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Exploration of Electro-Infiltrated Magnetic Nanocomposites

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Title:
Exploration of Electro-Infiltrated Magnetic Nanocomposites
Creator:
Wen, Xiao
Place of Publication:
[Gainesville, Fla.]
Florida
Publisher:
University of Florida
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Language:
english
Physical Description:
1 online resource (121 p.)

Thesis/Dissertation Information

Degree:
Doctorate ( Ph.D.)
Degree Grantor:
University of Florida
Degree Disciplines:
Electrical and Computer Engineering
Committee Chair:
ARNOLD,DAVID P
Committee Co-Chair:
YOON,YONG KYU
Committee Members:
GILA,BRENT P
ANDREW,JENNIFER

Subjects

Subjects / Keywords:
electro-infiltration -- exchange-coupling -- exchange-spring -- magnet -- mems -- microfabrication -- nanocomposite
Electrical and Computer Engineering -- Dissertations, Academic -- UF
Genre:
bibliography ( marcgt )
theses ( marcgt )
government publication (state, provincial, terriorial, dependent) ( marcgt )
born-digital ( sobekcm )
Electronic Thesis or Dissertation
Electrical and Computer Engineering thesis, Ph.D.

Notes

Abstract:
This dissertation introduces a scalable and process-integrable manufacturing method, electro-infiltration, for creating microstructures of magnetic nanocomposite materials. Magnetic nanomaterials are first consolidated into a photoresist mold and then a magnetic metal matrix is electroplated to bind the nanomaterials and form a dense nanocomposite microstructure. The magnetic moments in the inclusion phase and the matrix phase can interact through the interfaces by exchange coupling. The nanoscale dimensions and the large interfacial surface area combine to yield hybrid magnetic properties that differ from the constituent phases. Compared to the conventional magnetic materials, the exchange-coupled nanocomposites show promise to provide tunable and improved magnetic properties, such as a higher maximum energy density, low eddy current loss and extended operational frequencies. The electro-infiltration process enables fabrication of nanocomposites more rapidly than the vapor deposition methods, making it efficient and cost-effective to fabricate tens-of-microns-thick films. Electro-infiltrated Fe2O3/Fe-Co and Ni0.5Zn0.5Fe2O4/Ni-Fe nanocomposites are studied for potential application as high performance microinductor cores operating at hundreds of megahertz. In order to reduce the eddy current loss, high-resistivity Fe2O3 and Ni0.5Zn0.5Fe2O4 nanoparticles are embedded into soft magnetic Fe-Co or Ni-Fe matrices to form the nanocomposite material. The Ni0.5Zn0.5Fe2O4/Ni-Fe nanocomposite exhibits a high saturation magnetization of 1.15 T, a coercivity of 250 A/m and a relative permeability of 320 up to 93 MHz, indicating the promising potential of the electro-infiltrated nanocomposites. The peak value of imaginary part of the permeability decreases in nanocomposites, implying energy loss reduction. The Fe2O3/Fe-Co nanocomposite exhibits a 5x improvement on the product of permeability and cutoff frequency over the Fe-Co film. Exchange-coupled Fe-Co/CoPt nanocomposites are developed to explore the capacity of forming high energy density permanent magnet using electro-infiltration. The Fe-Co nanoparticles with high magnetization embedded in and magnetically coupled to the hard magnetic CoPt matrix results in an exchange coupled magnet. The nanocomposite yields a saturation magnetization of 0.97 T, a remanent magnetization of 0.70 T and a coercivity of 127 kA/m. However, the maximum energy density of the nanocomposite is 21.8 kJ/m3, which is slightly lower than that of single phase CoPt. As the exchange coupling between the soft phase and the hard phase is manifested in the delta-M and FORC diagram. The improvement on maximum energy density is highly possible in optimized nanocomposite materials. The developed electro-infiltration process provides a platform to fabricate nanocomposite materials. It allows to customize and optimize the material properties of nanocomposites, and offers a new way to explore novel functions of nanocomposite materials. ( en )
General Note:
In the series University of Florida Digital Collections.
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Includes vita.
Bibliography:
Includes bibliographical references.
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Description based on online resource; title from PDF title page.
Source of Description:
This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Thesis:
Thesis (Ph.D.)--University of Florida, 2017.
Local:
Adviser: ARNOLD,DAVID P.
Local:
Co-adviser: YOON,YONG KYU.
Statement of Responsibility:
by Xiao Wen.

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Source Institution:
UFRGP
Rights Management:
Applicable rights reserved.
Classification:
LD1780 2017 ( lcc )

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EXPLORATIONOFELECTRO-INFILTRATEDMAGNETICNANOCOMPOSITESByXIAOWENADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOLOFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENTOFTHEREQUIREMENTSFORTHEDEGREEOFDOCTOROFPHILOSOPHYUNIVERSITYOFFLORIDA2017

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c2017XiaoWen

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ToLaoMa

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ACKNOWLEDGMENTSIwouldliketothankDr.DavidArnoldforbeingagreatadvisor,whoencourages,supportsandguidesmeonmyjourneyofresearch.I'mgratefultoDr.JenniferAndrewforthejoyfulandvaluablediscussionsinweeklymeetings,toDr.Yong-KyuYoonfortheinspiringandwarmconversations,andtoDr.BrentGilaforhandingmethekeystocooltoolsandbeingsogladtojoinmycommittee.Ithanktheprofessorsforbeingmycommitteemembersandtakingtimetosharetheirexpertiseandexperiences.IacknowledgetheNRFtechnicians,Mr.DavidHays,Mr.AlOgden,Mr.BillLewis,Mr.AndresTrucco,andDr.BrentGilaagain,fortheirdedicatedhelpinsideandoutsidecleanroom.IalsothanktheMAICtechnician,Mr.WayneAcree,forhisassistanceincharacterization.IthankmycolleaguesinDr.Arnold'sgroupfortheendlessdiscussionswhichfrequentlytriggersparksofnovelideas.IenjoythecolloborationwithDr.KellyStefan,MattBauerandDr.JustinStarrinDr.Andrew'sgroup,beinggratefulfortheexperiencesoftheinterdisciplinaryresearch.IthankalltheIMGmembersforhavingbuiltsuchanenergeticandcheerfulcommunity.IpraisemyfriendsandmyfamilyfortheirunderstandingandlastingsupportandoweatriptoJune.IacknowledgeChinaScholarshipCouncil,NationalScienceFoundation(CMMI-1451993andIIP-1439644),OceofTechnologyLicensingatUFandMISTCenterforprovidingnancialsupport.Otherwise,thisworkwillnotproceed. 4

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TABLEOFCONTENTS page ACKNOWLEDGMENTS ................................. 4 LISTOFTABLES ..................................... 7 LISTOFFIGURES .................................... 8 ABSTRACT ........................................ 11 CHAPTER 1INTRODUCTION .................................. 13 1.1ProspectsofNanocomposites ......................... 13 1.2ConceptofElectro-Inltration ......................... 17 1.2.1BenetsofElectro-InltrationProcess ................. 17 1.2.2MicrofabricatedMagneticDevices ................... 19 1.3ResearchGoals ................................. 21 1.4DissertationOutline .............................. 22 2BACKGROUND ................................... 24 2.1FundamentalsofMagnetism .......................... 24 2.1.1MagnetizationProcessandHysteresisLoop .............. 26 2.1.2Micromagnetics ............................. 30 2.1.3MagnetizationofParticle ........................ 35 2.1.4RandomAnisotropyModel ....................... 38 2.1.5FerromagneticResonance ........................ 38 2.1.6EddyCurrent .............................. 41 2.2HighFrequencySoftMagneticMaterials ................... 43 2.2.1SoftMagneticAlloy ........................... 43 2.2.2SoftMagneticFerrites .......................... 45 2.3MicrofabricatedPermanentMagnets ..................... 46 2.4ReviewofFabricationTechniquesofNanocomposite ............. 46 2.4.1Multilayer ................................ 46 2.4.2Inclusion-Matrix ............................. 49 2.4.2.1Dielectric-coatednanoparticles ............... 49 2.4.2.2Vapordeposition ....................... 51 2.4.2.3Electrochemicaldeposition .................. 52 2.4.3Summary ................................. 54 3ELECTRO-INFILTRATIONPROCESS ...................... 56 3.1SubstratePreparation ............................. 56 3.2FabricationofNanoparticles .......................... 58 3.3ParticleConsolidation ............................. 61 5

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3.3.1EvaporativeDeposition ......................... 62 3.3.2ElectrophoreticDeposition ....................... 65 3.4InltrativeElectroplating ............................ 67 3.5PlanarizationandCleaning .......................... 72 3.6Annealing .................................... 73 3.7ThicknessMeasurement ............................ 74 3.8Summary .................................... 74 4SOFTNANOCOMPOSITEMAGNETS ...................... 76 4.1CharacterizationMethod ............................ 76 4.2NanoparticleFilms ............................... 79 4.3ElectroplatedAlloy ............................... 80 4.4SoftMagneticNanocompositeMagnets .................... 83 4.4.1-Fe2O3=Fe67Co33Nanocomposite ................... 86 4.4.2Ni0:5Zn0:5Fe2O4=Fe67Co33andAl2O3=Fe67Co33Nanocomposites ... 89 4.4.3Ni0:5Zn0:5Fe2O4=Ni-FeNanocomposites ................ 92 4.5ComparisonofHighFrequencyMagneticProperties ............. 95 4.6Summary .................................... 96 5HARDNANOCOMPOSITEMAGNETS ...................... 98 5.1TheoreticalModel ................................ 99 5.2FePt/Fe-CoNanocomposites .......................... 104 5.3Fe-Co/CoPtNanocomposites ......................... 107 5.4Summary .................................... 108 6CONCLUSION .................................... 110 6.1Summary .................................... 110 6.2FutureWork ................................... 111 6.2.1FirstDirection:StructureStudy .................... 112 6.2.2SecondDirection:SoftMagneticNanocomposites .......... 112 6.2.3ThirdDirection:HardMagneticNanocomposites .......... 112 6.2.4FourthDirection:SuperlatticeMetaconductor ............ 113 REFERENCES ....................................... 114 BIOGRAPHICALSKETCH ................................ 121 6

