G=15m G=20m G=5m G=10m W=17m W=25m W=34m W=11:5m W=23m Linesm: Linesm: Linesm: Linesm: Linesm: 8000,7000,5900 8000,7000,5900 8000,7000,5900 8000,6400,3300 8000,6400,3300 3000,2000,1500 3000,2000,1500 3000,2000,1500 2200,1700 2200,1700 615,400 615,400 615,400 670,420 670,420 A-sapphire A-sapphire A-sapphire A-sapphire A-sapphire straightline straightline bentline bentline bentline G=15m G=20m G=5m G=10m G=15m W=35m W=48m W=11:5m W=23m W=35m Linesm: Linesm: Linesm: Linesm: Linesm: 8000,6400,3300 8000,6400,3300 8000,6400,3300 8000,6400,3300 8000,6400,3300 2200,1700 2200,1700 2200,1700 2200,1700 2200,1700 670,420 670,420 670,420 670,420 670,420 A-sapphire 100-MgO 100-MgO 100-MgO 100-MgO bentline straightline straightline straightline straightline G=20m G=5m G=10m G=15m G=20m W=48m W=10:5m W=21m W=32m W=43m Linesm: Linesm: Linesm: Linesm: Linesm: 8000,6400,3300 8000,6300,3200 8000,6300,3200 8000,6300,3200 8000,6300,3200 2200,1700 2100,1600 2100,1600 2100,1600 2100,1600 670,420 660,410 660,410 660,410 660,410 100-MgO 100-MgO 100-MgO 100-MgO bentline bentlineline bentlineline bentlineline G=5m G=10m G=15m G=20m W=10:5m W=21m W=32m W=43m Linesm: Linesm: Linesm: Linesm: 8000,6400,3300 8000,6300,3200 8000,6300,3200 8000,6300,3200 2200,1700 2100,1600 2100,1600 2100,1600 670,420 660,410 660,410 660,410 Table3.4.Maskdesignparameters:G-gapbetweencentralandgroundconductor,W-widthofthecentralconductor,Lines"-delaylinesforTRLcalibration.37

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a b Figure3.13.CPWmaskdesigndetails:aCPWline,bIDC.38

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a b Figure3.14.CPWmaskdesign:amaskwithCPWlinesandinterdigitatedcapacitors,bmaskwithbent"CPWlines.39

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3.4.3Micro-CircuitFabricationMicrocircuitfabricationwasperformedinsideaclass1000cleanroomatUSF-NNRC.ThemaskswerefabricatedusingtheNegativePhotoresistNR1-3000PYanddevelopedinResistDeveloperRD6.TheresistwasremovedbyResistRemoverRR4,allfromFuturrex,Inc.Therecommendedprocedureoutlinedoncompany'sweb-site[5]hadtobemodiedforsapphireandMgOsubstrates.Thesesubstratesareverysmallcm1cm,andarepoorheatconductors.Thebakingtimehadtobeincreased3.5timesthatofsiliconwafers.Thedevelopmenttimewaskeptat25sec.Themicrolithographyprocedureisillustratedingure3.15,andstep-by-stepfabricationprocessisdescribedbelow.Microcircuitfabricationprocedure:Cleanthesubstrateswithacetoneandmethanolanddryinnitrogen.Spin-coatthephotoresistontothesubstrateat3000rmpfor30secondswith3secondsrampingtime.Bakeonahotplateat1550Cfor165seconds.ExposetoUVlightfor60seconds.Postbakeonahotplateat1100Cfor165seconds.DevelopinRD6for25seconds.GentlyrinseindistilledwateranddrywithN2.PlacethesampleintheRF-PlasmaEtchforabout45secondsat75W.ThisisnecessarytoremoveanyremainingphotoresistthatwasnotremovedbytheRD6developer.DeposittheCr/Aumetalusingthermalevaporatorthedetailsaregivenbelow.Lift-othephotoresistintheresistremoverRR4byheatingat1150Cfor25minoraslongasnecessarytoremoveallphotoresistfromthesubstrate.Ifnecessary,ultrasonicateinacetonefor30secondsandrepeatthepreviousstep,untilallphotoresistisremoved.40

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Finally,cleanthesamplesinacetone,rinsewithmethanolanddrywithN2.AnotherRF-Plasma-Etchmaybeneededtoremoveanyresiduesonthesubstratesurface. Figure3.15.Microlithographyprocedurewithnegativephotoresist.TheCr/Aumetalwasdepositedinathermalevaporator.Priortodeposition,thecham-berwasevacuateddowntolowtorrs.First,athinchromelayer15nmwasdepositedtopromotetheadhesionofgold.Thenthegoldlayerwasdepositedtoathicknessofabout1m,whichwasconrmedwithaprolometer.Immediatelyaftermetaldeposition,thephotoresistwasremoved.Themicrolithographyprocesswasveryproblematic.Oneofthemainissueswasthesmallsizeofthesample.Afterthespin-coatingofthephotoresistonthesubstrate,theedgesoftheresistwerealwaysslightlyhigherthanthemiddle,whichonasmallsubstratecanhaveaverypronouncedeect-seegure3.16a.Thisresultedinunevendevelopmentoftheresistandcreatedvariousdefectsinthefabricatedcircuits-seegure3.16b.Thesolutionwastoreducethedevelopmenttime,butthismadesomeoftheresisttoremain41

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a b Figure3.16.Problemswiththefabrication:aanexaggeratedillustrationoftheheightproleofspin-coatedphotoresistlayer,bdefectsontheresistmaskafterdeveloping.undevelopedandstayontheplacesfromwhichitshouldhavebeenremoved.Thatcausedsomeofthesamplestobecomecontaminated"byphotoresistlyingunderneaththemetallayer.Apartialsolutionwastoexposethesubstrateswithmasktorf-plasmaetchbeforeandaftermetalization.Thisstepremovedsomeoftheunwantedresistfromunderneathandbetweenthemetallayers.3.4.4NetworkAnalyzerMeasurementsNetworkanalyzerisaninstrumentthatanalyzesthereectionandtransmissionofelec-tromagneticsignalsinelectricalnetworks.NetworkanalyzersareusedathighfrequenciesMHzandGHz,andcanbeeitherscalarmeasureamplitudepropertiesonlyorvector42

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networkanalyzersmeasurebothmagnitudeandphaseproperties.Asillustratedingure3.17a,anetworkanalyzerconsistsofasource,signalseparationdevices,anddetectors.Itcanindependentlymeasurethetwoincomingwavesa1anda2,andthetworeversetrav-elingwavesb1andb2.Theseinputandoutputwavesforatwo-portnetworkareshowningure3.17b.Therelationshipbetweentheseparameterscanbedescribedbyscattering-orS-parameters:264b1b2375=264S11S12S21S22375264a1a2375.2Thecomponentsofthescatteringmatrixarecalled:S11:inputreectioncoecientS21:forwardtransmissioncoecientgainS12:reversetransmissioncoecientisolationS22:outputreectioncoecient Figure3.17.Networkanalyzermeasurements:aablockdiagramofanetworkanalyzer[23],batwoportnetwork.3.4.4.1ProbeCalibrationBeforemakingmeasurementsoftheS-parameters,anaccuratetwo-portcalibrationwasperformedinordertoeliminatesystematicerrorsassociatedwiththeVNA,cables,and43

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probes.Foron-wafermeasurements,calibrationstandardsareneededtocorrectforthelossesintheprobes.Twocommonmethodsofcalibrationwereusedinthisstudy:SOLTandTRL.Short-Open-Load-ThruSOLTcalibrationisthemostcommonlyusedcalibration,andithasbeenusedforthemostpartinthisstudy.Fourknownstandardsareneededshort,open,loadandthru,whicharefoundonacommerciallyavailableceramiccalibrationsubstratefromGGBCompany.Forthiscalibration,the'referenceplane'islocatedattheprobetips,meaningthaterrorcontributionsfromtheVNAuptothispointarecalibratedout. Figure3.18.SOLTcalibration.Thru-Reect-LineTRLcalibrationisanalternative,so-calledself-calibration"method,developedbyresearchesattheU.S.NationalInstituteofStandardsandTechnologyNIST.TheadvantageoftheTRLcalibrationisthatcalibrationstandardscaneasilybefabricatedonanysubstrateunderinvestigation-seegures3.13and3.14.The'referenceplane'inthiscaseisplacedatthe'centeroftheThru',seegure3.13a.The'Line'sectionsweredesignedtobecloseto1/4wavelength0fordierentfrequencies,rangingfrom1to80GHz.Theshorter'Lines'correspondtohigherfrequencies.OneofthedisadvantagesofTRLcalibrationislargelengthoflinesatlowfrequencies.AnotherproblemwiththeTRLcalibrationinthisstudywasthatDUTaswellasthecalibrationstandardsdependonthechangesinthecharacteristicimpedanceoftheunderlyingBaM-BSTthinlms.Thisproblemcouldberesolvedbycalibratingatdierentvoltagesandmagneticeldsbeforeeachmeasurement,butsuchprocedureisquitetedious.44

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TRLcalibrationwasattemptedafewtimes,butintheendSOLTcalibrationwasusedformeasurementsinchapter7.3.4.5FerrimagneticResonanceMeasurementsFerrimagneticresonanceFMRisaspectroscopictechniqueusedforprobingspinwavesandspindynamicsofaferrimagneticmaterial.TheFMReectcomesfromtheprecessionalmotionofthemagneticmomentofelectronswhenexposedtoanexternalmagneticeld.ConventionalFMRexperimentsareusuallyperformedinsidearesonantcavityplacedinanexternalmagneticeld.Thecavityresonatesataspecicfrequencydeterminedbyitsdimensions.Thesampleunderinvestigationisplacedatspecicregionswherethemag-neticeldlinesaremaximum,gure3.19.Asthemagneticeldischanged,absorptionofthemicrowavesinsidethecavityismeasured.Whentheprecessionfrequencyofmagneticmomentsisthesameastheresonantcavityfrequency,amaximumabsorptionisobserved. Figure3.19.Schematicofaresonantcavity.3.5SummaryThischaptergaveanoutlineoftheexperimentalmethodsandproceduresusedinthiswork.Highqualitythinlmswerepreparedusingrfmagnetronsputteringandpulsedlaserdeposition.StructuralcharacterizationwasstudiedusingX-raydiraction,scanning45

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electronandatomicforcemicroscopy.Measurementsofmicrowavepropertiesrequiredfabricationofcoplanarwaveguidesusingmicrolithography,andassemblyofahome-mademicrowaveprobestationinamagneticeld.Scatteringparametersweremeasuredus-ingavectornetworkanalyzer.Inaddition,ferrimagneticresonancemeasurementswereperformedthroughcollaborationusingamicrowaveresonantcavity.Followingchapterselaborateontheresultsobtainedfromtheseexperiments.46

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CHAPTER4BAM-BSTCOMPOSITETHINFILMS1Thischaptercontainsabriefsummaryoftheauthor'sworkforhismaster'sthesis[47]andthepublicationsthatfollowed[48,49].Muchofthisdissertationisbasedontheresultsandexperiencesofthiswork.Thelmpreparation,Cobalt-implantationandsomestructuralcharacterizationweredoneinDr.N.J.Dudney'slaboratoryattheOakRidgeNationalLaboratory,duringasummerinternshipin2001.MagneticcharacterizationwasdoneattheUniversityofSouthFlorida.4.1CompositeThinFilmsCompositethinlmsofBST/BaMweresynthesizedusingRFmagnetronsputteringandtheirmagneticresponsewasstudiedusingPPMS.DierentvolumetricproportionsofBSTandBaMphasesweresputteredfromstoichiometricceramictargetmaterialsBaMandBSTontoaluminasubstrates,andthetotallmthicknesswasabout2m.Duringthesimultaneoussputteringfromtwotargets,thesubstrateholderwasrotatedinordertoobtainuniformlmdeposition.Thedepositionconditionsoutlinedbelowfollowedseveralrecipesavailableintheliterature[75,95]andwerevariedmethodicallyuntilhigh-qualitylmswereobtainedconrmedbyx-raydiractionscans. Gasandpressure Argon,10mTorr Depositionrate Varyingfrom2-5A/min Annealing 10hat1000oCinO2 Thickness 2m Table4.1.DepositionparametersofthincompositelmssputteredatORNL. 1ThecontentsofthischapterhavebeenpublishedinJAP2003[48]andICF-Proceedings2005[49].47

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Thelmsweredepositedatambienttemperatureandpost-annealedinowingoxygen.FilmthicknesswasdeterminedbyaprolometerandthecrystalstructureandcompositionbyXRDandSEM.Followingvolumetriccompositionsweremadeandtheirpropertiescompared:pureBaM,pureBST,25%BaM-75%BST,50%BaM-50%BST,75%BaM-25%BST.Theguresbelow.1and4.2showthemeasuredpropertiesofthe50%-50%BaM/BSTcomposite. Figure4.1.XRDof50%-50%BaM/BSTsputteredcompositethinlmonalumina. Figure4.2.SEMof50%-50%BaM/BSTsputteredcompositethinlmonalumina.48

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Figure4.3.Magnetizationof50%-50%BaM/BSTsputteredcompositethinlmonalu-mina.TheXRDscanshowsapolycrystallinelmwithallthepeaksaccountedforbyBSTandBaMphasesandtheirorientationisappropriatelylabeledusingpowderdiractiondatabase[1,2,3].Thepeakswithoutlabelsareattributedtothealuminasubstrate.Thesharpnessofthepeaksandtheabsenceofotherimpurityphasesindicatethatthesetwophasescoexistwithoutmutualdegradation.ThisisanimportantresultwhichdemonstratestheformationofBST/BaMcompositesinthin-lmform.TheSEMscanshowsacoarsecrystallitestructureofthelmsurface.UsinglocalchemicalanalysiswithanenergydispersivespectroscopycapableSEM,itwasconrmedthatthebrightercrystallitesareFerich,indicatingthatthesearethehexaferritecrystallitesthatareembeddedinamorecontinuoussurroundingBSTmatrix[47].FormationofelongatedcrystallitesandnetworkstructureasseenintheSEMimageistypicalandhasbeenobservedinotherhexaferritelms[93].Thegure4.3showstheM-Hhysteresisloopforthe50%BaM/BSTcompositemeasuredat300KusingPPMS,afterthediamagneticbackgroundhadbeensubtractedtoobtaintheintrinsicmagneticparametersoftheferritealoneduetopresenceofaluminasubstrateandBST,therawmagnetizationcurveshowslargenegativeslope,typicalofadiamagneticmaterial.Atroomtemperature,acoercivityHcofnearly6kOeisobtained49