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LISTOFTABLES Table page 2-1Criticaldimensionsforsuperparamagneticparticles ................ 36 2-2Magneticcorematerials ............................... 44 2-3Microfabricatedpermanentmagnets ........................ 47 3-1Summaryoffabricationprocessesfornanocomposites ............... 57 3-2TestofphotoresistadhesiononDay39 ....................... 58 4-1Magneticpropertiesofparticlesandthinlms ................... 79 4-2Magneticpropertiesofelectroplatedlms ..................... 81 4-3Choiceofmaterialsinthesoftmagneticnanocomposites ............. 86 5-1Roomtemperaturemicromagneticparametersformagneticmaterials ...... 103 5-2Criticaldimensionsforexchange-couplednanocomposites ............. 103 5-3Choiceofmaterialsfortheexchange-coupledhardmagneticnanocomposites .. 104 5-4Elementalatomiccompositionsformagneticlms ................. 107 7

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LISTOFFIGURES Figure page 1-1Illustrationofnanocompositestructures ...................... 15 1-2Illustrationoftheelectro-inltrationprocess .................... 18 1-3Illustrationofprocessparametersforpermalloy .................. 19 2-1Sketchofmagneticeldlinesaroundamagneticmoment ............. 25 2-2Schematicsoftheorderingofmagneticmomentsinferromagneticandferrimag-neticmaterials. .................................... 27 2-3Schematicsofhysteresisloops ............................ 27 2-4Schematicofthehysteresisloopsofapermanentmagnetandasoftmagnet ... 29 2-53Dillustrationofmagneticmomentsina180Blochdomainwall ........ 33 2-6Angleofmagneticmomentsina180Blochdomainwall ............ 34 2-7Hypotheticmagnetizationconguration ...................... 35 2-8Nucleationeldasafunctionofparticlesize .................... 37 2-9SEMimagesofthenanolaminatedpermalloycore ................. 48 2-10PermeabilityspectraofNi-Fe=SiO2nanocompositeswithdierentsilicaweightfractions ........................................ 50 2-11SEMimagesofthepermalloynanocrystalswithsilicacoatingandtheFe=SiO2laminates ....................................... 51 2-12SchematicoftheFePt=Fe80Ni20vapordepositionsystemandTEMimageofthefabricatednanocomposite .............................. 52 2-13CoFe2O4=CoandFe-Co/FePtnanocompositesmadebyelectrophoreticdeposi-tionandelectroplating ................................ 54 3-1TypicalsizedistributionofthesynthesizedNiZnferritenanoparticles ...... 60 3-2Schematicoftheevaporativedepositionprocess .................. 63 3-3OpticalandSEMpicturesofthethinlmsofmagnetitenanoparticlesandNiZnferritenanoparticles ................................. 64 3-4OpticalimagesoftheNiZnferritenanoparticlelms ............... 65 3-5Setupforelectrophoreticdepositionusingthesubstrateascathode ....... 66 8

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3-6OpticalandSEMimagesoverlookingaluminananoparticlelmpreparedbyelectrophoreticdeposition .............................. 66 3-7Opticalimagesofpatternedlmsofnickel-zincferritenanoparticles ....... 67 3-8Schematicsoftheinltrativeelectroplatingprocess ................ 68 3-9ImagesofthesurfaceofaNiZnferrite/Fe-Colm ................. 70 3-10SEMandTEMimagesshowingthecross-sectionsoftheelectro-inltrated-Fe2O3/Fe-CoandNi0:5Zn0:5Fe2O4/Fe-Cosamples. ...................... 71 3-11SEMimagesatcleavedcross-sectionsofCoFe2O4nanoparticlelms,aCoPtlmandFe-Co/CoPtnanocompositelm ...................... 73 4-1Pictureofthemicrostriplinepermeametertestxtureloadedwitharectangu-larsample. ....................................... 78 4-2HysteresisloopsofmaghemiteandNiZnferriteparticlelms. .......... 80 4-3Permeabilityspectrumofamaghemiteparticlelm ................ 81 4-4ThicknessprolesofelectroplatedNi-Felms ................... 82 4-5m-HhysteresisloopsofelectroplatedNi-Felms .................. 84 4-6HysteresisloopsofelectroplatedNi-Felmsgroupedbyloopshapes ....... 84 4-7PlotsofNi-Fepropertiesandelectroplatingparameters .............. 85 4-8HysteresisloopsofNi-Felmelectroplatedina90mTmagneticeld ...... 86 4-9Hysteresisloopsof-Fe2O3lm,Fe-Colmandtheircomposites ........ 88 4-10Permeabilityspectraof-Fe2O3particlelm,Fe-Colmandtheircomposites 89 4-11HysteresisloopsofNi0:5Zn0:5Fe2O4/Fe-Conanocomposite,Al2O3/Fe-Conano-compositeandFe-Colm. ............................. 91 4-12PermeabilityspectraofNi0:5Zn0:5Fe2O4/Fe-Co,Al2O3/Fe-CocompositelmsandFe-Colm. .................................... 91 4-13HysteresisloopsofaNiZnferrite/Ni-Fenanocomposite,apermalloylm,andaNiZnferritenanoparticlelayer ............................ 93 4-14ComplexpermeabilityspectrumofNiZnferrite/Ni-Fenanocompositeandpermal-loylm ........................................ 94 4-15Graphicrepresentationofcutofrequencyandpermeabilityforelectro-inltratedsoftmagneticnanocompositesamples. ....................... 96 5-1Schematicofmagnetizationprocessofahard-softbilayer ............. 100 9

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5-2OpticalimagesatthesurfaceofannealedFePt/Fe-Conanocomposites ..... 105 5-3Them-HhysteresisloopsofFePt/Fe-ConanocompositescomparedwithFePtandFe-Colms .................................... 106 5-4HysteresisloopsandenergydensityplotoftheannealedCoPtlmandtheFe-Co/CoPtnanocompositelm ............................ 108 5-5FORCdiagramandMplotoftheFe-Co/CoPtnanocompositelm ....... 109 10

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AbstractofDissertationPresentedtotheGraduateSchooloftheUniversityofFloridainPartialFulllmentoftheRequirementsfortheDegreeofDoctorofPhilosophyEXPLORATIONOFELECTRO-INFILTRATEDMAGNETICNANOCOMPOSITESByXiaoWenAugust2017Chair:DavidP.ArnoldMajor:ElectricalandComputerEngineering Thisdissertationintroducesascalableandprocess-integrablemanufacturingmethod,electro-inltration,forcreatingmicrostructuresofmagneticnanocompositematerials.Magneticnanomaterialsarerstconsolidatedintoaphotoresistmoldandthenamagneticmetalmatrixiselectroplatedtobindthenanomaterialsandformadensenanocompositemicrostructure.Themagneticmomentsintheinclusionphaseandthematrixphasecaninteractthroughtheinterfacesbyexchangecoupling.Thenanoscaledimensionsandthelargeinterfacialsurfaceareacombinetoyieldhybridmagneticpropertiesthatdierfromtheconstituentphases.Comparedtotheconventionalmagneticmaterials,theexchange-couplednanocompositesshowpromisetoprovidetunableandimprovedmagneticproperties,suchasahighermaximumenergydensity,loweddycurrentlossandextendedoperationalfrequencies.Theelectro-inltrationprocessenablesfabricationofnanocompositesmorerapidlythanthevapordepositionmethods,makingitecientandcost-eectivetofabricatetens-of-microns-thicklms. Electro-inltrated-Fe2O3/Fe-CoandNi0:5Zn0:5Fe2O4/Ni-Fenanocompositesarestudiedforpotentialapplicationashighperformancemicroinductorcoresoperatingathundredsofmegahertz.Inordertoreducetheeddycurrentloss,high-resistivity-Fe2O3andNi0:5Zn0:5Fe2O4nanoparticlesareembeddedintosoftmagneticFe-CoorNi-Fematricestoformthenanocompositematerial.TheNi0:5Zn0:5Fe2O4/Ni-Fenanocompositeexhibitsahighsaturationmagnetizationof1.15T,acoercivityof250A/mandarelative 11

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permeabilityof320upto93MHz,indicatingthepromisingpotentialoftheelectro-inltratednanocomposites.Thepeakvalueofimaginarypartofthepermeabilitydecreasesinnanocomposites,implyingenergylossreduction.The-Fe2O3/Fe-Conanocompositeexhibitsa5ximprovementontheproductofpermeabilityandcutofrequencyovertheFe-Colm. Exchange-coupledFe-Co/CoPtnanocompositesaredevelopedtoexplorethecapacityofforminghighenergydensitypermanentmagnetusingelectro-inltration.TheFe-ConanoparticleswithhighmagnetizationembeddedinandmagneticallycoupledtothehardmagneticCoPtmatrixresultsinanexchangecoupledmagnet.Thenanocompositeyieldsasaturationmagnetizationof0.97T,aremanentmagnetizationof0.70Tandacoercivityof127kA/m.However,themaximumenergydensityofthenanocompositeis21.8kJ=m3,whichisslightlylowerthanthatofsinglephaseCoPt.AstheexchangecouplingbetweenthesoftphaseandthehardphaseismanifestedintheMandFORCdiagram.Theimprovementonmaximumenergydensityishighlypossibleinoptimizednanocompositematerials. Thedevelopedelectro-inltrationprocessprovidesaplatformtofabricatenano-compositematerials.Itallowstocustomizeandoptimizethematerialpropertiesofnanocomposites,andoersanewwaytoexplorenovelfunctionsofnanocompositematerials. 12