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withasomewhatmodestsaturationmagnetizationMs80emu/cm3.ThemoststrikingfeatureseenintheM-Hdataisthepresenceofadistinctdoubletransitionevidentfromtheshapeoftheloop,whichisonlyobservedintheBST/BaMcompositelmsandnotinthepureBaMlmsgrownunderidenticaldepositionconditions.So,itappearstoexclusivelybeapropertyincompositesandsuggestsastrongpossibilityofinuenceofBSTontheshapeandorientationofhexaferritecrystallitesandoverallmagneticproperties.Thedoubletransitionwasalsoobservedincompositesofotherratios%and25%ofBaMtoBST,andcomesfromthepresenceoftwotypesofBaMcrystalliteswithdierentshapesandorientations.Amoredetailedexplanationisgiveninchapters6.4and6.5whereitwillbeshownthatPLDdepositedcompositethinlmsfromasinglecompositetargetalsoshowasimilarbehavior.4.2Cobalt-ImplantationofCompositeThinFilmsFromtheabovementionedcompositelms,twosetsweregrownunderidenticalcondi-tions.OneofthemwassubjectedtohighenergyCoionimplantationusingtheion-beamfacilityattheORNL.Surfacebombardmentwithhigh-energyionsandimplantationhasbeenknowntoalterthecrystallitestructureandpossiblybreakdowntheaveragecrystal-litesizefromthemicrontonanometerlengthscale.Ithasalsobeenreportedthattheiontrackscreatelocalstressesandthuschangethelocalmagneticproperties[26].ThetablebelowliststhekeyparametersusedduringtheCoionimplantationprocess. 59Co+ Energyofthebeam BeamFlux Thinlmheatedbythebeam Beam1 450keV 81016/cm2 900C Table4.2.Cobalt-implantationprocessparameters.Acomparativeanalysisofthemicrostructureandmagneticpropertiesinbareandion-implantedBST/BaMcompositelmsispresentedbelow.ThemicrostructurevariationinCoion-implantedcompositelms,incomparisonwithsampleswithoutanysurface50

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modication,isshowninFig.1.ThesearetypicalSEMimagesforoneparticularcasewiththeferritecomponentbeing25%.a b Figure4.4.SEMof25%-75%BaM/BSTsputteredcompositethinlmonalumina:apurecompositelm,bCo-implantedcompositethinlm.ThetrendseenintheseimagesisalsopresentinsampleswithdierentratiosoftheBaMtoBST.ThebrightregionscorrespondtotheBaMcrystallitesandthisfactwasconrmedbydoinglocalenergydispersivespectroscopyEDSonindividualcrystallitestoascertainthechemicalphase[47].FromacomparisonofthetwoSEMimages,asignicantreductioninthesizeoftheferritecrystallitesisevidentintheCo-implantedlmwiththesizerangeanywherebetween40and150nm.Aswillbeseeninthelaterpartofthediscussion,thismodicationinthemicrostructurehassignicantimpactonthemagneticproperties.TheexpectationwasthatwithCo-ions,theradiationdamagecombinedwithpossiblemagneticcontributionsfromCoitselfcouldbeusedtotunethemagneticresponseinthehexaferritesamples.However,itshouldbenotedthattheuenceofCoionsinthebeamuxisprettylowandunlikelytoplayanysignicantroleinalteringthelocalmagnetization.Rather,itisexpectedthatanychangeinmagneticresponsewouldprimarilybearesultofthemodicationoftheBaMcrystallitestructurecausedbytheionimplantation.Radiationdamageinpermanentmagneticmaterialshasbeenstudiedinthepastanditisgenerallyunderstoodthatthiswillreducetheoverallmagnetization[59].Themechanismisduetothealteredlocalmicrostructure.LocalheatingbytheintenseionbeamsabovetheCurie51

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temperatureresultsinstabilizingregionswithoppositemagneticorientationsresultinginanoveralldemagnetizationeect.Whilethereductioninsaturationmagnetizationcanbereconciled,itwouldbeofinteresttoseehowthision-beammodicationwouldaectthecoercivityandtheshapeofhysteresisloops.a b c Figure4.5.M-HloopsofCo-implantedBaM/BSTsputteredcompositethinlmsonalu-mina:a75%BST-25%BaM,b50%BST-50%BaM,c25%BST-75%BaM.Thegure4.5showstheM-Hhysteresisloopsfordierentcompositesat300Kand5K.Adiamagneticbackgroundwasseeninallcompositesandthiswascorrectedinthedatashownhere.Allthecurvesshowsizablecoercivityconsistentwithahardmagnetic52

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BaMphase.Adistinctfeatureisasharpdouble-transitionintheM-Hdataseeninthecompositelmsthatwasdiscussedearlier.ThecoercivityHc,saturationmagnetizationMs,andtheremanentmagnetizationMrdecreaseastheBSTcontentincreases.ThedecreaseinHcwithadditionofBSTwasalsoreportedrecentlyinbulkcompositesamplesofZ-typehexaferriteBa3Co0:4Zn0:62Fe23:4O41andattributedtothedecreaseineectivemagnetocrystallineanisotropy[56].Forallthecomposites,withincreaseintemperaturethesaturationmagnetizationdecreasesbutthecoercivityincreases.Thisisoppositetowhatonewouldnormallyexpectandcanbeunderstoodbytakingintoaccountthecompetitionbetweentheshapeandmagnetocrystallineanisotropyinthesystem[74].TheM-Hcurvesandthedouble-transitionfeatureinCo-implantedlmsarequalita-tivelysimilarinshapetothatinbarecompositesbutquantitativedierencescanbenoted.Thereisapinchingo"nearzeroelds,leadingtoadecreaseinthecoercivityintheion-implantedcompositelms.However,the75%BSTsampledoesnotshowthistrendandinsteadhasalargercoercivityfortheCo-implantedcase.ThisreectsthesignicantrolethatBSTmatrixplaysinmediatingthemagneticinteractionsbetweenferritecrystallites.Theremanenteldalsoissignicantlyreducedbecauseoftheshapeoftheloop.Thede-creaseinMswithirradiationisalsoobservedinbubblegarnetlms[34]andisattributedtothedecreaseoftheferrimagneticallyorderedvolumeduetotheformationofnucleartracks.Thesemagneticparametersforallthesamplesstudiedaretabulatedintable4.3. BeforeCo-Implantation Composition HCTesla MRemu*10)]TJ/F16 7.97 Tf 6.587 0 Td[(3 MSemu*10)]TJ/F16 7.97 Tf 6.586 0 Td[(3 5K 300K 5K 300K 5K 300K 25%BST-75%BaM 0.58 0.67 4.05 2.32 7.68 4.12 50%BST-50%BaM 0.16 0.13 0.83 0.40 2.66 0.96 75%BST-25%BaM 0.07 0.11 0.68 0.35 5.95 1.19 BeforeCo-Implantation 25%BST-75%BaM 0.53 0.53 2.50 1.48 4.40 2.78 50%BST-50%BaM 0.13 0.12 0.53 0.21 1.89 0.71 75%BST-25%BaM 0.20 0.65 1.09 0.65 6.50 1.33 Table4.3.MagneticparametersoftheCobalt-implantedcompositethinlms.53

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Thechangesinmagneticpropertiesareclearlyduetotheinuenceofthestrongcou-plingbetweenthemicrostructuralmodicationswiththemagnetism.ThedecreaseincoercivityisexpectedjustfromtheconsiderationofthesmallercrystallitessizeseenfromtheSEMimagesoftheCo-implantedsamples.Innanostructuredmaterials,itiswellknownthatasthecrystallitesizeisreduced,thereisinitiallyanincreaseinthecoercivityfollowedbyarapiddecreaseasthesizeisreducedfurther[46].Therearealsoadditionalstress-inducedchangesinlocalpermeabilitythatoneshouldconsiderinion-implantedma-terials.Thishasbeenobservedbyothergroups[26].TheseobservationsclearlyshowthatthecoercivityandremanencecanperhapsbecontrollablyvariedevenwiththeoverallM-Hshapestillpreserved.Thiscouldhaveimportantimplicationsforrecordingmediawherematerialswithhighanisotropylikehexaferritesareneededbutlowereldsaredesirablefromapracticalpointofview.4.3SummaryStructuralandmagneticpropertiesofsputteredferrite-ferroelectriccompositethinlmswereinvestigated.AsetoflmswasalsosubjectedtohighenergyCobalt-ionimplantation.Hysteresisloopsexhibitedadoubletransitionthatisobservedonlyinthecompositelmsandnotinpurehexaferritelmsgrownunderidenticaldepositionconditions.TheunusualfeatureintheM-HdataisascribedtotheexistenceofBaMcrystalliteswithdierentshapesandorientationsinuencedbythesurroundingBSTmatrix.ThemoststrikingfeatureintheCo-implantedlmswasthereductionoftheferritecrystallitesizeseenintheSEMscans.ThathadadirectinuenceonthereductionofthecoercivityandsaturationmagnetizationofthesamplesasseenintheM-Hloops.54

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CHAPTER5SPUTTEREDBAM-BSTTHINFILMMULTILAYERS1Afterthesuccessofthesputteredcompositethinlms,theresearchonBaM-BSTsystemcontinued,andmultilayeredthinlmsofBaMandBSTweregrownusingRFmagnetronsputteringduringanotherinternshipatORNLin2003,aswellasduringseveralresearchvisitstoProfessorK.Coey'slaboratoryattheUCF-AMPACfacilitiesin2004and2005.ThischapterisasummaryoftheworkonsputteredBaM/BSTmultilayerswhichresultedintwopublications[41,109].5.1ThinFilmMultilayersSputteredatORNLTheworkonthesputteredthinlmmultilayerswasdonetogetherwithN.FreyandDr.S.Sanyadanam,onlypartialresultsthatarerelevanttothisdissertationarepresentedhere.Moredetaileddiscussioncanbefoundinthereferredpublications.MultilayersofBSTandBaMweredepositedonaluminaandsiliconsubstratesusingRFmagnetronsputteringinthepresenceofhighpurity.999%Argas.Theas-grownlmswereamorphousandweresubsequentlyannealedat10000CinowingO2for10htoobtaincrystallinesinglephases.Filmsonsiliconsubstratesdisplayedpooradhesionandakedoduringannealing.Itwasfoundthatheatingthesubstratecouldsurmountthisproblemwhilesputtering,resultingingoodadhesionandahigh-qualitylm.Thethick-nessoftheentirelmsconsistingofastackoffouralternatinglayersofBSTandBaMwasoptimizedtobearound1.5m.Thetable5.1summarizesthegrowthconditionsforeachlayer.Thebasepressureineachcasewaslessthan6.8*10)]TJ/F16 7.97 Tf 6.586 0 Td[(6Torr.Technicaldicultieslimitedthesubstrateheatingto3500Cduringthesputtering.However,thistemperature 1ThecontentsofthischapterhavebeenpublishedinJAP2005[109]andMRB2005[41].55

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wasnotsucienttogetthepropercrystallinephasesandannealingwasnecessary.Mul-tilayersweredepositedonpolycrystallinealuminaandorientedSisubstrates. BasePressure=6.8*10)]TJ/F16 7.97 Tf 6.586 0 Td[(6Torr FirstLayer SecondLayer ThirdLayer FourthLayer Depositiontemperature3500C BST BaM BST BaM ArgonpressuremTorr 20 20 20 20 Argonowsccm 57.0 56.7 56.6 56.4 PowerW 71 70 71 70 DCBiasV 146 242 148 245 Depositionrate/min 60.0 39.2 71.6 46.4 Table5.1.Multilayersputteringparameters. Figure5.1.X-raydiractionofBaM-BSTmultilayers.BaMandBSTphases,aswellasthepeaksofaluminasubstrateareindexed.Figure5.1showstherepresentativeX-raydiractionpatternofthemultilayeralu-mina/BST/BaM/BST/BaM.Thespectrumindicatesthepresenceofthedesiredcompos-itemultiphaseconsistingofreectionsfromBSTandBaMandalsothoseofthealuminasubstrate.Notracesofimpuritywerefound,howeverpossibilityofinter-diusionattheinterfacesofthemultilayerstructurecannotberuledout.MultilayersgrownonSidisplayedpoorcrystallographictexture.TracesofimpurityphaseswerefoundnotshownhereindicatingtheinteractionofSrwiththeSisubstratesformingSr3Si3O9.Itisbelievedthatthisimpuritychemicalphaseshouldshownodiscernibleinuenceonthefunctionalmagneticpropertiesofthemultilayerstructure.56

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MagnetizationmeasurementswereperformedatvarioustemperaturesusingPPMS.Measurementsweredoneonallsampleswithmagneticeldappliedparallelin-planeandperpendicularout-of-planetomultilayerplaneat10and300K.Ingure5.2a,thein-planehysteresisloopsmeasuredat10and300KforsamplesonaluminaandSiareshown.Presenceoflargecoercivityat10KHc1450Oeandat300KHc2300OeformultilayeronSiandforaluminasampleat10KHc2200Oeandat300KHc1550OeisconsistentwiththehardmagneticnatureoftheBaM.a b Figure5.2.MagnetizationofBaM-BSTmultilayer:aTemperaturedependenceofin-planemagnetizationofBaM-BSTmultilayers.,bAnisotropyofBaM-BSTmultilayersat10K.Figure5.2b-topshowsthemagnetizationhysteresisloopsofthemultilayeronSiwithappliedeldparallelinplaneandperpendicularout-of-planetothelmplane.TheperpendicularcoercivityHc?andthein-planecoercivityHcjjat10Kare1250Oeand1450Oe,respectivelyandthesquarenessStheratioofremanencemagnetizationtosaturationmagnetizationoftheperpendicularandparallelloopsare0.45and0.6,respectively.Thedierenceinmagnetizationintwodierentdirectionsofthemultilayerplaneindicatesthatthereisapreferentialorientationofmagnetizationalongafavoreddirection,whichisacharacteristicofbariumferritethinlms.TheBSTlayersandtheSisubstratecontributetoadiamagneticbackground,whichisreectedinthenegativeslopesoftheM-Hcurvesathighereldsandhadbeensubtractedinthedatashown.Incan57

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HcOe HcOe Sample In-plane Out-of-plane In-plane Out-of-plane K K K K BaM/BST/Si 1460 1250 2300 2200 BaM/BST2/Al2O3 2595 2595 3900 4000 BST/BaM2/Al2O3grownatRT 2260 2600 1550 1900 Table5.2.CoercivitiesofthemultilayerthinlmsbeseenthatwithincreasingtemperatureHcincreasesandthemagnetizationdecreases.Thistrendisoppositetotheexpectedbehaviorinbulkmagneticmaterialswherelargercoercivitiesaregenerallyobservedatlowertemperature.Incontrast,sampleonaluminagure5.2b-bottomshowsnosuchdeviationuponchangingorientationsofthemagneticeld,becausepolycrystallinealuminasubstratedoesnotallowanycrystalliteorientation.ThevalueofHcasaresultremainsthesameinbothdirectionsandthevalueofHc?=Hcjjat10Kis2600Oe.TheresultspresentedaboveareconsistentwiththosereportedforpureBaMlmsdepositedusingPLDheatedinsituat9000ConSisubstrates[85].TheHcvaluesofvariousmultilayersamplesat10and300Karesummarizedintable5.2.TheincreaseinHcinthepresentcasewithtemperaturemaybeduetothecompetitionbetweentheshapeandmagnetocrystallineanisotropies.Thetheoreticalcoercivityofarandomarrayisshownintable5.3[74].HC=0.48K=MS-NMS2K/MS-crystalanisotropyNMS-shapeanisotropy)]TJ 9.963 9.963 Td[()]TJ 8.421 8.418 Td[()1000(@@@RTable5.3.DependenceofthinlmcoercivityonanisotropyAsMsdecreaseswithincreaseintemperature,thecontributionfromtheshapeanisotropydecreasesandHcincreaseswithtemperature.TheshapeoftheBaMcrystallitesdependsverymuchonthetypeofsubstrateandsubstratedepositiontemperature[85].Thelms58

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hereweregrownatmoderatetemperatures3500Candnotsucientlyhighenoughtocausecrystallization.Thistemperature-dependentgrowthandcrystallitestructureofBaMmayexplainthevariationsincoercivityvaluesobservedhere.59

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5.2ThinFilmMultilayersSputteredatUCFFollowingtheinspiringresultsfromthesputteringoftheBaM/BSTmultilayersatORNL,severalresearchtripstoUCF-AMPACweremadeinordertodepositmorethinlmsusingRFmagnetronsputtering.Thisfacilitydidnothaveanoptionofheatingthesubstrateduringthedeposition,thusseveralotherdepositionandpostannealingmethodsweretriedout.Thetable5.5liststhedepositionparametersusedinthisprocess. UCFmagnetronsputteringdepositionparameters RFmagnetronsputteringunitfromAJAInternational. Power:BaM=70W,BST=80W. Gaspressure:20mtorr,80%Ar+20%O2. Substratetemperature=ambienttemperature. Targets:3"powder-pressedBaMandBST. Substrates:10mmx10mmone-sidepolishedC-sapphire,A-sapphireand100MgO. Substrate-targetdistance=2cm. Postannealinginfurnaceat8000C11000CinowingO2. Filmsthicknessvaries0.5-1.5microns. Table5.4.UCFmagnetronsputteringdepositionparametersThinlmmultilayersofdierenttotalthicknessweredeposited.Thistime,insteadofaluminasubstrates,orientedsapphireC-andA-orientationandMgOsubstrateswereused.BaMwasalwaysdepositedrstonsapphiresubstrate,whileBSTwastherstlayeronMgO.Orientedthinlmsgrownonthesesubstratesusingsputteringwerereportedbyseveralauthors[57,58,75,93,95].Unfortunately,inallthesestudiessubstratewasheatedduringthedepositiontopromotelmgrowthandlmadhesion.Inordertoovercomethis,postannealingwasdoneatseveraltemperaturesandatvariousannealingandheating/coolingtimes.Inaddition,verythinseed-layersafewnmoftherstlayerweredeposited,thenannealed,followedbyadepositionofamultilayer,whichagainwaspostannealed.TherewerenoproblemswiththelmadhesionthatwasseeninearlierdepositionsonSi,orwithformationofdierentphases.Themultilayershadingeneral60

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sharptransitionswithoutmuchintegrationasseenfromtheSEMscanofonesampleonsapphireingure5.3. Figure5.3.Cross-sectionalSEMscanofBaM-BSTmultilayeronsapphire.Themainprobleminthelmpreparationwasthecrystallization.Mostofthelmswereamorphousorshowedpoorcrystallizationevenafterannealing.Thetable5.5outlinessomeoftheresultsoftheseexperiments. >1.5m 1:0m <0.5m BaMmostly001-oriented NoBaMpeaks VerygoodBaM001 C/BaM/BST BSTpolycrystalline BSTweakly orientation withstrong110peak polycrystalline NoBSTpeaks BST100-orientation BSTstrong100 BSTverygood100 MgO/BST/BaM orientation orientation BaMmostly110oriented BaMweak110peak NoBaMpeaks Table5.5.PartialresultsfromsputteringatUCF.AlllmswerepostannealedinO2.Asitcanbeseen,mostofthelmswerepolycrystallineorevenweaklypolycrystallineonlyverysmallandbroadpeaksdetectedbytheXRDscan,someevenamorphous.Theseresultsindicatethatsubstrateheatingplaysveryimportantroleinobtaininghighqualitylms,anditcannotbereplacedbyusingseed-layersandpostannealing.5.3SummaryThinlmmultilayersofBaMandBSTweresputteredontodierentsubstratesundervariousconditions.ThelmsdepositedonheatedaluminaandSiwerepolycrystallineand61

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showedsomeunusualmagnetizationpropertiesthatwereexplainedascompetitionbetweenmagnetocrystallineandshapeanisotropy.ThesputteredlmsonsapphireandMgOatambienttemperatureshowedpoorcrystallinity,evenafterannealing.Themainconclusionisthatthedepositiontemperature,choiceofsubstrateaswellaspresenceofBSTstronglyinuencethestructuralandmagneticpropertiesofBaMlms.62

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CHAPTER6PULSED-LASERDEPOSITEDBAM-BSTBILAYERSANDCOMPOSITETHINFILMS1Theresultsfromthepreviouschaptershowedthatsubstrateheatingaswellasthesubstratechoiceareessentialfactorsforobtaininghighqualityorientedthinlms.InadditiontoRFmagnetronsputtering,pulsedlaserablationPLDisanothersuccessfultechniquefordepositionofqualitythinlmswithexcellentstoichiometry[25].Inthischapter,theresultsfromthePLDdepositionofBaM/BSTbilayersandcompositethinlmsarepresented.ThedepositionwasdoneattheLaboratoryforAdvancedMaterialsandTechnologyLAMSATatUSF,andallcharacterizationwasdoneatUSFfacilities.Thetable6.1liststhetypicaldepositionparametersusedinthisstudy. Pulsed-laserdepositionparameters ExcimerlaserKrF=248nm: Energydensity=4-9J/cm2. PLDrateincreasedgraduallyfrom1to10ppsinrst10minutes. Substratetemperature=6500Cwithsilverpaste. Gas:highpurityO2at250mtorr. Targets:2"powder-pressedBaM,BSTandBaM-BST%-50%byweight. Substrates:10mmx10mmone-sidepolishedC-sapphire,A-sapphireand100-MgO. Substrate-targetdistance=5.5cm. Filmscooledafterdepositionin1atmO2at50C/min. Postannealinginfurnaceat9000CinowingO2. Totallmthicknessabout1micron. Table6.1.Pulsed-laserdepositionparametersBilayersaswellascompositeBaM/BSTlmsweredepositedontopolishedsapphireA-andC-orientedand-orientedMgOsubstrates.Priortothedeposition,theas1Thecontentsofthischapterarepublishedin[50,51]63

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receivedsubstrateswereultrasonicallycleanedinacetone,rinsedwithmethanol,anddriedwithnitrogen.KrFpulsedexcimerlaserwasusedtoablatethematerialfromtherotatingtargettothesubstrate,asillustratedinthegure3.2.Thelaserenergydensitywasfocusedtoabout8.7J/cm2forBaM,4.3J/cm2forBSTand6.5J/cm2forthecompositetarget2.Thecompositetargetwasmadebymixing50%byweightBaMandBSTpowders,andpressingthemixtureintoa1"pallet.Thesubstrateswerepositioned5.5cmawayfromthetargetandparalleltothetargetsurface.TheidealdepositiontemperatureforBaMonsapphireisreportedtobearound9200C[105,107],whiletheidealdepositiontemperatureforBSTvariesfrom7750Cto8200C[19,62].Themaximumdepositiontemperaturewiththisset-upwas6500Cusingaquartzlamp,thusthistemperaturewasalsochosenasthedepositiontemperature.Athinlayerofsilverpastewascarefullyappliedtothebackofthesubstratesinordertopromoteuniformheatowfromthesubstrateheatertothedepositedlmduringtheheateddeposition.Beforedeposition,thechamberwasevacuatedforseveralhourstoapressurebelow10)]TJ/F16 7.97 Tf 6.586 0 Td[(5torr,andthenhighpurityoxygen.999%wasintroduced.AlldepositionsweredoneatconstantO2pressureof250mtorr.Afterthedeposition,thelmswereslowlycooledin-situtoroomtemperatureatalmost1atmgaspressure.AlllmswerealsopostannealedexsituinaquartztubefurnaceinowingO2.Afterseveralpostannealingtrials,themostsuccessfulannealingprocedureintermsofcrystallographicstructurewasdeterminedtobeasfollowing:Increasethetemperaturefromroomtemperatureto9000Cin3hrs.Annealat9000Cfor8hrs.Decreasethetemperaturefrom9000Ctoroomtemperaturein3hrs.AllsubsequentPLDlmswerepreparedthisway.Atthebeginningofthedeposition,thelaserpulseratewasincreasedfrom1ppsto10ppsgraduallyoveratimeperiodof10min.Thisstepwasneededtopromoteinitialcrystalgrowthasreportedbyotherauthors 2Duetotheageandthequalityofthelasersystemandtheoptics,thereallaserenergycouldhavebeenlessthanreportedhere.64

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[107].Afterthat,theratewaskeptat10pps.Dependingonthetargetmaterialsanddesiredlmthickness,thetotalnumberofpulsesvaried.Whendepositingmorethanonelayer,thechamberhadtobeopened,thetargetwaschanged,andtheaboveprocedurewasrepeated.Thetextureandsurfacemorphologyanalysiswasperformedinordertostudythestructuralqualityofthelms.ThemagnetizationmeasurementswerealsostudiedusingPPMS.Theseresultsarepresentedforeachlmtypeinthefollowingsections.ThecrystalstructuresofBaMandBSTwerealreadydescribedinthechapter2,BSThavingacubicandBaMhavingahexagonalcrystalstructure.Dependingonthecrystallo-graphicorientationofthesubstratesandtheunderlyingthinlms,thegrowthorientationsofthelmonthetopvaried.Thereasonisthatduringtheinitialstageofthedeposition,thelmorientationisadjustedtothesubstratetominimizelatticemismatch.Sometypicalorientationsofthecubicandhexagonalcrystallatticesareshowningure6.1.Dependingonthepreferredlmorientation,Bragg-reectionsfromdierentplanesarevisibleintheX-raypole-plot.a b Figure6.1.Crystalplaneorientations:acubiclatticebhexagonallattice.65

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6.1BaM-BSTbilayeronC-sapphireBaM/BSTbilayersweredepositedonC-orientedsapphiresubstratesandcomparedwithpureBaMlmsdepositedonthesamesubstrate.Figure6.2illustratesthearrange-mentoflayersandshowstheirpreferentialorientation.TherstlayerBaMwasde-positedusing45,000pulses,resultinginathicknessof500nm,andthesecondlayerBSTneeded61,000pulsesforthesamethickness.ThepureBaMlmwasdepositedusing46,000pulses.Foursamplesweredepositedsimultaneouslyandusedfordierentstudies.Theas-depositedlmswerecrystalline,butitwasdeterminedthatpostannealingfurtherimprovedthecrystallographicstructure.a b Figure6.2.PLDdepositedthinlmsonC-sapphiresubstrate:aBaMsinglelayerbBaM-BSTthinlmbilayer.TheX-raydiractogramsoftheC/BaM/BSTbilayerareshowningure6.3.Themaingureshowsa)]TJ/F15 10.909 Tf 11.612 0 Td[(2scanofthesample.Eachpeakislabeledaccordingtoitscrystallographicorientation.IncaseoftheBaMlm,thethird"indexisusuallyomittedforsimplicatione.g.insteadof06.Itcanbeseenthatallpeakscomeeitherfromthesubstrate,BaMorBST.TheBaMlmshowsonlypeaksbelongingtothelgroupofpeaks,indicatingthatthec-axisofthehexagonalstructuregrowsoutofthethinlmplane.Ontheotherhand,thetwosharpBSTpeaksandshowastrongalignmentinthehhhdirection.Theanalysisofthedegreeoflmalignmentfromthe!-scansrockingcurvesrevealslowdispersionandepitaxialnatureofthebilayers:thefull-width-at-half-maximumisFWHM=0.370forBaMpeakand0.410forBSTpeak.Thesevaluescorrespondtohighlyorientedepitaxiallms.BaMthinlmsdepositedundersimilarconditionsbutathighertemperaturesshowslightlylowervaluesofFWHMaround0.150[105,107].TheBSTlayer,asitwillbeseenlater,alsoinuences66