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CHAPTER1INTRODUCTION Magnetsareenablingcomponentsforavarietyofhighperformancesystems,suchasgenerators,motors,speakers,powerelectronicsandharddiskdrives.Thequalityofmagneticmaterialsisthekeyfactorfortheimprovementofsystemeciencyandreductionofsizeandcost.Theemergingnanomaterialsandmicrofabricationtechnologieshaveoeredanopportunitytoproducenanostructuredmaterials.Thisdissertationintroducestheelectro-inltrationprocesswhichcombinestheconventionalmagnetsandthemagneticnanomaterialstocreatenanocompositesforbetterperformancesandnewfunctions. 1.1ProspectsofNanocomposites Ananocompositeisamultiplephasematerialinwhichoneormorephaseshavedimensionsonthenanometerscale.Whileamultilayerstructurewithnanometerthicknessesisconsideredasatwo-dimensionalnancompositematerial,ananocompositewithcontrollablearrangementinthreedimensionsisathree-dimensionalnanocomposite.Thestructureofnanomaterialsembeddedinamatrixisacommonexampleofthree-dimensionalnanocomposites. Forseveraldecades,nanomaterialshavebeensparselydopedintoceramic,metalandpolymermatricestoimprovemechanical,electricalpropertiesandthermalstability.Forexamples,addinghexagonalboronnitrideintosiliconnitrideceramiccanincreasefracturestrengthoverabroadtemperaturerangefromroomtemperatureupto1500C[ 1 ]andincorporating7%ofAl2O3nanoparticlesbyvolumeinaluminumcanincreasetheyieldstrengthbymorethan100%[ 2 ].Forpolymers,allkindsofproperties,suchasmechanical,electrical,optical,catalyticandbiologicalproperties,alterbyaddingvariousnanomaterials[ 3 ]. Adistinguishedpropertyofnanomaterialsistheirlargesurfacetovolumeratio.Forthenanocompositescontainingalargevolumefractionofnanomaterials,theinterface 13

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areaishugeandtheeectofinterfaceonmaterialpropertiesbecomessignicant.Somephysicalstates,suchasstrainandorientationofmagneticmoments,propagateviainterfaces.Theirinteractionsamongdierentphasesofnanocompositesenablenewfunctionsandenhancesproperties.Forinstance,thenanocompositewithapiezoelectricphaseandamagnetostrictivephasecoupledbystresseectivelyactsasamagnetoelectricmaterial[ 4 ]. Thepropertiesofnanocompositesareinuencedbytheshapeanddimensionofnanomaterialsaswell.Whiletheintrinsicphysicalquantitiesofacomposite,likemass,relateonlytothevolumeaverageofallincludedphases,otherquantitiesdependonthestructuraldetails.Forinstance,givenametal/dielectriccompositewiththesamevolumefractionofmetal,thestructureofdielectricinductionsinametalmatrixhashigherelectricalconductivitythanthestructureofmetallicinclusionsinadielectricmatrix.Ananisotropicstructuretypicallyleadstoanisotropicproperties,suchastheorientationofgoldnanorodsinapolymerdeterminesthepolarizationofabsorbedlightinthenanocompositemedium[ 5 ]. Becauseoftheinterfaceeectsandthedimension-dependentphenomena,nano-compositeshavepromiseforimprovementofmaterialpropertiesandpotentiallynewfunctions.Fig. 1-1 showssomeexamplesofnanocompositestructuresthatcanbemadebyelectro-inltration.Fig. 1-1 aillustratesthestructurewithnormalparticleswhichhaveshapeirregularityandsomesizedistribution.Astheparticlesdonothavetobesinglephasematerials,Fig. 1-1 bsketchesanexampleofcore-shellparticles.Formonodisperseparticles,itispossibletoformasuperlatticeinthecompositeandexhibitanisotropicandfrequency-dependentproperties,asshowninFig. 1-1 c.Theparticleswithanisotropicshapeincompositeexhibitanisotropicpropertiesaswell,suchasalignednanoneedlesornanowiresinFig. 1-1 d. Magneticnanocomposites .Enlightenedbytheirpromisingphenomena,thisresearchfocusesonthedevelopmentofnanocompositematerialsformagneticapplications. 14

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Figure1-1. Illustrationofnanocompositestructureswith(a)normalirregularparticles,(b)core-shellparticles,(c)monodisperseparticlesinasuperlatticeand(d)alignednanoneedles. Nanocompositesareexpectedtoimprovefunctionalpropertiesthatallowstomakebettermaterials,suchasstrongpermanentmagnetsandhighfrequencysoftmagnets. Currently,neodymiumironboron(Nd2Fe17B)isthecommerciallyavailablemagneticmaterialthatoersthehighestenergyproduct.Ifanevenstrongermagnetisavailable,itwouldhavehigherremanentmagnetizationandcorrespondinglylargercoercivity.Thediscoveryofabettermaterialispossible,butdicult,sincethedominanceofNd2Fe17Bhasbeenobservedforover20years.Anotherpossibilityistoproducecompositesfromexistingmaterials.Becausesoftmaterialsusuallyhavehighermagnetizationthanpermanentmagnets,itistemptingtocombineapermanentmagnetandasoftmagnettocreateamaterialthatbenetsfrombothhighcoercivityandhighmagnetization.Theoreticalcalculations[ 6 ]showedthatagiantenergyproductof1090kJ=m3(137MGOe)canbeachievedinnanostructuredtwo-phaseSm2Fe17N3=Fe65Co35magnets,resultingintwicethestrengthasthebestNd2Fe17Bmagnets.Thisimprovementreliesontheeectofexchangecoupling,whichdescribesthealignmentofadjacentmagneticmomentsonthenanometerscale.Theeectivenessofexchangecouplingbetweenthehard 15

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magneticphaseandthesoftmagneticphaseissensitivetotheirdimensions.Thus,thestructureofthesenanocomposites,eitherstackedlayersorinclusionsinamatrix,needtobepreciselydesignedandfabricated. Eddycurrentlimitssoftmagnetperformanceinhighfrequencyapplications.Atabove100kHzeddycurrentlossdominatestheenergylossinmagneticcores[ 7 ].TheeddycurrentpreventsanACmagneticeldfrompenetratingintothemagnet,resultingintheskineectthatonlythesurfaceofthemagnetcaneectivelyconductandenhancetheACmagneticeld.Theperformanceofthesoftmagneticiscompromisedandthegeneratededdycurrentdissipatesenergytoheat.Theenergylossfromtheeddycurrentdeterioratesasfrequencyincreases.Softmagneticnanocompositesaredesignedtoreduceeddycurrentloss,byaddingdielectricsorotherhighresistivitymagneticmaterialstoencapsulatemagneticmetalsandblocktheeddycurrent[ 8 ].Adielectricmatrixincludingsoftmagneticinclusionsisanidealstructurewhereineddycurrentisconnedinsmallseparatespaces.Inanotherway,themultilayerstructureiscommonlyused,whereinsulationlayerisolateseddycurrentowinmagneticlaminates.Incorporatingdielectricsorhighresistivitymagneticmaterialsinamagneticmetalmatrixisalsoacandidatestructure.Althougheddycurrentisnotlocallyconnedinmetalmatrix,theinclusionsincreasetheeectiveelectricresistivity. Skineectappliestoanormalnon-magneticconductoraswellandresultsinACcurrentonlyowingbeneaththesurfaceoftheconductor.Thereducedcross-sectionwherethecurrentowsthroughincreasestheresistivityofconductorathighfrequencies.Toavoidskineect,magneticmaterialswithnegativepermeabilitycanbeincludedintheconductortoformananocompositewithzeropermeability.Insuchasmaterial,themagneticeldiscanceledandtheskineectiseliminated.Cu=Ninanocompositesinmultilayerstructurehasdemonstratedthesuppressiononeddycurrentloss.Thefeasibilityofinclusion-matrixstructureremainsunknown. 16

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Insummary,magneticnanocompositeshavepotentialtoimprovemagneticpropertyandintroducenewfunctionthatisdiculttoacquireinconventionalmaterials.Thedirectcontrolofmaterialstructureinthefabricationofnanocompositeoersthecapacitytotailormagneticproperties. 1.2ConceptofElectro-Inltration Theelectro-inltrationprocesscomprisestwocriticalsteps:nanomaterialconsolida-tionandinltrativeelectroplating.Acompositethinlmisformedsothatnanomaterialsareembeddedinametalmatrix. Atypicalprocessofelectro-inltrationisillustratedinFig. 1-2 .Aphotoresistmoldisrstphotolithographicallypatternedonasubstratewithaseedlayer.Thennanomaterialsareconsolidatedintothetrenchesofphotoresist.Ametaliselectroplatedtollthevoidspacesbetweenthenanomaterialsandbindtheminplace.Afterexcessnanomaterialormetalonthelmsurfaceisremoved,thephotoresistisstrippedandtheseedlayerisetchedaway,leavingananocompositemicrostructure. Theelectro-inltratednanocompositehastwophases,namelytheinclusionphaseandthematrixphase.Theentirestructureisideallyfullydenseandtheinclusionphasehasalargevolumefractionof>30%. 1.2.1BenetsofElectro-InltrationProcess Electro-inltrationasalmdepositiontechniquefornanocompositesissuitableforbatchmanufacture.ItappliestoMEMSmicrofabricationandiscompatiblewiththeCMOSprocess.Sincethefabricationofnanomaterialsisseparatefromtheelectro-inltrationprocess,hightemperatureannealingandotherunconventionalmethodscanbeusedtomakehighperformancenanomaterialsbeforetheyare\integrated"onthesubstrate.Thedevelopmentofelectro-inltrationprocessintendstobridgethegapbetweenthewell-developedsynthesistechniquesofmagneticnanomaterialsandpracticalapplications.Becausetheelectro-inltrationprocessisbasedonphotolithographyandelectroplating,itwillbeeasytoadoptinindustry. 17