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theorientationoftheBaMlmattheinterface.BSThasacubiccrystalstructureasdescribedinchapter2.2,withthelatticeparameteraround3.95A.ItscrystalstructureissimilartoMgOthathasalatticeparameteraround4.2AinnextsectionitwillbeseenthatBSTgrowsepitaxiallyonMgO.Itwasshownin[110]thatMgOthinlmdepositedonaC-orientedsapphiresubstrategrowsinpreferential111orientation,withFWHMof2.40,indicatinglargemosaicity.ThissuggeststhatifMgOgrowsonC-sapphire,andBaMgrowsepitaxiallyonC-sapphire,thatBSTwouldalsogrowonBaM,whichisindeedthecasehere.Inaddition,bothC-orientedBaMaswellasBSTlmshavebeensuccessfullygrownonMgO[89,91,94],thusitwasintuitivetotrygrowingBSTlmsofC-orientedBaM.IthasalsobeenreportedthatBSTlmsexhibitsimilarpropertiesaslmsatmicrowavefrequencies[91].ThesmallFWHMintherockingcurveoftheBSTlmssuggestsabetterlatticematchcomparedtotheMgOgrownonC-sapphire,probablydueasmallerdierenceinthelatticeparameter.Growthconditionsplayasignicantroleaswellindeterminingtheoverallqualityofthelms.ThevaluesofFWHMofthisBSTlmareevenbetterthantheFWHMvaluesoftheBSTlmgrowndirectlyonMgOasseeninthenextsection.470andBSTlmsgrownonandMgO.540and0.510,respectivelyreportedby[90].Thegures6.4aandbrevealthecrystallographictextureanalysisoftheC/BaM/BSTbilayer.BecauseplanesofBaMcrystallitesandplanesofBSTcrystallitesareparalleltothelmsurface,adierentsetofplanesneedstobechosenforthepoleplotand-scan[105].Figure6.4aisanXRDpoleplotofBaMpeakandBST200peak,bothbeingthestrongestpeaksinthe)]TJ/F15 10.909 Tf 9.333 0 Td[(2scanoftheX-raydiractionpattern[2,3].Thegure6.4bisa-scanat=350forBaMandat=550forBST.TherearesixpeaksinthepoleplotoftheBaMlmandalsosixpeaksinthepoleplotoftheBSTlm.-BSThasathree-foldsymmetryabouttheaxisperpendiculartotheplane[110].OccurrenceofsixpolesintheBSTlmindicatespresenceoftwosetsofepitaxialsinglecrystallitesrotatedby600toeachother-[;;; 200; 00; 0].Thisphenomenoniscalled67

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Figure6.3.-2scanofBaM-BSTbilayeronC-sapphiresubstrate.Inset:rockingcurvesofBaMandBSTpeaks.twinninganddevelopsduringlmgrowth,orcoolingfromhigh-temperaturedepositiontoroomtemperature,whenthereisalargelatticemismatch[70].The-scans,whichareperformedathigherresolution,actuallyconrmthisandshowanadditionalsetofthreepolesseparatedby1200.ThisresultindicatesthattheBSTlmisnotpurelysinglecrystalline,butepitaxiallypolycrystalline.OnesetofthreepolesandonesetofsixpolesofBSTlmgrownonMgOwasalsoobservedby[91].InthecaseofBaMlm,thesixpeakscomefromthesixsetsofplanesassociatedwiththesixfacesofthehexagon.Everytimethesampleisrotatedby600aBragg'sconditionissatised,resultinginapeak.ThisisconsistentwithepitaxialgrowthofBaM,withcrystallineaxisofBaMbeingparalleltotheaxisofsapphire,alsoreportedby[57,105].The-scanrevealsthattherearealsotwosetsofthreepeaksseparatedby1200-oneat-1200,00,1200,andtheother,verysmalloneat-600,600,1800.ThiswasalsoobservedinthesinglelayerBaMlmnotshownhere.Thisindicatesapresenceofothersetsofcrystallites,tiltedfromtheC-axisduetothetwinning,aswillbeevidentinthemagneticpropertiesofthesample.Thisdefectmostlikelyhastodowithinsucienttemperatureduringdepositionprocess.Otherlmsgrownathighertemperaturesdonotshowthis[57,105,107].At68

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a b Figure6.4.CrystallographictextureanalysisofofBaM-BSTbilayeronC-sapphiresub-strate:aXRDpolegureofBaMpeakat2=32:2920andBSTpeakat2=45:9440,bazimuthalscanofBaMpeakat=350andBSTpeakat=550.temperaturesabove9000C,thelatticemismatchbetweentheBaMandsapphiresubstrateislessthanattheroomtemperature.Thelatticeparametersofthea-axisofBaMandsapphireare5.89Aand4.75A,respectively,whiletheexpansioncoecientsarea=9:99ppm/0CforBaManda=4:9ppm/0Catroomtemperatureanda=15:6ppm/0Cat5000Cforsapphire[33].ThelatticeparameterexpansioncoecientofsapphireismuchsmallerthanforBaMatroomtemperature,butitincreaseswithtemperature,andonlyatelevatedtemperatures,thegapbetweenthelatticeparametersstartstoshrink.Duetothecapabilitiesofcommercialsubstrateheaters,maximumtemperaturesforthinlmdepositionareusuallyaround900-10000C,thusthisrangewasalsoestablishedbyothersasoptimalgrowthtemperatureforBaMlms[105,107].Figures6.5aandbpresentroomtemperaturemagnetichysteresisloopsM-HcurvesoftheBaMsinglelayeraandtheBaM/BSTbilayerb,withmagneticeldappliedparallelin-planeandperpendicularout-of-planetothelmsurface.ThediamagneticcontributionsmanifestedbyanegativeslopeintheM-HcurvescomingfromthesubstrateandBSTlmhavebeensubtracted.Alargeout-of-planeanisotropyisobserved,typicalforout-of-planegrowncrystallites,witheasymagnetizationaxisperpendiculartothelm69

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a b Figure6.5.In-planeandout-of-planemagnetizationofPLDdepositedthinlmsonC-sapphiresubstrate:asinglelayerBaMthinlmbBaM-BSTthinlmbilayer.plane.Thein-planemagnetizationdoesnotreachthesaturationatthemaximumeldof12kOe,becauseoflargeanisotropyeld,whichisaround17kOe[77].Thesquarenessandthecoercivityoftheout-of-planeloopofthesinglelayeraresmall0.22and545Oe,respectively,makingitmagneticallysoft,whichisgoodforlowlosshigh-frequencyapplications.Thebilayerismagneticallyharder,withsquarenessof0.62andcoercivityof1850Oe.ThisismostlikelycausedbytheBaM/BSTinterfaceandisalsonoticedinthebilayergrownonA-orientedsapphiresamples.Apossibleexplanationisgiveninsection6.3.TheM-HloopsofasinglelayerBaMlmarecomparablewiththedataby[105]ingure6.8-rightpanel,althoughtheirlmsweredepositedatahighertemperature.Aslightbroadeningofthein-planehysteresisloopisobservedinbothsingleBaMaswellasBaM/BSTbilayer.Thisiscausedbyapresenceofasmallamountofin-planeorientedcrystallites,alsoevidentfromtheX-raydiractionanalysis.Theatomicforcemicrographsingures6.7revealthesurfacemorphologyofBaMsinglelayerthinlm.Hexagonalcrystallitesareseengrowingoutofthelmplane,consistentwiththemagnetizationstudies.Onecanalsoobservethepresenceofsmallparticles"atthecrystalliteedges,roughly2-5nmindiameter.ThesearelikelyBaMatomicclusters,comingfromunstablegrowthandmound"formationduringdepositionprocess,asillustratedingure6.6.Themoundsoccurwhenthediusingadatomsarereectedfromthedescending70

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stepedgesandattachtotheascendingsteps[35].TheBaMlmgrownat9200Casreportedby[105],experienceslargeshearduetothelatticedistortionofBaMattheinterfacewiththesubstrate.Thiscreatesscrewlocationsthatpropagatefromtheinterfaceallthewaytothelmsurface,creatingspirallywindingstepsandterracesseegure6.8,leftpanelatthebottom.ThelmgrowsbydepositingBaMatomsontothesesteps.Duetothelowerdepositiontemperatureinthiscase,someoftheseatomsseemedtohavenucleatedatthecrystalliteedges. Figure6.6.Bilayerthinlmgrowthduringdeposition[35].GrowthandcharacterizationofPLDdepositedBaMthinlmsonC-orientedsapphirewasthoroughlyinvestigatedby[105],anditisverycomparablewiththedatapresentedhere,consideringthatthegrowthtemperatureherewasaround6500C,andthelmswerepostannealedat9500C.Themagnetizationfromgure6.5aforsinglelayerBaMlmtswellwiththedataingure6.8-betweengurescandd,ontherightpanel.TheAFMscansingure6.7showingout-of-planegrowncrystallitesalsocorrespondwellwiththeAFMscansintheleftpanelandillustrationsinthemiddlepanelofgure6.8forthetemperaturebetween7000Cand9200C.Thisshowsrepeatabilityofthelmpreparation,evenwithsomesmalladjustmentsinthegrowthconditions.71

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a b Figure6.7.AtomicforcemicroscopescanofsinglelayerBaMthinlmonC-orientedsapphiresubstrate:a2.5mx2.5mareascan,b1.0mx1.0mareascan. Figure6.8.ResultsfromPLDdepositedBaMlmsonC-sapphireby[105].72

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6.2BaM-BSTbilayeron-MgONextsampleisaBST/BaMbilayergrownon-orientedMgOsubstrateundersameconditions.MgOisanexcellentdielectricsubstrateformicrowaveapplicationswithdielec-tricconstant=9.5andlosstangenttan=110)]TJ/F16 7.97 Tf 6.587 0 Td[(5beinglowerthanothersubstratesusedforBSTgrowthLaAlO3,alumina,sapphire.Ithasacubiccrystalstructurewithlatticeparameterofa=4:214A,andrepresentsarelativelygoodsubstratematchtoBST,whichhasa=3:947A,althougha6.4%latticemismatchsuggeststhattheepitaxialgrowthmightbediculttoachieve.HighqualityBSTlmsgrownon-MgOhavebeenre-portedbyothers[20,22,62,90,99].Firsta0.5mBSTlayerwasdepositedontheMgOsubstrateusing60,000pulses,andsubsequentlyaBaMlayerofsamethicknesswasdepositedusing45,000pulses. Figure6.9.BST-BaMbilayerthinlmson100-MgO.The)]TJ/F15 10.909 Tf 10.192 0 Td[(2scaningure6.10revealsonlyh00peaksintheBSTlmandlpeakscomingfromtheBaMlm.Nosecondaryorientationswereobserved,suggestingthatthelmsareunidirectionalandsinglephase.The!-scansshownintheinsetconrmagooddegreeofout-of-planeorientation-FWHMofBaMpeakis0.550andofFWHMofBSTpeakis0.470.Figure6.11showsatextureanalysisoftheBSTlm.BaMlmcouldnotbeanalyzedbecausereectionsfromallpossibleplaneslietooclosetothereectionsoftheBSTplanes,whicharemuchlargerinmagnitude.ThepoleplotoftheBSTpeakat2=67.0120indicatesapresenceoftwosetsoffourpoles,oneat=250andtheotherat=450,consistentwiththecalculationoftheinterplanarangles.Foracubicsystem,theanglebetweentheatomicplanescanbeexpressedas[37]:73

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Figure6.10.-2scanofBST-BaMbilayeronMgOsubstrate.Inset:rockingcurvesofBaMandBSTpeaks.=cos)]TJ/F16 7.97 Tf 6.586 0 Td[(1h1h2+k1k2+l1l2 p h21+k21+l21h22+k22+l22Thereareonlytwosetsoffourpolessatisfyingthisrelationshipforthegivenangles-[; 20; 2 20; 220]and[; 220;; 20].The-scaningure6.11btakenat2=67.0120and=450indicatesexcellentin-planeepitaxy,asnoadditionalpeaksareobserved.Bothpoleplotand-scanconrmthattheBSTlmgrowsepitaxiallyontheMgOsubstrate.EventhoughtheX-raydiractionscansshowgoodqualityBaMlmwiththec-axisgrowingoutoflmplane,thein-planeandout-of-planemagnetizationloopsingure6.12donotshowalargepreferentialorientation,asitwasseenearlierfortheBaMlmdepositedonC-orientedsapphire.Thesaturationmagnetizationalongtheaxisperpendiculartothelmplaneisonlyslightlysmallerthanalongtheaxisparalleltothelmplane.Thiscanbeexplainedbylookingatthecomponentscontributingtothemagneticanisotropy.Thereisacompetitionbetweencrystalanisotropytryingtoalignthemagnetizationperpendiculartotheplaneandtheshapeanisotropytryingtoalignthemagnetizationinthelmplane.74

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a b Figure6.11.CrystallographictextureanalysisofBSTinBaM-BSTbilayeronMgOsubstrate:aXRDpolegureofBSTpeakat2=67:0120,bazimuthalscanofBSTpeakat=450.TheAFMscaningure6.13showsapresenceofacicularshapedBaMcrystallites,orientedinthelmplane.ThiskindofcrystallitegrowthinBaMlmshasalsobeenobservedbyothers[24,64,82,84,112,119],andtypicallyoccurswhenthegrowthisnotepitaxial,e.g.onsubstratessuchasSi/SiO2,Pt,polycrystallinealumina,etc.Itwasalsoreportedby[82,112]thatthepresenceoftheseelongatedin-planeorientedcrystallitesfavorsmagnetizationparalleltothelmplane.Theeectiveanisotropycanbecalculatedfromtheareasenclosedbythehysteresisloops[97]:Keff=K?)]TJ/F18 10.909 Tf 10.909 0 Td[(Kk=ZH?dM)]TJ/F24 10.909 Tf 10.909 14.849 Td[(ZHkdM=0:52106erg cm3whereH?andHkaremagneticeldsappliedperpendicularandparalleltothelmplane,respectively.Themagnetocrystallineanisotropyconstantcanbeexpressedas:K1=Keff+2M2s=0:52106erg cm3+0:6106erg cm3=1:12106erg cm3withMs315emu cm3beingthesaturationmagnetization,afteraroughestimateofthevolumeofthesampleV=710)]TJ/F16 7.97 Tf 6.587 0 Td[(6cm3.AlargedierencebetweenKeffandK1indicates75

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thattheshapeanisotropyplaysanimportantroleindeterminingoverallmagnetizationresponseinthisthinlm[97].Approximatevaluesofthemagnetizationparameterscoer-civeeldandsquarenessforin-planeandout-of-planehysteresisloopsaregivenintable6.2: HcOe Mr/Ms IN-PLANE 2045 0.63 OUT-OF-PLANE 2000 0.58 Table6.2.MagneticcharacteristicsofBST/BaMbilayergrownonMgOsubstrate.Thesevaluesindicatethatthelmhasahighdegreeofmagnetichardness,bothinpar-allelandperpendiculardirectionofthemagneticeld,andthesevaluesarewellcomparabletotheotherreportedvaluesofisotropicBaMthinlms[97,105,113].ThecrucialresultofthissectionisthatagoodqualityBaMlmwithhardmagneticpropertiescanbegrownonhighqualityBSTlm.ItwillbeseeninthenextchapterthatthisbilayerBST/BaMlmshowslargeelectricaltunabilityinthefrequencyrange1-65GHz,andmagnetictunabilityintherange40-60GHz.76

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Figure6.12.M-HloopofBaM-BSTbilayeronMgOsubstrate. Figure6.13.AtomicforcemicroscopescanofBST/BaMbilayeronMgOsubstrate-thetoplayerseenhereisBaM.Scanarea=2.5mx2.5m.77

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6.3BaM-BSTbilayeronA-sapphireAnotherBaM/BSTbilayerlmwasgrownonA-orientedsapphiresubstrateundersameconditionsasthebilayeronC-sapphiresubstrateinsection6.1.Itwasreportedby[57,131]thattheBaMlmdepositedonthissubstratehasitsc-axisinthelmplane,exactlyoppositetotheBaMlmgrownonC-sapphire.a b Figure6.14.PLDdepositedthinlmsonA-sapphiresubstrateandtheircrystallographicorientation:aBaMsinglelayerbBaM-BSTthinlmbilayer. Figure6.15.-2scanofBaM-BSTbilayeronA-sapphiresubstrate.Inset:rockingcurvesofBaMandBSTpeaks.Theappearanceoflargeh00typediractionpeaksforBaMinthe-2scanindicateshighlyorientedsingle-phaseBaMlayer.ThediractionpeaksofBST,111,211,and222revealapolycrystallinenatureoftheBSTlayer,with211peakbeingthemostprominent.Planarorientationwasexaminedusingrockingcurve!-scananalysis-78

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a b Figure6.16.CrystallographictextureanalysisoftheBaM-BSTbilayeronA-sapphiresub-strate:aXRDpolegureofBaMpeakat2=63:0600andBSTpeakat2=45:9440,bazimuthalscanofBaMpeakat=300andBSTpeakat=650.seeinsetofgure6.15.FullwidthhalfmaximumoftheBaM200peakis0.50and0.890forBSTpeak.Thisconrmsafairlygoodorientationofbothlayers.Thepoleplotandthe-scaningure6.16describethecrystallographictextureofthebilayer.TheBaMhasonesetoftwolargepeaksat=900and2700,asitisexpectedfromcrystalliteswithc-axisinthelmplane-thereareonlytwopossiblereectionsfromtheplanesparalleltoand 1planes,seegure6.1.Thereisalsoanothersetoftwosmallerpeakscomingfromanothersetofin-planeorientedcrystallitesrotatedby700.Thisisalsoconrmedinthe-scan.Noout-of-planeorientationwasrecorded,indicatingaverygoodcrystallographicqualityoftheBaMlm.ThepolegureoftheBSTlmshowsapresenceofmultiplecrystallites,consistentwiththe)]TJ/F15 10.909 Tf 10.537 0 Td[(2scan.Obviously,thereisapoorlatticematchbetweentheBaM-planeandtheBST,causingacreationofmultiplecrystallites.Thegures6.17aandbshowin-planeandout-of-planemagnetizationofsinglelayerBaMlmaandtheBaM-BSTbilayerb.Asimilartrendcanbeseenasingure6.5,withthedierencethatinthiscasetheeasyaxisofmagnetizationliesinthelmplane,79

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consistentwiththeorientationoftheBaMC-axis.Largeout-ofplanemagneticanisotropyisobserved,comparabletotheresultsof[130].Largesquarenessisalsonoticeableinthein-planehysteresisloops.87forthesinglelayerand0.91forthebi-layer,makingthembothmagneticallyhardinthelmplane.Acomparisonbetweentheout-of-planemagnetizationofthesingleBaMlayerandtheBaM/BSTbilayer,showsthatthereissomedierenceinthemagnetizationcharacteristics,justlikeinthecaseofthebilayeronC-sapphire.ItappearsthattheBSTlayerhasaninuenceontheshapeoftheM-Hcurveagain.Forexample,thecoercivityofthebilayerishighercomparedtothatofthesingleBaMlayer.Thecoercivitiesofthein-planeM-Hloopsareabout1820Oeand1870Oeforsingleandbilayer,whilethecoercivitiesoftheout-of-planeM-Hloopsare230Oeand1525Oeforthesingleandthebilayer,respectively.Thisisanimportantnding,indicatingpossibleinteractionofthelayerspresumablymediatedbystrainattheBaM/BSTinterface.ThebottomoftheBSTlayerlikelyinuencestheorientationoftheBaMcrystallitesclosetoitsinterfacecausingthemtobedierenti.e.c-axisorientedout-of-planecomparedtotheoverallnatureoftheBaMgrowthasseenintheXRDandalsoAFMimageswhichfavorsthein-planegeometry.Theatomicforcemicroscopesurfacescanisshowngure6.18.Itisnotaseasytorecognizethecrystalstructureasintheprevioustwocases,yetin-planeorientationofthecrystallitescanbeclearlyseen.80

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a b Figure6.17.In-planeandout-of-planemagnetizationofPLDdepositedthinlmsonA-sapphiresubstrate:asinglelayerBaMthinlmbBaM-BSTthinlmbilayer. Figure6.18.AtomicforcemicroscopescanofsinglelayerBaMthinlmonA-orientedsapphiresubstrate-scanarea=2.5mx2.5m.81

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6.4BaM-BSTcompositeonC-sapphireThisandthefollowingsectiondescribethepropertiesofthecompositeBaM/BSTlmsdepositedonC-andA-orientedsapphiresubstrates.ThecompositedepositedontheMgOsubstratewasamorphousevenafterannealing. Figure6.19.BaM-BSTcompositethinlmonC-sapphire.ThecompositelmwasdepositedontoC-sapphiresubstratebytwodierentmethods.Intherstmethod,thecompositelmwasdirectlydepositedusing62,000pulsesundersameconditionsasdescribedbeforedepositionandannealing,resultinginanapproximatethicknessof0.5m.Thesecondmethodinvolvedadepositionofaseedlayer-athinBaMlayer20nmdepositedat6500Cusing1100pulses,thenthecompositethinlmwasdepositedusing62,000pulses.Theresultsofthesetwomethodsarecomparedwitheachotherandwiththeresultsobtainedforbilayerthinlms.Thedepositionofaseedlayermadealargeimpactonthecrystallographicpropertiesofthecompositethinlms.Additionalcrystallographicphaseswereseeninthecompositelmswithouttheseedlayer,whichdisappearedinthecompositesgrownontheseedlayer.ThefollowingX-raydiractionanalysiscontainsonlytheresultsofthecompositelmswiththeseedlayer.The-2scaningure6.20showsa00lorientedBaMlmandhhhorientedBSTlm,withexceptionofsomeadditionalpeaks:BaMandBST.TherockingcurveoftheBaM008peakintheinsetshowsnearlysimilarresultsfortheBaM/BSTbilayerFWHM=0.390,seegure6.3.The!-scanoftheBSTpeakshowsalargerdegreeofplanardistributionFWHM=0.970.ThepoleplotsofthecompositethinlmarealsocomparablewiththeresultsofthebilayerBaM/BSTlmonC-sapphireingure6.4.ThereisonesetofsixpolesandonesetofthreepolescomingfromBaMandonesetofsixpolescomingfromBST.Theone82

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Figure6.20.-2scanofBaM-BSTcompositeonC-sapphiresubstrate.Inset:rockingcurvesofBaMandBSTpeaks.setofthreepolesofBaMhasmuchstrongerreectionsthaninthebilayer,probablyduetoapresenceoflargeramountofBaMcrystallitestiltedwithrespecttotheC-axis.TheoccurrenceofaBaMpeakinthe-2scancouldalsohaveanimpactonmagneticpropertiesdescribedbelow.Thepoleplotandthe-scancomingfromtheBSTarenearlythesameasinthebilayer.Theseareveryinterestingresults,indicatingthatBaMandBSTfavoraparticulargrowthinbothcompositeandthebilayerform.Mostlikely,thereisapresenceofseparateBaMandBSTregionsinsidethecompositelms.Intheseregions,BaMandBSTgrowseparately,buttheirorientationisinuencedbytheirinterfaceandtheinterfacewiththesubstrate.ThusthegrowthofbothBaMandBSTmightbecontrolledbytheinterface,indicatingapossibilityofadepositionofmultilayersconsistingofseveralalternatinglayersofBaMandBST,anideawhichmightbepursuedinaseparatestudy.Figure6.22showstheM-HloopsofthecompositeswithoutaandwithbtheBaMseedlayer,withmagneticeldsappliedinandoutofthelmplane.Eventhoughthecrys-tallographicanalysisofthecompositewiththeseedlayerdoesnotshowmuchdierencecomparedtothebilayerlm,themagnetizationpropertiesshowasignicantchange.Thecompositelmwithouttheseedlayershowsasmallanisotropysimilartothemutlilayeron83

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MgO,whilethelmontheseedlayershowsalargeranisotropicbehavior,verysimilartotheC/BaM/BSTbilayeringure6.5.AdistinctfeatureclearlyseeninallM-Hloopsisapresenceofadouble-hysteresis.Thiswasalsoreportedforsputteredpolycrystallinecom-positeBaM/BSTthinlmsonaluminainchapter4.Occurrenceofthedouble-hysteresisloopinBaMlmsgrownonsapphireandSi/SiO2/Ptunderdierentconditionswasalsoreportedby[84,111].TheirexplanationwasapresenceoftwodierenttypesofBaMcrystallites-platelet-shapedcrystallitespreferringout-of-planeorientationandacicularcrystallitespreferringin-planeorientation.Inthepreviouscasereviewedhere,theBaMgrownonC-sapphirehasout-of-planeorientedplatelet-crystallites,BaMgrownonA-sapphirehasin-planeorientedplatelet-crystallites,andBaMgrownonMgO/BSThasin-planeorientedacicularcrystallites,butnoneoftheselmshavebothplateletsandacicularcrystallites.Thisisprobablyonlythecaseinthecompositelms.Adeeperunder-standingrequiresamoredetailedstudyofthemicrostructure,andtherearenoindicationsofthepresenceofasecondmagneticphasethatcouldexplainthis.84

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a b Figure6.21.CrystallographictextureanalysisofofBaM-BSTcompositeonC-sapphiresubstratewiththeBaMseedlayer:aXRDpolegureofBaMpeakandBSTpeak,bazimuthalscanofBaMandBSTpeaks.a b Figure6.22.In-planeandout-of-planemagnetizationofPLDdepositedcompositethinlmsonC-sapphiresubstrate:acompositelmbcompositelmwith20nmBaMseedlayer.85

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6.5BaM-BSTcompositeonA-sapphireTheBaM/BSTcompositethinlmwasalsodepositedontoA-orientedsapphiresub-strateundersameconditionsdescribedintheprevioussection,bothwithoutandwiththeBaMseedlayer. Figure6.23.BaM-BSTcompositethinlmonA-sapphireTheX-raydiractionanalysisingures6.24and6.25revealsthatthecompositewiththeseedlayeralsohasnearlysamecrystallographicpropertiesastheBaM/BSTbilayerfromsection6.3,whilethecompositewithouttheseedlayernotshownherehasothercrystallinephases.TheBaMpeaksfromthe-2scanshowadistincth00orientation,whileBSThasastrongpeak.Therockingcurvesarealsosimilartothebilayer,withsmallFWHMofBaMpeakandabroaderFWHMforBSTpeak.Sincethetotalthicknessisabout1/2m,theeectofthesubstrateandtheseedlayerisdominant,particularlyforthegrowthofBaMconstituent.Thepoleandazimuthalscansarealsoverysimilartothebilayer,althoughinthiscasetheBaMshowsthreesetsoftwopoles,suggestingalargeramountofdierentin-planeorientedcrystallites.Thepeaksinthepoleplotandthe-scanarealsomuchbroaderthaninthebilayer,indicatingapoorertexturequalityofthelms.Themagnetizationpropertiesingure6.26,justasintheprevioussection,showalargedierencebetweenthecompositelmswithoutandwiththeseedlayer,aswellascomparedtotheBaM/BSTbilayeronthesamesubstrate.Herealso,thereisaclearpresenceofadouble-transitionhysteresisloop.ThecompositewithouttheseedlayerlooksnearlythesameasthecompositeonC-sapphirewithoutseedlayeringure6.22a,indicatingthatinternaldistributionoftheBaMandBSTregionsinthecompositeswithouttheseedlayer86