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Figure1-2. Illustrationoftheelectro-inltrationprocess.(a)Consolidatenanomaterials,(b)electroplatemetalmatrix,and(c)removephotoresistandseedlayer. Thethicknessoftheelectro-inltratedlmcoversarangefromsub-microntotensofmicrons.Itisgoodformicrofabricationofmagneticdevice,sincetheintegrateddevices,suchasmicroinductorsandmicroacutuators,requiremagneticlmstobetens-of-microns-thick.Inthetop-downmethods,amicromagnetismadebymachiningbulkmaterialsdowntoasub-millimetersize.Assemblyofthemicromagnetsontodevicesrequiresconsiderableeort.Ontheotherhand,forthebottom-upmethodswhichdepositlmsonsubstrates,thevapordepositionisecientonlyforalmthinnerthanafewmicrons.Screenprintingandelectroplatingareecientforlmthicknessoftensofmicrons.Butthepropertyofthescreenprintedmaterialsarenotasgoodaselectroplatingunlesspost-processingisinvolved.Sincehighdepositiontemperatureandpost-annealingareusuallyrequiredforpermanentmagnet,thedecouplingbetweenthesynthesisofhardmagneticnanomaterialsandthestructure-deningelectro-inltrationisvaluableforpost-CMOScompatibility. 18

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Figure1-3. Illustrationofprocessparametersforpermalloy.Typicalrangesofdepositionrateandlmthicknessarecompared,basedondatapointsfrom[ 9 { 11 ]forelectroplatingand[ 12 { 14 ]forsputtering. Asanexample,permalloyisacommonsoftmagneticmaterialandcanbedepositedbysputteringandelectroplating.AsshownisFig. 1-3 ,electroplatingprovidesmorerapiddepositionandthickerlmscomparedtosputtering.Thecommercialrolledpermalloysheetisavailableforathicknessdowntohalfamicron,butthecostofpatterningandassemblymaynotbepractical.Fortherequisitelmthicknessinthetensofmicronsrange,asindicatedingurebytheshadedarea,electroplatingisthepreferredprocessintermsofthicknessanddepositionrate. 1.2.2MicrofabricatedMagneticDevices Softmagnetswithloweddycurrentlossarecriticalforhighfrequencymagneticcomponents,suchasinductors.IntegrationofmicrofabricatedmagneticdeviceswithCMOScircuitswillleadtomorepowerfulandecientsystems-on-chip. Theintegrationofmicroinductorscanlowerthecostandreducethesizeofpowermanagementcircuits.TheinductorisakeycomponentinswitchmodeDC-DCconvertercircuits,whichecientlyconvertvoltagefrompowersources,likebatteryandrectied 19

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powergridelectricity,intothevariousworkingvoltagesofunits,suchasCPUandDRAM.Currently,discreteinductorsareindividuallysolderedonPCBboardsincommercialproductsofpowermanagement.Inadditiontothecostlypackagingandassemblyofdiscretecomponents,theyoccupyasignicantportionofboardareaandlimitsystemminiaturization.Theintegrationofinductorswouldenablefullyintegratedconvertersthatshrinkthefootprint,andreducethecostofpowermanagementforsensornetworksandInternetofThings. Theintegratedconverterhelpssaveenergyandimprovecomputingperformance.Whenmultipleconvertersareavailableonasinglechip,everyunitcanbeservedbyapoint-of-loadconverterandtheconverterwouldbeoptimizedbasedontheattributesoftheunit.Atdevicelevel,bothvoltageandoperationfrequencywouldscaleaccordingtothechangingworkloads.Anintegratedconverterallowsdynamicvoltagefrequencyscaling(DVFS)percoreofprocessorthatoersnevoltagecontrolwithrapidresponseonthesub-microsecondscale.DVFSexhibitssignicantenergysavingandoersthespaceforimprovingperformanceintheprocessors[ 15 ]. The4thgenerationofIntelCorepowerarchitecturehasadoptedfullyintegratedvoltageregulators(FIVR)inthesamepackagewiththecircuitsubstrateoperatingat140MHz.Aircoreinductorsinconvertercircuitsareimplementedusingthebottommetallayersoftheip-chippackage[ 16 ].Comparedtoamotherboardvoltageregulator,FIVRimprovesbatterylife,increasespeakpoweranddecreasesthemotherboardthickness[ 16 ],thusenablingultrathinlaptopcomputers.Forfuturescaling,magneticcoreswillbeusedtoenhanceenergydensityofintegratedinductorsandreducearea. Thedevelopmentofmagneticcorematerialsmeetsthetrade-oofvariousmaterialproperties,includingsaturationmagnetization,coercivity,permeabilityandresistivity,aswellasthecapabilityoffabrication.Mathuna[ 17 ]pointedoutthatthekeytrade-ointhematerialpropertiesformicroinductorcoresisbetweensaturationmagnetizationandresistivity,whichtightlyrelatetothepowerhandlingcapabilityoftheinductors,the 20

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powerlossandtheoperationalfrequency.AbenchmarkmaterialissputteredCoZrTa[ 18 ],whichhasasaturationmagnetizationof1.52T,acoercivityof1.2kA/m,apermeabilityof850{1100,andaresistivityof100cm.Theresistivityseemstobeitsshortcutanditneedslamination.Electro-inltrationaimsatpushingforwardthetrade-olineofthehighfrequencysoftmagneticnanocomposites. Ontheotherhand,strongpermanentmagnetsarealwaysdesirableformagneticactuators.Ithasbeenarguedthatatthemicroscalemagneticactuationoutperformselectrostaticactuationwhenthemovingrangeislargerthanonemicron[ 19 ].Butthedicultyliesinthemicrofabricationofgoodpermanentmagnets.Magneticactuationdevicesrequirepermanentmagneticmicrostructurestobetens-of-microns-thick[ 20 ].Ifthefabricationbarriercouldbeovercome,magneticactuationisenvisionedtohavequiteafewadvantages,suchaslongactuationrange,rapidandcontactlessactuation,andformationofbistablesuspensionswithoutpowerconsumption[ 21 ].Theelectro-inltratedpermanentmagnetswithlargemaximumenergydensitiesaresuitableformagneticactuationdevices. Recently-developedmagneticenergyharvesters[ 22 ]andelectrodynamicwirelesspowertransfersystems[ 23 ]utilizepermanentmagnetsasactuationdevicesinaweakexcitationeldsofafewmillitesla.Theremanentmagnetizationisthekeyparameterwhilethelargecoercivityisnotthemajordemand.Theelectro-inltratedpermanentmagnetwithtailoredmagneticpropertiesmayoerbetterperformancesthanconventionalrareearthpermanentmagnets. 1.3ResearchGoals IntegratingthefabricationofmagneticdeviceswiththeCMOScircuitryisalong-termgoalforthedevelopmentoftheelectro-inltrationprocess.Towardsthat,researchgoalofthisdissertationisspeciedinthreetasksattheprocessandmateriallevels. 1.Developmentofelectro-inltrationprocessforproducinginclusion-matrixnanocomposites.Developtheparticleconsolidationandtheinltrative 21

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electroplatingprocesses.Adjusttheelectro-inltrationprocessfordierentcombinationsofmagneticmaterialstoformmagneticnanocomposites. 2.Fabricationandcharacterizationofsoftmagneticnanocompositesformagneticcores.Investigatethefabricationprocessesforferritenanoparticlesandsoftmagneticalloys.Studythehighfrequencymagneticpropertiesofthenanocompositesandverifythereductioneectofeddycurrentloss. 3.Fabricationandcharacterizationofexchange-coupledhardmagneticnanocomposites.Developelectro-inltrationandannealingprocessforexchange-couplehardmagneticnanocomposites.Studytherelationshipbetweenmagneticpropertiesandnanocompositestructures.Verifytheexchangecouplingbetweenthesoftandhardphases.Improvetheprocessparametersbettermaterialstructuresandmagneticproperties. Researchcontributions .Thecontributionsofthisprojectinclude: 1.Developmentofelectro-inltrationprocessforpreparationofmagneticnano-compositesanddeterminationoftheinuenceofmaterialstructureonmagneticproperties; 2.Firstdemonstrationofnanocompositesutilizingtwomagneticphasesforsoftmagneticradiofrequencyapplications; 3.Fabricationofsoftmagneticnanocompositeswithimprovedhighfrequencypropertiescomparedtotheconstituentphases; 4.Firstdemonstrationofnanocompositesutilizingasoftmagneticphaseandahardmagneticphaseforexchangecoupledpermanentmagnetsusingelectro-inltrationprocess. 1.4DissertationOutline Thisdissertationisorganizedinsixchapters.Thecurrentchapterintroducestheconceptandsignicanceoftheelectro-inltrationprocessandmagneticnanocomposites,andthenspeciestheresearchgoals.Thesecondchapterdiscussesthefundamentalsofmagnetismandreviewsthefabricationapproachesformagneticnanocomposites.Thethirdchapterintroducesthegeneralfabricationmethodsoftheelectro-inltrationprocess. 22

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ThefabricationandcharacterizationforsoftmagneticnanocompositearediscussedinChapter4,andthoseforhardmagneticnanocompositesarepresentedinChapter5.FinallyinChapter6,thedevelopmentofelectro-inltrationprocessandtheapplicationsofmagneticnanocompositesaresummarizedandfutureworkishighlighted. 23

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CHAPTER2BACKGROUND Thischapterintroducesthefundamentalsofmagnetism.Theintroductionelaboratesmorethannecessarydetailstoprovideasolidtheoreticframework.Thenthepropertiesofconventionalmagneticmaterialsarediscussed.Finally,areviewoffabricationtechniquesformagneticnanocompositesispresented. 2.1FundamentalsofMagnetism Electricalcurrentandmagnetsarethesourcesofmagneticeld.Inamorefunda-mentalway,amagneticeldissetupbythemotionofcharges.Inamagnet,theorbitalmotionofelectronsandthespinofelectronsandnucleigeneratethemagneticeld.Theorbitalmotionandspinofelectronsinthemeanwhiledynamicallyreacttothemagneticeldappliedtothem. Aphysicalquantityofmagneticmomentisadoptedtobridgethemotionofelectronsandnucleusanditsmagneticbehaviors.Themagneticmomentispresumedtobethesourceofmagneticeld.Sincethefar-elddistributionsofthemagneticeldssetupbyacurrentlooporelectronmotionareidentical,thiselddistributionisusedtodenethemagneticeldofamagneticmoment.Inthisway,acurrentlooporelectronmotioncanbeseenasmagneticmomentswithcertainvalues,whenthemagneticeldiscalculatedorthedynamicresponseissimulated.Themagneticmomentusesavectornumbertosummarizethemagneticbehaviorsofelectronmotionandthefurtherdetailsofelectronmotionbecomesnegligible. Themagneticeldofamagneticmomentmisdescribedas B=0 4jrj3(3(m^r)^r)]TJ /F7 11.955 Tf 11.95 0 Td[(m):(2{1) whereristhepositionvector,andBstandsforthemagneticuxdensitywhichdescribesthemagneticeldandwillbeintroducesinthenextsection.Inthesphericalcoordinates, 24

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Figure2-1. Sketchofmagneticeldlinesaroundamagneticmomentpointingupwardsatthecenter. themagneticuxdensityofamagneticmomentalignedinthezdirectioniswrittenas B=0m 4r3(2cos^r+sin^):(2{2) AschematicofthemagneticmomenteldisplottedinFig. 2-1 Themagneticmomentofanatomisprimarilycontributedbythemotionofelectrons.Themagneticmomentmisproportionaltotheangularmomentumoforbitalmotionlandspinsoftheelectron. m=)]TJ /F3 11.955 Tf 18.05 8.09 Td[(e 2me(l+2s)(2{3) Theproportionalityfactor=)]TJ /F3 11.955 Tf 9.3 0 Td[(e=2meisknownasgyromagneticratio.Themagneticmomentofelectriccurrentowinginaloopequalstheproductofthecurrentmagnitudeandthelooparea. m=IS(2{4) Aquantityofmagnetization(M)isdenedasthevolumetricdensityofmagneticmoments,whichcanbeusedtocomparethestrengthofmagneticmaterialsindierentdimensions.Theunitofthemagnitudeofmagneticmomentistheelectromagneticunit, 25

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emu,whichisequivalenttoAm2.Thecommonunitofmagnetizationisemu=cm3,whichisequivalenttoamperepermeter,kA/m. Theinteractionofmagneticmomentandthemagneticeldappliedtoitisspeciedintermsofmagnetostaticenergy.ForamagneticmomentminamagneticeldB,themagnetostaticenergyis E=)]TJ /F7 11.955 Tf 9.29 0 Td[(mB(2{5) Theforceandtorqueappliedtothemagneticmomentareexpressedas F=r(mB)(2{6) =mB(2{7) 2.1.1MagnetizationProcessandHysteresisLoop Ferromagneticandferrimagneticmaterialsareofinteresttomagneticapplications,becauseoftheirhighmagnetization.Theatominthesematerialshaveanetmagneticmo-mentfromtheelectronmotion.Thenetmagneticmomentofamagnetisdeterminedbytheorderingofmagneticmomentsinallatoms.Themagneticmomentsofadjacentatomsarestronglycoupledinthesematerials,butindierentways.Inaferromagneticmaterial,themagneticmomentstendtoorienttowardsthesamedirection.Inaferrimagneticmaterial,themagneticmomentspointtotheoppositedirectioninterchangeably,asshowninFig. 2-2 .Becausethemagneticmomentspointingtoonedirectionarestrongerthanthosepointingtotheopposite,theferrimagneticmaterialexhibitsanetmagnetization. Thequasi-staticmagnetizationprocessofamagneticmaterialisusuallycharacterizedbythemagnetizationcurves.Eachpointonthecurverepresentsastateofmagnetizationofthematerialunderanexternalorappliedmagneticeld.Thedistincteectsofmagnetizationinferromagnetsandferrimagnetsarehysteresisandsaturation.Themagnetizationcurvesofthesematerialsarealsoknownashysteresisloops.InthehysteresisloopsshowninFig. 2-3 ,thexaxisdenotestheappliedmagneticeld(H 26

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Figure2-2. Schematicsoftheorderingofmagneticmomentsinferromagneticandferrimagneticmaterials. Figure2-3. Schematicsofhysteresisloops,(a)M-Hloopand(b)B-Hloop. eld)andtheyaxiscandenoteeitherthemagnetization(M)orthetotalmagneticeld(Beld). Thetotalmagneticeldisalsoreferredasthemagneticuxdensity. B=0(M+H)(2{8) IntheCGSunitsystem,HandMsharetheunitofoersted(Oe)andBeldusestheunitofgauss(G).Theproportionalityfactor,whichisknownasthevacuumpermeability0,equals1,thusthevalueofBeldissimplythesumofHandM.IntheSIunitsystem,whichisadoptedinthisdissertation,HandMhaveunitsofemu=m3andA/m,andB 27

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eldusestesla(T).Thevalueofthevacuumpermeabilitybecomes410)]TJ /F4 7.97 Tf 6.59 0 Td[(7henry(H),orVs=Am. Themagnetizationofademagnetizedmagnetincreasesastheappliedeldincreasestoacertainpoint,thenthemagnetizationsaturates.Inasaturatedmagnet,allthemagneticmomentsorienttowardsthedirectionoftheappliedeld.Theassociatedmagnitudeofmagnetizationiscalledsaturationmagnetization(Ms).IfthequantityisshownintheunitofT,itiscalledsaturationinduction,Js=0Ms. Whentheappliedeldisremoved,magnetsmaykeepmagnetized.Thislevelofmagnetizationischaracterizedastheremanentmagnetization(Mr),andtheremanence(Br)intheunitoftesla.Inordertofullydemagnetizeamagnet,inotherwords,tomakethetotalmagnetizationzero,amagneticeldhastobeappliedinthereversedirection.Thestrengthofthereverseeldthatperfectlydemagnetizesthemagnetiscalledintrinsiccoercivity(Hci).IfthezeroBeldisusedasthestandardofdemagnetization,whereinthemagnetizationcancelstheappliedeld,therequiredstrengthofthereverseeldiscoercivity(Hc),whichisslightlylowerthantheintrinsiccoercivity.Ahigherreverseeldremagnetizesmagnetinthereversedirection.Asthestrengthofthereverseeldincreasesevenmore,themagnetizationgrowsandnallyreachesthesaturationmagnetization. Becauseofthehysteresisphenomenon,themagnetizationdependsontheappliedeldatpresentandthehistoryofthemagnetizationstatus.Thusthemagnetizationcurveturnsintotheformofhysteresisloop.Theareaenclosedbythelooptellstheconsumedenergyineachmagnetizationcycle. Basedontheeasinessofbeingmagnetizedanddemagnetized,ferromagneticandferrimagneticmaterialsareroughlydividedintotwogroups,hardmagnetsandsoftmagnets.Ahardmagnetsetsupamagneticeldpermanentlyoncemagnetizedinastrongmagneticeld,anditismoreformallyreferredasapermanentmagnet.Asoftmagnet,likeapieceofiron,enhancesthemagneticeldappliedtoit,sinceitcanberemagnetizedandsaturatedbyarelativelyweakeld.Apermanentmagnethasawide 28

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Figure2-4. Schematicofthehysteresisloopsofapermanentmagnetandasoftmagnet,(a)M-Hloopand(b,c)B-Hloops. hysteresisloopduetoitshighcoercivityandasoftmagnethasanarrowonewithalowcoercivity,asinFig. 2-4 .Softmagnetshavestrongersaturationmagnetizationthanpermanentmagnetsinthecomparisonofhighqualityproducts. Thegureofmeritforcomparingpermanentmagnetsisthemaximumenergydensity(BH)max.ThequantitymeansthemaximumproductoftheappliedeldHandthemagneticuxdensityBonthemagnetizationcurveinthesecondquadrant.Halfofthevalueofthemaximumenergydensityequalsthemaximumenergythatcanbestoredinthemagnetinaunitvolumeandalsotheenergyofmagneticeldsetupbythemagnet. Sincesoftmagnetstypicallyhavelowcoercivity,theirhysteresisloopsbeforesaturationcanbeapproximatedbyanobliqueline.Itsslopeisthepermeability,whichshowsthecapabilitytoenhanceamagneticeld.Relativepermeabilitywithrespecttovacuumpermeability,r==0,iscommonlyusedfortheconvenienceofcomparison.r=+1,whereissusceptibility,theslopeinaM-Hloop. Thepropertyofamagnetcanbeanisotropic,duetotheshapeandthelatticestructureofthemagnet.Thehysteresisloopsdierinshapewhentheyaremeasuredindierentdirections.Alongtheso-calledeasyaxis,thehysteresisloopistypicallymoresquareandhasahighercoercivity,comparedtotheloopmeasuredalongthehardaxis.Theeasyaxisandthehardaxisarenormallyperpendiculartoeachother.Magnetizingananisotropicpermanentmagnetalongtheeasyaxisispreferred,becauseahigherremanent 29