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Figure6.24.-2scanofBaM-BSTcompositethinlmonA-sapphiresubstrate.Inset:rockingcurvesofBaMandBSTpeaks.andtheinteractionbetweenthemplaysagreaterroleindeterminingtheoverallmagneticresponsethantheeectofthesubstrate.Obviouslyinthecompositelms,themicrostrucutureandthemagneticpropertiesarehighlyconnected,allowingcustomizationofmagneticpropertiesthroughcontrolledchangesinthemicrostructurethiswasalsoobservedinthesputteredcompositesinchapter4.Thisopensapossibilityofcreatingarticialsingle-layerferrimagnetic-ferroelectricthinlmsthroughcontrolleddeposition.IftheBaMandBSTregionscanbecontrollede.g.depositionthroughamask,itmightbepossibletocustom-designsomeofthemagneticandferroelectricproperties,andevenexploreapossibilityofmagnetoelectriccoupling.Sucheectswereobservedinferroelectric-ferromagneticnanostructuredcompositethinlmin[133].87

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a b Figure6.25.CrystallographictextureanalysisofBaM-BSTcompositethinlmonA-sapphiresubstratewithBaMseedlayer:aXRDpolegureofBaMpeakat2=63:0200andBSTpeakat2=45:9440,bazimuthalscanofBaM220peakat=300andBSTpeakat=650.a b Figure6.26.In-planeandout-of-planemagnetizationofPLDdepositedcompositethinlmsonA-sapphiresubstrate:acompositelmbcompositelmwith20nmBaMseedlayer.88

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6.6SummaryThischapterdescribedcrystallographic,structuralandmagneticpropertiesofsinglelayer,bilayerandcompositeBaMandBSTthinlmsfabricatedbypulsedlaserdeposition.HighqualityBaM-BSTbilayersweregrownonC-andA-sapphireandMgO.Themeasuredpropertiesareconsistentwiththeresultsfromtheliteratureforsinglelayerlms.Thecompositelmsgrownonsapphireshowedapresenceofadouble-transitionhysteresisloop.89

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CHAPTER7MICROWAVEPROPERTIESOFBAM-BSTBILAYERSANDCOMPOSITETHINFILMS1ThischapterpresentstunableelectricalandmagneticpropertiesofPLDdepositedthinlmsamplesdescribedearlier.Thedatawastakenintherange1-65GHzusingtheexperimentalset-updescribedinchapter3.4.7.1CoplanarWaveguidePhaseShiftersCoplanarwaveguidephaseshifterisadevicethatincludesasubstrate,tunabledielec-tricmaterialandacoplanarwaveguide,andchangesthetransmissionphaseanglephaseofS21ofthepropagatingwavebyelectric,magneticormechanicalcontrol.Phaseshiftersndlargestapplicationsinphasedarrayantennasystems[44,126].Phasedarrayantennasarepartofaradarsysteminwhichthephaseofalargearrayofantennasiscontrolledsothattheradiationisreinforcedinoneandsuppressedintheotherdirection.Electricalcontrolofthephaseshiftcanberealizedbyusingnonlineardielectricssuchasbarium-strontium-titanate,inwhichthedielectricpropertiesvarywiththechangeofappliedvolt-age.Formagneticcontrol,materialslikeyttrium-iron-garnetorbarium-hexaferritecanbeused.Thephysicalpropertiesofthesematerialschangeinmagneticeldatmicrowavefrequencies.Mechanicaltunabilityisachievedbyphysicalmovementofspecicpartsofthetransmissionlinee.g.inMEMS.Allofthesethreetypesofphaseshiftershavelimita-tionsanddrawbacksintermsofamountofshift,losses,powerhandling,responsetimeandmanufacturingcosts.Recently,therehasbeenanincreaseinresearchanddevelopmentofnewclassoftunablemicrowavedevices,someofitwasmentionedinchapter1.1. 1Thecontentsofthischapterarepublishedin[50,51]90

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Phaseshiftersincorporatingferroelectric[12,19,20,22,40,63,72,86,92,104,127,126]orferrimagnetic[32,55,73,96,121,134,135]materialshavebeenstudiedinthepast.Theiroperationisbasedonthedependenceofthesample'spermittivityandpermeabilityontheappliedelectricandmagneticelds,asexplainedinchapter2.Therearetwowayshowelectricalandmagneticpropertiesofadevicecanbetuned.Oneisbyapplyingelectriceldtotunetheelectricalproperties,andapplyingmagneticeldtotunethemagneticproperties.Theotherway,whichisanewandgrowingresearchtopicisbytuningmagneticpropertieswithelectriceldandviceversa.Thisisbasedonthemagnetoelectriceect[38].Anexampleofapossibleuseofmagnetoelectriceectistuningferrimagneticresonancebyelectriceld[14,15].ElectricaltuningofFMRatmicrowavefrequenciescouldpotentiallyleadtodevelopmentofnewtypesofmicrowavedevices.Thus,lookingatthefrequencydispersionofthephaseshiftwithappliedelectricandmagneticeldsnotonlygivestheinformationabouttheelectricalandmagnetictunability,butitcouldalsoprovideamethodtodetectnovelmagnetoelectricphenomena.Thedatapresentedinthefollowingsectionsdescribesthephaseshiftofpureferritelms,ferrite-ferroelectricbilayersandferrite-ferroelectriccompositethinlmsunderappliedelectricandmagneticelds.ThephaseshiftofthetransmissioncoecientS21isdescribedas[68,90]:=2)]TJ/F18 10.909 Tf 10.909 0 Td[(1=360fl cq 2eff2eff)]TJ/F24 10.909 Tf 10.909 12.316 Td[(q 1eff1eff;.1withftheoperatingfrequency,lthelengthofthecpwline,cthespeedoflight,and"and"arethetwodierentstatese.g.withoutandwithbiaseld.Itcanbeseenthatthephaseshiftincreaseswithincreasingfrequency,linelength,andthedierenceineectivematerialpermittivitieseffandpermeabilitieseff.91

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7.2ElectricalTunabilityElectricaltunabilityofthecoplanarwaveguideswasdeterminedbymeasuringthescat-teringparameterswhileelectricbiasbetweenthecentralandthegroundconductorwasappliedusingthebiastees-seegures3.13aand7.1.Thesemeasurementswereper-formedattheUSFWAMILaboratoryonaprobestationcapableofmeasurementsupto65GHz.Beforethemeasurement,SOLTcalibrationonacalibrationsubstratewasper-formedintherange1-65GHz.Eventhoughinterdigitalcapacitorsweredesignedandfabricated,theywerenottestedduetosomefabricationandmeasurementdiculties.Onlythephaseshiftofthecoplanarwaveguideswasmeasured.Beforeandafterthemea-surementsunderappliedelectriceld,theresistancewasmeasuredbetweenthecentralandthegroundconnectorusingabiasteeandamultimeter.Figure7.1illustrateshowabiaselectriceldisappliedintheCPWgeometryusingabiastee.AbiasteeisadeviceusedtoapplybothDCandRFsignalstoadeviceundertestDUT.Thevoltagewasappliedinfoursteps-12.5V,25.0V,37.5V,and50V,andthesevaluesweretransformedintokV/mmbydividingbythewidthofthegap,whichwas5m. Figure7.1.AnillustrationofthebiasingoftheCPWphaseshifter.Thebiaseldisappliedbetweenthecentralandgroundconductorsviaabiastee.ThemainproblemduringthemeasurementofthebilayerswithBSTontopsap-phire/BaM/BST,wasthattheresistancebetweenthecentralandthegroundconductors92

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wouldstartdroppingwhenthebiasvoltagewasturnedon,andthemetallayerwouldappeartostartoxidizinganddegradingthewaveguideconductorlayerwouldliterallybecomebrownishandgreenishnearthecontactswiththeprobes,andifthevoltageisnotturnedoimmediately,itwillcontinueacrossthewholeareaofthewaveguide.Thereasonforthisisprobablyoccurrenceoftheleakagecurrentthatisoneofthemainfailuremechanismsinhigh-permittivitythinlms[13,102].Afterthemeasurementwasdone,andthebiasvoltageturnedo,theresistancewasimmediatelymeasuredacrossthegapusingthebiastee.Itwasnotedthatthevaluewasmuchlowerthanbeforethemeasure-ment,anditwasrisingslowlytowardtheinitialvalue.SuchresistancedegradationanditsreturntonearlyinitialvalueshasbeenreportedinBSTlmsbyothers[13,83,102].Duetothefatigueeect,theresistancedoesnotreturncompletelybacktoitsoriginalvalue.Althoughleakagecurrentshavenotbeenfullyunderstoodyet,itisbelievedthattheelectrode/ferroelectricinterfaceplaysasignicantrole.ForthebilayerswithBSTontop,theresistanceoverthe5mgapwasintheorderofMsbeforethemeasurement.ThebilayerwithBaMontopMgO/BST/BaMdidnotshowanydropinresistanceafterthemeasurementevenafterthemaximumappliedvoltageof50V,andnodegradationofthemetallayerwasobserved.TheresistancewasintheorderofGs.Thus,thisexperimentalschemehasresultedinanimportantobservationshowingthatahighlyinsulatinglayerwithlowmicrowavelossescanbedepositedonthetopoftheBSTandagoodtunabilitycanstillbeobtained,minimizingtheleakagecurrents.Anothermethodwasdemonstratedin[66],whereinsteadofapplyingmetalizationdirectlyontheBSTlm,a0.1mSiO2layerwasusedasabuerlayer,whichreducedthepossibilityofcreatinglargecurrentcrowdingdepthsinsidetheBSTthatincreasethelosses[21].Thatpaperdidnotaddresstheleakagecurrents,butobviouslytheirideaofalow-dielectricbuerlayermighthavereducedtheleakagecurrentssimilartothegeometrywiththeBaMontop.Increasedcareduringthemanufacturingprocesswasalsonecessarytoeliminatesomeproblemscausedbyresidualphotoresistonthelmsurfaceinbetweentheelectrodes.Very93

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smallmicroscopicamountofthephotoresistcanremainintheCPWgapafterprocessing,increasingtheriskofdevicefailure.Thesamplesneededtobethoroughlycleanedinultrasoniccleaner,RFPlasmaEtcher,andvisuallyinspectedunderthemicroscope.Thelimiteddevelopmenttimeandlongerbakingtimesexplainedinchapter3.4.3couldbeacauseforthis.AnotherimportantmatteristhattheBSTlayerduetoitslargepermittivitycausedthelineimpedancetobemuchlowerthan50.Whenthecircuitsweredesigned,theoptimumdimensionswerecalculatedusingonlythesubstrateproperties,andtheinuenceofthelmswasnottakenintoaccount.DuetothislargemismatchbetweentheDUTandthemicroprobesZin=50,thereectionlossS11waslarge,andthereforethetransmissioncoecientS21wasverysmall.ThepureBaMsamplesdidnotexperiencesuchlargeimpedancemismatch,mainlyduetotheabsenceoftheferroelectriclayer.PhaseshiftersareusuallycharacterizedbyagureofmeritFOMwhichisdenedas[118]:FOM=phaseshiftindegrees=magnitudeofinsertionlossindB7.2Onlywhenanetworkiswellmatched,theinsertionlossisequaltoS21[100].Sincethereisalargeimpedancemismatch,usingS21forinsertionlosswouldresultinlowervaluesoftheFOM.Despitethat,inordertocomparetheseresultswiththeexistingdatafromtheliterature,agureofmeritofbilayerlmsiscalculatedusingS21valuesandplottedingures7.2a,b,andc.IfthedimensionsofthecircuitsweredesignedtoadjustfortheimpedancemismatchduetotheBST,thusminimizingthemismatch,thereectioncoecientwouldhavebeensmallerandthepropagationcoecientwouldhavebeenlarger-thedBvaluesoftheS21wouldhavebeensmaller.ThiswouldmakethecorrectFOMprobablytwiceaslargeastheestimateshere.ThehighestFOMisseenintheMgO/BST/BaMbilayers,inwhichtheBSTlayerisdepositeddirectlyontheMgOsubstrate,andexperiencesthebestqualityintermsofthecrystallographictexture,asseeninthepreviouschapter.Itspreferredorientationwas.TheBSTlmdepositedonC/BaM,showedanexcellentcrystallographicorientationindirection,althoughthe94