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magnetizationandahighermaximumenergydensitycanbeobtained.Magnetizingasoftmagnetalongitshardaxisiscommon,sinceithasalowercoercivitythatimpliesalowerenergyconsumptioninanappliedACeldandtheslanthysteresisloopprovidesaconstantpermeabilityoveralargerrangeofmagnitudeoftheappliedeld. 2.1.2Micromagnetics Whiletheoverallmagneticpropertiesofamaterialcanbeoutlinedbythehysteresisloopandafewparameterssuchasmagnetizationandcoercivity,therelationshipbetweenamaterialstructureanditsmagneticpropertiesistypicallyexplainedandmodeledundertheframeofmicromagnetics,whichcanbeutilizedtopredictthepropertiesofnanocompositesaswell.Micromagneticsevaluatethefreeenergyassociatedwiththestatesofmagnetizationonthemicroscale,wheretheassumptionofcontinuousmediumisstillvalid.Likemagneticeld,thedistributionofmagnetizationcanbeviewedasavectoreldinamagnet.Themagnitudeofmagnetizationalwaysequalsthesaturationmagnetizationofthematerialatagiventemperatureanditsorientation(m=M=Ms)variestoreachastateofenergyminimum.Theevaluatedmagneticfreeenergyincludestheexchangeenergy,theanisotropyenergyandthemagnetostaticenergyoverthevolumeofmagnet,V[ 24 ]. E=ZV(A(rm)2+fAN(m;n))]TJ /F3 11.955 Tf 13.15 8.09 Td[(0 2MsHdm)]TJ /F3 11.955 Tf 11.96 0 Td[(0MsHamd3r(2{9) Thersttermexpressestheexchangeenergy.r(m)2isashorthandnotationforjrmxj2+jrmyj2+jrmzj2,whichdescribesthevariationofthemagnetizationorientation.TheparameterAiscalledtheexchangestinessconstantinanalogytothespringstiness.Itsummarizestheeectofshort-rangeexchangeinteractionandvariesfordierentelementsandlatticestructures,butA10)]TJ /F4 7.97 Tf 6.58 0 Td[(11J=mcanusuallybeagoodapproximation[ 24 ].Tokeeptheexchangeenergyminimum,theneighboringmagneticmomentstendtoalignwitheachother.Forthoseatomswhichshareelectronsandformhybridizedelectronorbitals,theantisymmetriccongurationonthespatialcomponent 30

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ofwavefunctionsofthecoupledelectronshasalowerenergythanthesymmetriccongurationinsomemagneticmaterials.Sincetwoelectronsmustnotoccupythesamestate,thespinfunctionshavetobesymmetricthatleadstotheelectronswithparallelspins[ 25 ].Itexplainsthealignmentoflocalmagneticmomentsandistheoriginofferromagnetism. Thesecondtermdescribestheanisotropyenergy.ThefunctionfANdepictstheenergydensityofthemagnetocrystalline,magnetostrictionandstressanisotropy.Sometimestheshapeanisotropyisalsoincluded.Dependingonthelatticestructure,theanisotropyismodeledindierentsymmetricpatterns,suchastheuniaxialanisotropyandthecubicanisotropy.Inacommonuniaxialanisotropymodel,theexpressionoftheenergydensityisexpandedtothesecondorder. fAN(m;n)=K0+K1sin2(2{10) Theangleliesbetweentheorientationmandtheanisotropyaxisn.Theanisotropyaxisnhasaspatialdistributioninthemagnet.WhentheanisotropyconstantK1>0,therearetwoenergyminimaat=0and=.Themagnetizationtendstoliealongtheanisotropyaxis,whichisnamedaseasyaxis.WhenK1<0,theenergyminimumliesinaplanewhere==2.Inthiscase,themagnetizationcanrotateinsidethiseasyplaneandstayattheenergyminimum.Whenthemagneticmomentliesintheeasyaxis,theeectofanisotropyenergycanbeviewedasamagneticeldappliedalongtheeasyaxis.Suchamagneticeldistheanisotropyeld. HAN=Hk=2K1 0Ms(2{11) ThelasttwotermsofEq. 2{9 expressthemagnetostaticenergy.HastandsfortheexternallyappliedmagneticeldandHdisthedemagnetizing(demag)eldgeneratedbythemagnetitself.Thedemageldresultsfromthemagneticmomentsinthemagnetand 31

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isthedivergentcomponentofthemagnetization. rHd=rM=r(Msm)(2{12) rHd=0(2{13) Thefactorof1/2isappliedintheequationbecauseRV)]TJ /F1 11.955 Tf 9.3 0 Td[(MsHdmd3rcalculatestheselfenergytwice.Thedemageldisreversetothemagnetizationorientationanddependentontheshapeofthemagnet.ThedemagfactorNdisusedtoapproximatethedemageld,whereHd=)]TJ /F3 11.955 Tf 9.3 0 Td[(NdM.Aspherehasademagfactorof1/3.Foranobjectwithahighaspectratioinshape,thedemagfactorissmallwhenmagnetizationliesinthelongaxis,butlargeinshortaxis.Intwoextremecases,thedemagfactorsofaneedlearecloseto0and1/2alongthelongaxisandtheshortaxisrespectively,andthevaluesforathinsheetarecloseto0and1inthein-planeandoutofplanedirection. Themagnetostaticenergyofthedemageldcanberewrittenintheformofanisotropyenergy,namedtheshapeanisotropy.Inthecaseofspheroid,thecorrespondinguniaxialanisotropyconstantis KM=0M2s 2(N?)]TJ /F3 11.955 Tf 11.95 0 Td[(Nk)=0M2s 4(1)]TJ /F1 11.955 Tf 11.95 0 Td[(3Nk)(2{14) whereNkandN?arethedemagfactorsalongandperpendiculartothesymmetryaxis.Theanisotropyaxisliesalongthesymmetryaxis. DomainWall .Magnetostaticenergyfavorsthealignmentofmagnetizationinsideamagnetwiththeappliedeld.Theanisotropykeepsthemagnetizationclosetotheeasyaxisortheeasyplane.Theexchangemaintainsthesmallspatialvariationofmagnetization.Inthecompetitionoftheexchangeenergy,theanisotropyenergyandthemagnetostaticenergy,themagnetizationndsbalanceandstaysinlocalenergyminima. Whenamagnetisnotsaturated,themagneticdomainsarecommonlyobserved.Amagneticdomainisaregioninthemicrometersizewherethemagneticmomentsrotate 32

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Figure2-5. 3Dillustrationofmagneticmomentssketchedasarrowsina180Blochdomainwallinthexyplane.Theprojectionofmagneticmomentsontothexandyaxesisshown. coherently.Theorientationtransitionofthemagneticmomentshappensonlyontheboundaryofthemagnetdomains,namelythemagneticdomainwall.Inthisconguration,theexchangeenergyandanisotropyenergycanremainminimuminsideadomain,andthehighenergydensityisassociatedwithdomainwalls.Domainwallsmoveinresponsetotheappliedeldandadjustthenetmagneticmomenttokeepthemagnetostaticenergyatalowlevel. Inthebalanceoftheexchangeenergyandtheanisotropyenergy,themagneticmomentsinadomainwallgraduallyrotatefromtheorientationinonedomaintotheother.The180Blochwallisagoodexampleofdomainwallwherethemagneticmomentsrotatewithinthewallplane,asdepictedinFig. 2-5 .Theenergydensitiesfromexchangeandanisotropyareequalinthedomainwall. Ad dz=fAN()(2{15) 33

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Figure2-6. Angleofmagneticmomentsina180Blochdomainwall. whereistheanglebetweentheorientationofmagneticmomentandtheanisotropyaxis.Sinceauniaxialanisotropyisassumedwiththeeasyaxislyingonthexaxisinthiscase,theexpressionsoftheorientationofmagneticmomentcanbeobtainedas[ 24 ] mx=sin=tanhz q A K1my=cos=0@coshz q A K11A)]TJ /F4 7.97 Tf 6.59 0 Td[(1(2{16) ThespatialfunctionofalongthezaxisisshowninFig. 2-6 .ThedomainwallproleischaracterizedbythedomainwallwidthWoutlinedbydashedline. W=r A K1(2{17)Domainwallwidthindicatesafeaturelengththatamagneticmomentcanaectbytherelayofexchangecoupling.ThecharacteristiclengthofexchangeisdenedaslW=p A=K1.ForsoftmagneticNiFe,W800nm,andW=4:5nmforpermanentmagneticCoPt. 34

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Figure2-7. Hypotheticmagnetizationconguration:(a)uniformmagnetization,(b)domainstructure,and(c)vortexstructure,after[ 24 ]. Theanisotropyenergyfavorsasmalldomainwallwidthbecauselessmagneticmomentsinadomainwalldeviatefromtheeasyaxisoreasyplane.Theexchangeenergyprefersawidedomainwallinwhichmagneticmomentsmildlyrotate.Thetotalenergyofdomainwallperunitsurfaceiscalculatedtobe[ 24 ] W=4p AK1(2{18) TheproportionofenergystoredinthewidthWistanh(=2)W=0:917W. 2.1.3MagnetizationofParticle Dependingontheparticlesize,dierentmagnetizationcongurationsarecandidatesofthelowestenergystate,suchasuniformmagnetization,domainstructureandcurling/vortex,asshowninFig. 2-7 .Forlargemagneticparticles,multiplemagneticdomainsareformedtoreducestrayeldandkeeplowmagnetostaticenergy.Asaparticleshrinkstothesizeofmagneticdomains,thesingledomainparticleisfavoredintermsofenergyratherthanformingdomainwalls.Thevortexstructurekeepssingledomainparticlesfreeofstrayeldatthecostofexchangeandanisotropyenergy.Whentheparticlesizefurtherdecreases,theuniformmagnetizedparticleisobserved,whichcreatesstrayeldbuthasnoexchangeandanisotropyenergy. Fortinymagneticparticlesinthesizeofafewnanometer,thethermaluctuationissucientlylargetorotatemagnetization.AccordingtoNeel,theaveragetimelengththattakestoipthemagnetizationstateofaparticleisgivenbyArrheniuslaw[ 26 ], =0eKeV kT(2{19) 35

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wherevisparticlevolumeandKedenotesaneectiveanisotropyconstantincludingmagnetocrystallineandshapeanisotropy,whichequalsKe=K1+1 20(N?)]TJ /F3 11.955 Tf 11.96 0 Td[(Nk)Ms2foranellipsoidalparticle.Thepre-exponentialfactor0istherelaxationtimeofmagneticresonanceintheanisotropyeld,whichisapproximatedby)]TJ /F4 7.97 Tf 6.58 0 Td[(10=(2))]TJ /F4 7.97 Tf 6.59 0 Td[(10He,whereisgyromagneticratioandHe=2Ke=0Ms. Forhardmagnets,thetransitionfromthermalunstabletothermalstabletakeplacesinanarrowrangeofparticlesize.Theparticlesizetoreachthesamethermalstabilityismuchlargerforthesoftmagnets.TwoexamplesarelistedintheTable 2-1 Table2-1. Criticaldimensionsforsuperparamagneticparticles MaterialJs(T)Ke(MJ=m3)Diameter(nm)for=0:1sDiameter(nm)for=109s(30year) CoPt1.014.94.35.3-Fe2O30.50-0.00538.550.7 Ifonemeasuresaparticlemadebyaferromagneticmaterialatatimescalemuchlargerthanits,themeasuredremanentmagnetizationandcoercivityiszero,becausethemagneticmomentswouldhavebeendriventorandomrotationbythermaluctuation.Thismagneticpropertyisclassiedassuperparamagnetism,andthetimelengthisnamedasblocktime.Themagnetizedstateofaparticleislikelytobe\blocked"fromchangeforthisperiod,andthenbecomesmorelikelytogetippedbythermaluctuationforaperiodbeyond. Howmagneticmomentsreactinareverseelddetermineshysteresispropertiesofamagneticparticle.Thedimension-dependentreversalprocessescanbecategorizedintocoherentrotation,curlinganddomainwalldisplacement,correspondingtothemagnetizationcongurationsofuniformmagnetization,vortexanddomainstructure. Applyingareversaleldtoauniformlymagnetizedparticle,atsomepointthedeviationstartstoappeartheuniformlymagnetizedstate.ThiseldisdenedasnucleationeldHn.Sincenucleationmusthappenbeforethereversalofmagneticmoment, 36

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Figure2-8. Nucleationeldasafunctionofparticlesize,after[ 26 ]. coercivityHc>Hn.Nucleationeldsetsatheoreticallowerboundforcoercivity.ThedependenceofnucleationeldonparticlesizeisshowninFig. 2-8 ForsmallparticleswiththesizesbelowacriticaldiameterDthassociatedwiththermalenergy,theparticlesaresuperparamagnetic. BeyondDth,coherentrotationoccursforsmallnanoparticleswheremagneticmomentsarealignedbyexchangecouplingandtheycoherentlyrotatewithrespecttoappliedeld.Singledomainparticleincoherentrotationmodehasthehighestnucleationeld. Incurlingmode,vortexstructureisgeneratedinparticleinaplanenormtoreversaleldsoastoavoidstrayeld.Thenucleationeldincurlingdecreasesquadraticallywiththeparticleradius.ThecriticaldiameterDcohofaparticlehasequalnucleationeldincoherentrotationandcurlingmode.ForasphereparticlewithA=10)]TJ /F4 7.97 Tf 6.59 0 Td[(11Jm)]TJ /F4 7.97 Tf 6.59 0 Td[(1,0Ms=1Tandassumedkc=1:3,Dcoh70nm. ThereexistsacriticaldiameterDsd,belowwhichaparticlewithsingledomainisenergeticallyfavored.ForaparticlelargerthanDsd,itbecomesworthytoformmagneticdomainwalls. Asparticleissucientlylargetoformmultipledomains,domainwalldisplacementbecomesthemechanismofmagnetizationreversal.Therequiredenergyformovingdomain 37

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wallsislowerthantheenergybarrierinmagnetizationreversal,thustheassociatednucleationelddrops. 2.1.4RandomAnisotropyModel Apieceofmaterialcanbetreatedasanassemblyofinteractedparticleswhenthemagneticpropertiesareevaluated.Inapolycrystallinematerial,eachcrystalliteorgraincanbeconsideredasaparticlewithcertainshapeandanisotropyaxis.ForamaterialwiththegrainsizeDsmallerthanthecharacteristiclengthofexchangecouplinglW,anumberofgrainsarecoupledbyexchange,N=(lW=D)3.Sincetheorientationofanisotropyaxisineachgrainisnormallyrandomlydistributed,thebarrierofanisotropyenergyissmoothedoutintheexchangecoupledvolume.Asaconsequence,theresultinganisotropyconstanttakesthemeanvalueoftheNgrains,[ 27 ] hKiK1 p N=K1D lW3=2=K1 Dr hKi A!3=2(2{20) NoticethattheanisotropyconstantinlWshouldalsobereplacedbyhKi,sothesmoothedanisotropyfurtherenlargesthevolumeofexchangecoupling.Finallytheanisotropyconstantequals hKiK41 A3D6(2{21) whichhasasixthorderdependenceongrainsize.Itleadstoadownfallofcoercivityandaboostofpermeabilityforthematerialswithsmallgrains. Hc/hKi 0Ms=K41D6 0MsA3/0Ms2 hKi=0Ms2A3 K41D6(2{22) 2.1.5FerromagneticResonance Softmagneticmaterialshavehighfrequencyapplicationsandtheirpropertieshighlydependonfrequency.Themagneticmomentsencounterresonanceandtheelectromagnetic 38

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eldarecoupledwiththeeddycurrentdescribedinMaxwell'sequation.Somesimplediscussionsaremadeforthesetwocases. Amagneticmomentexperiencesatorqueinamagneticeldinthedirectionperpendiculartoboththemomentandtheeld.Guidedbythetorque,themomentrevolvesaroundthedirectionoftheappliedeldinthephenomenonofLarmorprecession.TheangularfrequencyofLarmorprecessionisproportionaltothestrengthoftheappliedeldwithafactorofgyromagneticratio. !=B(2{23) Whilethemagneticmomentsprecede,adampingmechanismoccursbytransferringtheenergyofmomentmotiontothethermalenergyduetothecouplingofmomentmotiontospinwave,eddycurrent,latticevibration,andtheeectofcrystallinestructureanddomainstructure[ 28 ].Thelossofenergyresultsintherelaxationofmotionofmagneticmomenttowardstheequilibrium,whereinthemagneticmomentalignedwiththelocalelditexperiences.AphenomenologicalmodelofprecessionanddampingisdescribedintheLandau-Lifshitz-Gilbert(LLG)equation. @M @t=(1+)MH)]TJ /F3 11.955 Tf 15.68 8.09 Td[( MM@M @t(2{24) wherethediscretemagneticmomentsMaretreatedasacontinuousmagnetizationeldM(r;t),H(r;t)isthelocaleldexperiencedbythemagneticmoments,andisthedampingparameter.Theexternallyappliedeld,damagnetizingeld,andeectiveeldofexchange,anisotropyandmagnetoelasticityareinvolvedintheeectiveeldH.Ontherighthandsideofequation,thevariationofMisattributedtoHeldinthersttermwhichleadstoprecessionandthedampinginthesecondtermwhichresultsinrelaxation. Theferromagneticresonanceoccurswhenanoscillatingeldisappliedattheprecessionfrequency.GiventhatastaticexternaleldHaappliedinzdirectioneectivelysaturatesamagnet,andthedemagnetizingeldisconsidered,H=Ha+Hd,theresonant 39

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frequencyiscalculatedinthefollowingequation[ 29 ]. !0=0[(Ha+(Nx)]TJ /F11 11.955 Tf 11.95 0 Td[(Nz)M)(Ha+(Ny)]TJ /F11 11.955 Tf 11.96 0 Td[(Nz)M)]1 2(2{25) whereNx,NyandNzarethedemagnetizingfactorsintherespectivedirections. Forathinlm,!0=0(Ha)]TJ /F1 11.955 Tf 12.93 0 Td[(M)whenHaisperpendiculartotheplane,and!0=0p Ha(Ha+M),whenHaisinplane. Whentheuniaxialanisotropyisalongwiththedirectionoftheappliedeld,itisequivalenttoaddananisotropyeldontotheappliedeld.Foranin-planeHasucientlylargetosaturateamagnet,!0=0p (Ha+Hk)(Ha+Hk+Ms).Whenthein-planeanisotropiceldcanalignthemagneticmomentsandHkMs,whichcanbevalidforsoftmagnets,theequationofresonancefrequencyhasaconciseform,!0=0p HkMs. Becauseoftheferromagneticresonance,magneticmaterialscanbeanisotropicmediawithrespecttoelectromagneticwaves.Atensorpermeability[]withcomplexnumberelementisneededtodescribemediumbehavior. []=264j0)]TJ /F3 11.955 Tf 9.3 0 Td[(j0000375(2{26) wherethemagnetissaturatedinthezdirection.Aleft-handcircularlypolarizedwavehasadierentpropagationconstantinthemagnetcomparedtowhataright-handcircularlypolarizedwaveexperiences.Inthecaseofathinlmwithasmalldampingparameterandasmallin-planeoscillatingeldperpendiculartotheanisotropyaxis,theelementsofpermeabilityarederivedfromLLGequation[ 30 ]. 40

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0=1+0Ms(0Hk+0Ms+j!) (0Hk+0Ms+j!)(0Hk+j!))]TJ /F3 11.955 Tf 11.96 0 Td[(!2(1)]TJ /F3 11.955 Tf 11.96 0 Td[(2)2 (2{27)=1+0Ms 0Hk+j!1+!2(1)]TJ /F3 11.955 Tf 11.95 0 Td[(2)2 (0Hk+0Ms+j!)(0Hk+j!))]TJ /F3 11.955 Tf 11.96 0 Td[(!2(1)]TJ /F3 11.955 Tf 11.95 0 Td[(2)2 (2{28) 0=j0Ms!(1)]TJ /F3 11.955 Tf 11.96 0 Td[(2) (0Hk+0Ms+j!)(0Hk+j!))]TJ /F3 11.955 Tf 11.96 0 Td[(!2(1)]TJ /F3 11.955 Tf 11.96 0 Td[(2)2 (2{29) Snoek'sLimit .J.L.Snoekfoundasimpliedrelationshipregardingferromagneticresonanceinpolycrystallinespinelferritesin1947[ 31 32 ].Theproductoftheinitialsus-ceptibilityandtheresonantfrequency!0isproportionaltothesaturationmagnetizationMs. Foranaggregateofcubiccrystalswithrandomorientation,thestaticsusceptibilityis =2Ms 3Hk(2{30) Multiplyingbytheresonantfrequency!0,theresultantSnoek'sproductshowsalinearrelationshipwithsaturationmagnetization. !0=0Hk2Ms 3Hk=2 30Ms(2{31) Forthehighfrequencyperformanceofferrites,saturationmagnetizationbecomesalimitingfactor.Someexperimentaldataofnickelzincferriteandmanganeseferritehasconrmedthislinearrelationshipandgivenaproportionalityfactorof0.8[ 33 ],whichiscloseto2/3intheequation. 2.1.6EddyCurrent Whenatimevaryingmagneticeldisappliedtoamagnet,thevariationofmagneticeldcanbeenhancedbythevalueofpermeability.However,eddycurrentalsooccursinthemagnetduetothecouplingbetweenthetimevaryingmagneticeldandthe 41