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FOMissomewhatsmaller.TheBSTlmdepositedonA/BaMwaspolycrystallineandshowedalesserdegreeoftexture.Itwasalsohavingproblemswithmetaldegradationandresistancedropwhenvoltagewasapplied,asdiscussedearlier.Thisbilayershowedsmallerdegreeofelectricaltunability.Itwasshownin[91]thattheorientedBSTlmshowslesstunabilitythanorientedlm,butwithimproveddielectricqualityfactor.ItisalsoimportanttonoticethattheFOMinguresaandbhaveaatresponseinthefrequencyregionfrom20-50GHz,abovewhichthecalibrationbecomesbad.Thiscouldbeimportantinpossiblepracticalapplications.MaximumvaluesofFOMfoundintheliteratureforBST-basedphaseshiftersarearound40-50deg/dB[20,68,79,90].Thetheoreticalestimatesbasedoncalculationoflossesinaferroelectricdevicepredictthatthegureofmeritcouldbefurtherincreasedbyafactorofthreeifthelossesareminimized[118].Thesourcesoflossesincludeenergydissipationinaferroelectric,lossesinconductingelectrodes,andexternallossesofthecircuitdesign.Themainsourceoflossesinthiscasecomesfromthecircuitdesign,causingalargeimpedancemismatch,whichcanbeeasilyxedbyredesigningthecircuitdimensions.Inaddition,ithasbeenrecentlyreportedthatthesmallgapsize5mcanfurtherincreaselossesinthemetalelectrodes[20,118].Largergapsizewiththislmthickness.5mwouldresultinlesslossesinmetalconductorsbutalsoindecreaseintunability.ItwouldbeinterestingtostudytheeectoftheincreaseoftheCPW-gapandlmthicknessonthegureofmerit.Figures7.3showamoreaccurateplotofthetunabilityeectasaphaseshiftperunitlengthdeg/cm.Asdescribedbyequation7.1,thephaseshiftincreaseswithfrequencyandappliedvoltageincreaseinpermittivitydierence.Thedataisnoisyabove50GHz,probablyduetocalibrationissues.DuetoasmallsizeoftheCPWgapm,averylargephaseshiftwasobtainedwitharelativelysmallvoltagekV/mmcorrespondsto50V.OtherreportsonBSTbasedCPWphaseshiftersusemuchlargevoltagesduetolargergapsizeupto250V[20,68,79].Eventhoughasmallergapsizeintroducesadditionallosses,itprovidesabenetofusingsmallerbiaselds.ThephaseshiftofA-BaM-BSTstructure95

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ingure7.3cshowsawavynatureofthecurve.Thereasonbehindthisiseitherdielectricresonancesinthesubstrate-BaM-BSTstructureorpoorimpedancematch.Phaseshiftershouldbemanufactureddirectlyonthesubstrate,andthemicrowavemeasurementsshouldbeperformedtoseeifthereareanydielectricresonancesduetothesubstrate.Scatteringparametersofthebilayerthinlmsaredisplayedinthegure7.4.ItcanbeseenthatthereectionlossS11islargeduetopoorimpedancematch,ultimatelycausingthetransmissioncoecientS21tobeverysmall.Anothertrend,thathasbeenreportedbyothers[68,79,90],isthattheinsertionlossesofMgO-BST-BaMandC-BaM-BSTlmsincreasewithfrequencyandimprovewithincreasedappliedvoltage.TheA-BaM-BSTbilayershowsanoppositetrend,probablyduetotheproblemswithmetaldegradation.Anotherwaytominimizethedielectric,interfaceandconductorlossesisbymeasuringthepropertiesatcryogenictemperatures.Bilayerstructuresincorporatingthesupercon-ductingmaterialssuchasYBa2Cu3O7)]TJ/F16 7.97 Tf 6.587 0 Td[(3,ferroelectric/paraelectricandferrimagneticma-terialsholdagreatpromisefordevelopmentofcryogenicmicrowavedevices[17,39,53,60,78,81].Thereisalsoanimportantissuethatwasraisedby[42],andaddressesthequestionwhethermaterialsinmicrowavedevicesshouldbeusedinferroelectricorparaelectricstate.Sofar,exclusivelyparaelectricmaterialshavebeenutilizedfortunabledevices,buttherearedoubtswhetheratupperGHzfrequenciesthesematerialsareinparaelectricstateatall.Also,verylittleresearchhasbeendoneonmeasurementsofmicrowavepropertiesofmaterialsinferroelectricpolarstate.Themainreasonforusingparaelectricsisthatbelow10GHz,ferroelectricsinpolarstateexperiencelargelossesduetopiezoelectrictrans-formations,domainwallmovementsandhysteresiseects.Itturnsoutthatatmillimeterfrequencies,piezoelectrictransformationsanddomainwallmovementsdonotcontributetothehighfrequencylossesanexampleisferroelectricNa0:5K0:5NbO3[9].Inaddition,ferroelectricsbelowCurietemperaturehavemuchhigherpermittivitiesthaninthepara-electricstate.Thisindicatesthatferroelectricsinpolarstatecouldbeusedformicrowavetunabledevicesabove10GHz.Itwouldbeinterestingtomeasuremicrowaveproper-96

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tiesofothercompositionsofBa1)]TJ/F19 7.97 Tf 6.587 0 Td[(xSrxTiO3,belowandabovex=0.3,whichcorrespondstoferroelectric-paraelectricboundaryatroomtemperatureandhasamaximumpeakofpermittivity[80].AlsocryogenicmicrowavemeasurementsofBa0:5Sr0:5TiO3belowCurietemperaturecouldbeperformedtostudytheimpactoflossesatmicrowaveandmillimeterwavefrequencies.Anotherstudyofadual-tunabledevicealsomentionedinchapter1.1,reportsontheelectricalandmagnetictunabilityofBSTlmdepositedonYIGsubstrate[67].ThephaseshiftfromBSTlmwasonlyabout170/cmat10GHzwith2.1kV/mm,whichismuchsmallerthanthedataobtainedhere.Thegapoftheircoplanarwaveguidewas19m,whichcouldaccountforlowtunability,giventhesmallthicknessofthelm500nm.Apoorimpedancematch,resultinginhighreectionandinsertionlosseswasalsoreported.Microwavecharacterizationofcompositethinlmswasalsoperformed,buttheselmswerehavingverylowresistanceacrossthegapinks,andnoelectricaltunabilitycouldbemeasured.Thereasonforthisisprobablylargeleakagecurrentsthatalsocauseddegra-dationoftheelectrodes,asmentionedpreviously.Inaddition,thereisapossibilitythatsomeotherfaceswereformedinsmallquantitiese.g.Fe3O4thathavesemiconductingfeatures,butwerenotdetectedintheX-raydiractionscan.Furtherinvestigationsareneededtounderstandtheoriginofthelowresistivityinthecompositesandtoimprovetheirquality.97

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a b c Figure7.2.FigureofmeritforelectricaltunabilityofBaM-BSTbilayers:aMgO/BST/BaM,bC-sapphire/BaM/BSTandcA-sapphire/BaM/BST.98

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a b c Figure7.3.PhaseshiftofBaM-BSTbilayerswithappliedelectriceld:aMgO/BST/BaM,bC/BaM/BSTandcA/BaM/BST.99

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Figure7.4.S-parametersofbilayerthinlmsunderbiaselectricelds:aMgo/BST/BaM,bC/BaM/BST,cA/BaM/BST.100

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7.3MagneticTunabilityForthemeasurementsofthephaseshiftunderbiasmagneticelds,theCPWstructureillustratedingure7.5wasused.Theright-anglebentsallowforprobingfromthesides.Asimilarmethodwasalsousedby[27]forstudyofferromagneticresonanceinaferromagneticmetal.Inthisset-up,themagneticeldisalwaysperpendiculartothemagneticcomponentofthemicrowaveeldthatispropagatingalongawaveguideseealsogures3.9,3.10and3.12.ThisarrangementofACandDCmagneticeldsismandatoryforstudyingferrimagneticresonance. Figure7.5.Set-upformagnetictunabilitymeasurementThemainprobleminthisexperimentwasthatthecablesandtheprobesareofSMA-type"SubMiniatureversionA,whichisratedonlyforuseuptillabout24GHz.Abovethisfrequency,highermodesofpropagationareexcited,increasingthereturnloss.De-spitethisdrawback,anSOLTcalibrationwasperformedfrom1-65GHzonacalibrationsubstrate,andscatteringparametersofthethinlmswererecordedatdierentappliedmagneticandelectricelds.ThephaseshiftmeasuresthedierenceinthephaseofS21withandwithoutthebiaseld.ItturnsoutthatthephaseofS21isunaectedbytheincreasedreturnlossesduetoexceedingusablerangeofthecablesandconnectors.Addi-tionallossesduetoimpedancemismatchasdescribedintheprevioussectionareseeninthebilayerlms.Becauseofthat,themainfocusisdirectedtowardthemonolayerBaMlms,andsomeresultsofthebilayersandthecompositelmswillbepresented.101

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Figure7.6.MagnetictunabilityoftheBaMthinlmdepositedonA-orientedsapphiresubstrate:aS11,bS21,cphaseshift,dFMRpeak.Figure7.6showsmagnetictunabilityoftheBaMthinlmontheA-orientedsapphiresubstrate.Itisshowninchapter6.3thattheeasy-axisofmagnetizationofthislmliesinthelmplane.ThereectioncoecientS11shownin7.6aisalmostunaectedbythemagneticeld,andseemsquitenoisyabove20GHzduetopreviouslymentionedreasons.ThetransmissioncoecientS21atzeroeldshowsalargeincreaseoflossesatabout50GHzingure7.6b,indicatingthepresenceofthenaturalferrimagneticresonance.AtypicalshiftoftheFMRpeakpositiontohigherfrequenciesatappliedmagneticeldsisalsonoticeableseealsogure7.6d.Thefrequencyoftheferrimagneticresonanceforanisotropicthinlmswithin-planemagnetizationisgivenby[122,131]:102

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f= 2p HR+HA+4MSHR+HA.3whereisthegyromagneticratio,HRistheresonantmagneticeldappliedeld,HAistheanisotropyeld,andMSisthesaturationeld.Thepeakpositionfollowsastraightlineasseeningure7.6d,anditcouldbeeasilyttediftheanisotropyeldHAwereknown.Thisvaluecanbeobtainedusingtorquemagnetometermeasurements,aset-upnotavailableatthetimeofthiswriting.Asimilartof7mPLDgrownthickBaMlmwasreportedby[131].Foraroughestimate,onecantaketheanisotropyeldtobe17kOe,and=2=2.72GHz/kOe[131],andthesaturationmagnetizationisestimatedtobeseealsogure6.17a:Ms=3:210)]TJ/F16 7.97 Tf 6.587 0 Td[(4emu3:210)]TJ/F16 7.97 Tf 6.587 0 Td[(4 2:010)]TJ/F16 7.97 Tf 6.587 0 Td[(6emu cm3160emu cm3Then4MS2kOe,andusingtheequation7.3,onecantthepositionoftheresonantfrequencyingure7.6d.Eventhoughthisisanapproximation,therealvaluesofHcand4MSareprobablynotmuchdierent,andthecalculateddataseemstoagreewellwiththeexperimental,seegure7.7.ThisdemonstratesthattheferrimagneticresonantfrequencycanbepreciselydeterminedusingmeasurementsofS21. Figure7.7.Resonantfrequencyvs.appliedeldforBaMthinlmdepositedonA-orientedsapphire-experimentalandcalculateddata.103

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Figure7.8.MicrowavepropertiesofBaMthinlmsonC-orientedsapphiresubstrateunderbiasmagneticeld:aS11,bS21,andcphaseshift.Thefocusofthestudyisactuallydirectedtowardthephaseshiftasseenin7.6c.Theplotofthephaseshiftvs.frequencyatdierenteldsshowsthegreatpotentialofthisexperiment.Eventhoughthefrequencyexceededtherecommendedlimitsofthecablesandconnectors,andasitwillbeseenlaterinthecaseofthebilayers,thereturnlosswaslargeduetoimpedancemismatch,thephaseshiftwasnearlyunaectedbythis.Afterathoroughliteraturesearch,itwasconcludedthatthisisprobablytherstlookatthephaseshiftneartheFMRfrequencies,asthemagneticeldisvaried.AmoredetailedquantitativeanalysisthatrelatesthephaseshifttoFMRisneeded.Microwaveresponseinbaresapphiresubstrateswasalsomeasurednotshownhereandnoshiftwithappliedelectricormagneticeldswasseen.104

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Thegures7.8a-cshowthemicrowavepropertiesofBaMsinglelayerlmdepositedontheC-orientedsapphire,andhavingtheeasymagnetizationaxisperpendiculartothelmplane.EventhoughtheeectoftheappliedmagneticeldcannotbeclearlyseeninthemeasurementofS11andS21parameters,thephaseshiftshowsaclearchangewithmagnetization.Inthiscase,theeldwasrstappliedinonedirection,thenbroughttozero,thenappliedinopositedirection,andbroughtbacktozeroagain.Itcanbeseenfromthegure7.8cthattheeectisrepeatableinbothdirectionsofappliedmagneticeld,andalsothatthereisnoeectofremanentmagnetization.Thisismainlyduetothefactthatitishardtofullymagnetizethislmintheplanedirection.Anothervisibleeectisthedierenceintheshapeofthephaseshiftcomparedtothepreviousexamplewiththein-planeorientedBaM.Theexplanationliesinthedierentorientationofthethinlmmagnetizationswithrespecttotheoscillatingmagneticeld.ModelingofsuchlongitudinallybiasedmagneticphaseshifterwouldinvolvecalculationofTEMwavepropagationthroughananisotropicmedium,giventhevaluesfortheanisotropyeldHA,saturationmagnetizationMS,demagnetizationfactorsNkandN?,FMRlinewidthH,eectiveg-factorgeff,permittivityandthecircuitdimensions,aswellastakingintoaccountdierentmagnetostaticmodes[134].Someofthesevalueswerenotavailableatthetimeofthiswriting.Thefollowinggures7.9and7.10showthetunabilityofthebilayersontheA-orientedsapphireandMgO.ThebilayerontheC-orientedsapphireshowedaverysmallphaseshiftandthedatawasverynoisy,anditwillnotbepresentedhere.ItcanagainbenoticedthateventhoughthereisverylittlechangeinthemagnitudeoftheS-parameterswiththeappliedmagneticeld,thephaseofS21showsasignicantchange.ThephaseshiftofBaM-BSTbilayeronA-orientedsapphireingure7.9clooksquitedierentthanforthesinglelayerBaMlmingure7.6c.Thiscanberelatedtotheexistanceofsomeout-of-planegrownBaMcrystallographicphases,creatingamixtureofthephaseshiftresponseseeningures7.6cand7.8c.Itwasobservedthatthereisanincreaseoftheout-of-planeremanentmagnetizationingure6.17.Obviously,thereisastronginuence105

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Figure7.9.MicrowavepropertiesofBaM-BSTbilayeronA-orientedsapphiresubstrateunderbiasmagneticandelectriceld:aS11,bS21,cphaseshiftundervaryingmagneticelds,anddphaseshiftunderelectricandmagneticelds.ofinterfacialeectsbetweentheBaMandBSTlayer.Sucheectscouldbeimportantifoneislookingformagnetoelectriccoupling,otherwise,theseeectsshouldbeminimizedtoreduceunwantedresponseinmicrowavedevices.Onewaytodothisisbyintroducingintermediatebuerlayers,whichcouldreducethelatticemismatchbetweenthelayers.Figure7.9dshowsadual-tunabilityeectelectricalandmagnetic.First,thephaseshiftwasmeasuredwithappliedmagneticeldof5.5kOe,thenwhilethemagneticeldwason,theelectriceldof25Vwasappliedandthedatawasmeasuredagain.Theeectsoftheelectricalandmagneticshiftingures7.3cand7.9cgetsuperimposed,creatingadual-tunablephaseshifter.Thisisbelievedtobetherstdemonstrationofdualtunabilityinthisclassofmaterials.TheelectricelddoesnotseemtochangetheFMRposition,thus106

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Figure7.10.MicrowavepropertiesofBST-BaMbilayeronMgOsubstrateunderbiasmag-neticandelectriceld:aS11,bS21,cphaseshiftundervaryingmagneticelds,anddphaseshiftunderelectricandmagneticelds.thereislikelynomagnetoelectriccouplingbetweenthelayers.Unfortunately,nohigherelectriceldscouldbeappliedduetotheleakagecurrentproblemsmentionedearlier.ThephaseshiftoftheBST-BaMbilayeronMgOsubstrate,gures7.10candd,exhibitssomewhatsimilarbehavior,althoughthemagneticphaseshiftlooksmoreliketheonefromthesinglelayerBaMlmontheA-sapphiresubstrate.Thisbilayerlmshowsnearlyanisotropicmagneticbehaviorasseeningure6.12,thusthereisnopreferredaxisofmagnetization.Thedualtunabilityat5.5kOeandvariouselectriceldsalsoshowsasuperpositionofelectricalandmagneticeect.ItalsoconrmsthepreviousmeasurementsofelectricaltunabilitydoneondierentsampleandwithdierentCPWstructurestraight"vs.bent"CPW-comparewiththegure7.3a.107

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Figure7.11.Compositestunability:aC-BaM-COMP,bA-BaM-COMPFinally,themagneticallytunablephaseshiftwasalsomeasuredinBaM-BSTcompositethinlms.Eventhoughtheeectisquitesmallandnoisy,itstilldemonstratesthateventhesmallestchangesinmagnetizationatmicrowavefrequenciescanbepickedupbylookingatthephaseshift.7.4SimulationsUsingCommercialSoftwarePackagesSimulationsofmeasureddataS-parametersofCPWcircuitswasattemptedusingtwocommercialsoftwarepackages-HPAdvancedDesignSystemADSandAnsoftHighFrequencyStructureSimulatorHFSS.DuetothepresenceofBST,theeectivedielectricconstantofthestructuresubstrate+bilayerwaslarge,andneitherADSnorHFSSwereabletohandlesimulationswithlargedielectricconstant.Despitethislackofquantitativeanalysisandmodeling,theresultsobtainedherearewellcomparabletotheresultsobtainedbyothersforbothBaMandBSTlms.108

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7.5MicrowaveMeasurementsatOaklandUniversityThissectionpresentstheresultsofmicrowavemeasurementsperformedatProfessorG.Srinivasan'slaboratoryattheOaklandUniversityPhysicsDepartment.Theauthorisnotanexpertinthesemeasurementsandnodetailedinterpretationofthedatawillbepresented.Theintentisrathertoshowthemagnetictunabilityperformedonadierentexperimentalset-up.Inthisexperiment,thesampleswereplacedatthemetalshortendofaU-bandwaveguide-coaxialadapterandthereectedpowerwasmeasuredatdierentex-ternalmagneticeldsrangingfrom0to14kOeusinganelectromagnetandinthefrequencyrangeof40-60GHz.Figure7.12ashowsthereturnlossofBaM-BSTbilayerdepositedonA-sapphiresubstrateasafunctionofexternalmagneticeld,whilepositionofthemaxi-mumabsorptionpeakresonantabsortionasafunctionofeldisplottedingure7.12b.a b Figure7.12.MicrowaveabsorptionmeasurementsofBaM-BSTbilayeronA-sapphire:areturnlossandbPositionoftheresonantabsorptionasafunctionofexternaleld.Anoticeablefeatureinthissetofdataisthattherearedistinctjumpsintheminimumfrequencypositionatcharacteristiceldsabove8kOe.Thesejumpsareduetomoderepulsionofmagneticanddielectricresonances[51],andmoreworkisneededtointerpretthesecomplexfeatures.ThesefeatureswerenotobservedinsinglelayerBaMlmsonA-109

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sapphire.Thisisanotherdemonstrationthatthesemultilayersystemsproviderichphysicsaswellaspotentialapplications.7.6SummaryThischapterpresentedtheresultsofthemicrowavecharacterizationofPLD-depositedsingle-andbilayerthinlms.AcoplanarwaveguidephaseshifterwasfabricatedandthephaseshiftoftransmissioncoecientS21wasmeasuredasafunctionoffrequencyandappliedelectricandmagneticelds.Duetothepresenceofamaterialwithlargedielectricconstant,thecharacteristicimpedanceofthelinewassmallerthan50,causingalargemismatchattheports.Electricaltunabilitywasseeninalllms,andparticularlylargevaluesweremeasuredintheMgO-BST-BaMstructure,duetoanexcellentqualityofBST.AlsotheBaMlayerontopprovidedpossibilityofapplyinglargeelectriceldswithoutcausingdamagingleakagecurrents.Magnetictunabilityofthephasewasalsomeasuredinacustom-madeexperimentalset-up,andatypicalphaseshiftwasobservedattheferrimagneticresonantfrequency.DualtunabilityoftheA-BaM-BSTstructurewasalsoreported.110

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CHAPTER8CONCLUSIONANDFUTUREWORKThisworkpresentedgrowth,structural,crystallographic,magneticandmicrowaveanalysisofseveraldierenttypesofBaM-BSTthinlms.Thelmsweregrownincom-positeandmultilayerform,usingRFmagnetronsputteringorpulsedlaserablation.Theeectofsubstratesanddepositiontemperaturewasalsostudied.Severalinterestingphenomenawereobserved.Compositelmsshowedadouble-transitionhysteresisloop,andmagneticpropertiesinmultilayerlmswerealsoaectedbythepres-enceoftheBST.ApossibilityofcustomizingmagneticpropertiesthroughtheuseofBSTwasalsoobserved.HighqualityBaM-BSTmultilayersweregrownonsinglecrystalsapphireandMgOsubstratesprovidingapossibilityofdesigningmultifunctionalmagneto-dielectricthinlms.Anovelexperimentalstageformicrowavemeasurementsofthinlmswasbuiltandelectricallyandmagneticallytunablephaseshiftwasmeasuredinthefrequencyrange1-65GHz.Itwasseenthattheferrimagneticresonancecanalsobeobservedbymeasuringscatteringparametersofaphaseshifter.Futureworkonthesesystemsmightincludesomeofthefollowingideas:Growingoftri-andmultilayersofBaMandBSTusingPLDonsinglecrystalsub-strates.Increasingthethicknessofthelayers.Thicklmsaremorepreferableforpossibleapplications.AlsotheCPW-gapcanbelargerinthickerlmsminimizingpotentiallossesduetothesmallgapsize.Thegeometricllfactor-ratioofthelmsizetothetotalthicknesssubstrate+lmislargerforthickerlms,andtheelectromagnetic111

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eldsaremorecontainedinthethickerlms,havinglargereectsontheoverallelectromagneticwavepropagationproperties.Investigatingforapossiblemagnetoelectriccouplingandotherphenomenaattheinterfaceofthelms-neutronscattering,transmissionelectronmicroscopy,etc.DepositingbuerlayersbetweenBaMandBSTattheinterfacetominimizetheinterfacialeects.IncreasingtheBaMlmdepositiontemperaturetooptimumreportedtemperatureof9200C,andndingaoptimumdepositiontemperatureforBSTinthiscase.Morestudiesofcompositethinlms,inparticularthecorrelationsbetweenmi-crostructureandmagneticeects.Findingamorequantitativeexplanationofthedouble-hysteresisloopandapossibilityofcustomdesigningmagneticpropertiesthroughcontrolledmicrostructuraldesign.Designingthecircuitstoaccountforthechangeincharacteristicimpedanceandrepeatingthemeasurementsofgureofmeritinbilayerlms.Measuringofelectricalandmagnetictunabilitybeyond65GHz,astheresonantfrequencyexceeded65GHzinthisstudy.Isthereacut-o"frequencyforelectricaltunability?Studyingthegap-dependentelectricalandmagnetictunability,andoptimizingthegapsizetothelmthickness.Calculatingrelativepermeabilityandpermittivityandlookingforasoftwarethatcouldpossiblymodelaphaseshifterwithlargepermittivityandpermeability.Itwouldbealsointerestingtousethesemeasurementtechniquesfortestingbiologicalsamplesathighfrequenciesandextracttheircomplexpermittivityandpermeability.Coplanarwaveguidephaseshifterscouldbefabricatedonhigh-resistivitysubstratesandbiologicalsamplescouldbedepositedonthetopandtheirparametersmeasuredasafunctionoffrequency,andelectricandmagneticelds.112

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APPENDICES123

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AppendixAListofPublicationsR.Heindletal,Pulsed-laserdepositedferrite-ferroelectricthinlms,underprepa-ration.R.Heindl,H.Srikanth,S.Witanachchi,P.Mukherjee,T.Weller,A.S.Tatarenko,G.Srinivasanetal,Structure,magnetismandtunablemicrowaveprop-ertiesofPLD-grownBariumFerrite/BariumStrontiumTitanatebi-layerlmssub-mittedtoJ.Appl.Phys,2006.R.Heindletal,Ferrite-ferroelectriccompositethinlmsbypulsedlaserdeposition,underpreparation.N.A.Frey,R.Heindl,S.Srinath,S.Srikanth,N.J.Dudney,MicrostructureandmagnetisminBariumStrontiumTitanateBSTO-BariumHexaferriteBaMmul-tilayers,Mat.Res.Bull.40,1286-1293.S.Srinath,N.A.Frey,R.Heindl,H.Srikanth,K.R.Coey,N.J.Dudney,GrowthandcharacterizationofsputteredBSTO/BaMmultilayers,J.Appl.Phys.97,1-3.R.Hajndl,S.Srinath,andS.Srikanth,SurfacemodicationandmagnetisminCo-implantedBSTO/BariumHexaferritecompositelms,NinthInternationalCon-ferenceonFerritesICF-9,SanFrancisco,p.155-160,ISBN:1574982184,AmericanCeramicSociety,March1,2005.R.Hajndl,J.Sanders,H.Srikanth,andN.J.Dudney,GrowthandcharacterizationofBSTO/hexaferritecompositethinlms,J.Appl.Phys.93,7999-8001.H.Srikanth,R.Hajndl,B.Moulton,M.J.Zaworotko,Magneticstudiesofcrystal-engineeredmolecularnanostructuresinvited,J.Appl.Phys.93,7089-7091.B.Moulton,J.Lu,R.Hajndl,S.Hariharan,andM.J.Zaworotko,Crystalengi-neeringofananoscalekagomlattice,Angew.Chem.Int.Ed.41,2821-2824.124

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AppendixAContinuedH.Srikanth,R.Hajndl,C.Chirinos,andJ.Sanders,Magneticstudiesofpolymer-coatedFenanoparticlessynthesizedbymicrowaveplasmapolymerization,Appl.Phys.Lett.79,3503-3505.125

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AppendixBListofConferencePresentationsConferencePresentationsoral50thMagnetismandMagneticMaterialsMMMConference2005,SanJose.NinthInternationalConferenceonFerritesICF-9-August2004,SanFrancisco.AmericanPhysicalSocietyAPSMarchMeeting2003,Austin.47thMagnetismandMagneticMaterialsMMMConference2002,Tampa.AmericanPhysicalSocietyAPSMarchMeeting2002,Indianapolis.ContributedConferencePresentationsAmericanPhysicalSocietyAPSMarchMeeting2005,LosAngeles.49thMagnetismandMagneticMaterialsMMMConference2004,Jacksonville.AmericanPhysicalSocietyAPSMarchMeeting2004,Montreal.InternationalConferenceonMaterialsforAdvancedTechnologiesICMAT2001,Singapore.126

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ABOUTTHEAUTHORRankoHeindlwasbornisSarajevo,Bosnia-Herzegovina.In1992,hemovedtoGermanywherehenishedHigh-SchoolandattendedRuhr-UniversityinBochumfor2yearsandobtainedVordiplom".In1998,hecametoUSAandjoinedthePhysicsDepartmentattheUniversityofSouthFloridaUSF.HeobtainedB.S.andM.S.degreesinphysicsin2000and2002,respectively.BesidesworkingatUSF,healsomaderesearchvisitstotheOakRidgeNationalLaboratoryandtheAdvancedMaterialsProcessingandAnalysisCenterattheUniversityofCentralFlorida.Hepublishedseveralscienticpapersintheeldofmagnetism.HewasawardedTharp,SURAandIGERTfellowshipsovertheyears.WhenhebecameaUScitizenin2004,hechangedthespellingofhislastnamefromHajndltoHeindl.HewontheNRCPostdoctoralResearchFellowshipinFall2006,andwilljoinNIST,BoulderintheSpring2007.