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electriceld.theeddycurrentbecomessignicantinamagnetwithlowresistivityathighfrequency. AccordingtoLenz'slaw,theeddycurrentisgeneratedsothatthevariationofmagneticeldisprevented.Asaconsequence,thepenetrationofthemagneticeldintothemagnetislimited,exponentiallydecayingfromsurface.Similartoskineectwherethecurrentowisconstrainedinaconductor,thepenetrationofthemagneticeldinthemagnetischaracterizedbyaskindepthbasedonthegoverningMaxwell'sequations.Theskindepthforalossylmturnsouttobeinverselyrelatedtothepermeabilityandtheconductivityofthematerial. =1 p f(2{32) Foramagneticthinlmwhichisagoodconductor(!),thepropagationofmagneticeldcanbederivedfromtheMaxwellequations.TheexcitationeldisassumedtoapplyinthexdirectionandthustheboundaryconditionsatthinlmsurfaceissetasHx(z=d=2)=H0ej!t,wheredisthethinlmthickness.Solvingthefollowingequation, @2Hx @z2=j!H0ej!t(2{33) aproleofthemagneticeldalongthethicknessdirectionzisobtainedas[ 30 ] Hx=H0e(1+j)z=+e)]TJ /F4 7.97 Tf 6.59 0 Td[((1+j)z= e(1+j)d=2+e)]TJ /F4 7.97 Tf 6.59 0 Td[((1+j)d=2ej!t(2{34) Theaverageeectivepermeabilityofthelmisdeterminedbytheaveragemagneticeldpenetratedintothelm[ 30 ]. e=Rd=2)]TJ /F5 7.97 Tf 6.59 0 Td[(d=2iHxdz dH0ej!t=i2 (1+j)dtanh(1+j)d 2(2{35) whereiistheinitialpermeability.Whenlmthicknessissmallcomparedtotheskindepth,eectivepermeabilityequalstoinitialpermeability.Asfrequencyincreasesandtheskindepthbecomescomparableto,evensmallerthan,thelmthickness,theeectivepermeabilitysignicantlydegrades. 42

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Theeectsofferromagneticresonanceandeddycurrentmaybothappeartobesignicantatthefrequencyofinterest.Inthiscase,thepermeabilityelementintensordiagonalcanbeusedtoreplacethepermeabilityinEq. 2{32 andtheinitialpermeabilityinEq. 2{35 2.2HighFrequencySoftMagneticMaterials Targetinghighfrequencyapplicationsabovemegahertz,softmagneticmaterialsareexpectedtohaveadecentpermeability,ahighsaturationmagnetization,alowcoercivityandalargeresistivity.ThehighsaturationmagnetizationprovidesastrongpowerhandlingcapacityandextendstheoperationfrequencyrangeaccordingtoSnoek'slimit,asdescribedinSection 2.1.5 .Thelowcoercivitymaintainsalowhysteresislossandthehighresistivitysuppressestheeddycurrentloss. Commonmaterialsusedforsoftmagneticcorefallintotwocategories:alloyandferrite.Generally,magneticalloyhasahighersaturationmagnetization,beinghighlyconductive,comparedtoferrite.AselectedlistofcorematerialpropertiesissummarizedinTable 2-2 .Formagneticalloydeposition,sputteringandelectroplatingarewidelyinvestigated.Sputteringisamoreversatiletechniquewhichisavailableforcommonalloysandallowsadditionofnon-metallicelements.Thedepositionthicknessisusuallylimitedbyafewmicron.Electroplatinghasasimplersetupandprovidesamorerapiddepositionforthickerlmsoftensofmicrons. 2.2.1SoftMagneticAlloy Iron,cobaltandnickelarethethreeelementsthatshowsignicantmagneticbehaviors.Ironandcobalthavehighsaturationmagnetizations,2.15Tand1.81T,respectively.Ironandnickelhavelowcoercivities.CobalthasthehighestCurietemperature(1087C)inferromagnetsandthusagoodthermalstability.Bothcobaltandnickelcanformaprotectiveoxidesurfacesothattheyareconsideredcorrosion-resistant.Thesethreeelementsareusuallyusedintheformofalloyandtheirpropertiesaspuremetalshelppredicttheperformanceofalloy. 43

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Table2-2. Magneticcorematerials MaterialProcessRelativeperme-abilityrSaturationinductionJs(T)CoercivityHc(A/m)Resistivity(cm) PermalloyNi80Fe20[ 9 34 { 36 ]Electroplating1300)]TJ /F1 11.955 Tf 11.95 0 Td[(85001:0)]TJ /F1 11.955 Tf 11.95 0 Td[(1:147)]TJ /F1 11.955 Tf 11.96 0 Td[(15216)]TJ /F1 11.955 Tf 11.96 0 Td[(60PermalloyNi45Fe55[ 37 38 ]Electroplating2801:5)]TJ /F1 11.955 Tf 11.95 0 Td[(1:63245)]TJ /F1 11.955 Tf 11.96 0 Td[(48Ni6Fe70P24[ 36 ]Electroplating)]TJ /F1 11.955 Tf 59.92 0 Td[(1:2144170Fe65Co35[ 34 39 ]Electroplating)]TJ /F1 11.955 Tf 24.5 0 Td[(2:43)]TJ /F1 11.955 Tf 11.95 0 Td[(2:45320)]TJ /F1 11.955 Tf 11.95 0 Td[(13608Co65Fe12Ni23[ 40 ]Electroplating6002:19521Co915Zr4Ta45[ 18 ]Sputtering850)]TJ /F1 11.955 Tf 11.95 0 Td[(11001:521:2100CoZrO[ 41 42 ]Sputtering50)]TJ /F1 11.955 Tf 11.96 0 Td[(801:0)]TJ /F1 11.955 Tf 11.95 0 Td[(1:380)]TJ /F1 11.955 Tf 11.96 0 Td[(400600)]TJ /F1 11.955 Tf 11.95 0 Td[(1400MnZnferrite[ 43 ]Pressandsinter350)]TJ /F1 11.955 Tf 11.95 0 Td[(40000:32)]TJ /F1 11.955 Tf 11.95 0 Td[(0:545)]TJ /F1 11.955 Tf 34.59 0 Td[(108)]TJ /F1 11.955 Tf 11.96 0 Td[(1012NiZnferrite[ 43 44 ]Pressandsinter80)]TJ /F1 11.955 Tf 11.95 0 Td[(20000:26)]TJ /F1 11.955 Tf 11.95 0 Td[(0:38)]TJ /F1 11.955 Tf 30.36 0 Td[(1011)]TJ /F1 11.955 Tf 11.96 0 Td[(1013MnZnferrite/polyimide[ 44 ]Screenprinting)]TJ /F1 11.955 Tf 59.92 0 Td[(0:43)]TJ /F1 11.955 Tf 65.64 0 Td[(1010NiZnferrite/polyimide[ 44 45 ]Screenprinting60:2)]TJ /F1 11.955 Tf 11.95 0 Td[(0:29)]TJ /F1 11.955 Tf 65.64 0 Td[(1012 The80permalloy(Ni80Fe20)isapopularsoftmagneticmaterialwithnear-zeroanisotropyandmagnetostriction[ 34 ].Itsapplicationinthereadheadsofharddiskdriveshavepromoteditsprevalenceinmicrofabricateddevices.Permalloyhasahighpermeability,adecentsaturationmagnetization,alowcoercivityandalowresistivity.Itspropertiesareoftenusedasastandardforcomparison.However,afairlylargevariationinmagneticpropertieshasbeenobserved,dependingonthedetailsoffabricationprocesses.Heattreatmentcangreatlyenhancethepermeabilityofpermalloy,butitisnotpreferredintheCMOS-compatibleprocess.Increasingthecompositionofironcanraiseupthesaturationmagnetization,asin45permalloy(Ni45Fe55),butlowerdownthepermeability.Addingphosphoruscanincreaseresistivity,butthepermeabilityandthecoercivityseemtobecompromised. 44

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Fe65Co35hasthehighestsaturationmagnetizationof2.45T,butitscoercivityisrelativelylarge.Fe-Ni-Coternaryalloyprovidesmorechoicesinthetrade-oofmagneticproperties.Dr.OsakafromWasedaUniversityinvestigatedonelectroplatedalloyandfoundacompositionofCo65Ni12Fe23withahighsaturationmagnetizationandalowcoercivity[ 40 ].SputteredCo-Zr-Taalloyexhibitsaprettygoodcombinationofmagneticpropertiesandresistivity[ 18 ]. RFreactivemagnetronsputteringinoxygenandargonatmosphereallowstocontainoxygenelementinsidethedepositedlayer.SputteredCo-Zr-Olayershows10xhigherresistivitythanmetalalloy,withslightlyweakermagneticproperties[ 42 ]. 2.2.2SoftMagneticFerrites SoftmagneticferritesarewidelyacceptedbyindustryasmagneticcorematerialforinductorsandtransformersoperatingatkHzfrequencyrange,partiallyduetolowcost.Inmanufacture,pre-sinteredrawmeterialsaremilledintoparticles.Theparticleswithorganicbindersarepressedintodiesandsinteredtoformthedesireddimensionbeforenishing[ 43 ].Assemblyisneededtoplaceferritecoresintoinductorsandtransformers.Themagneticpropertiesofferriteislimitedbysaturationmagnetization.Thehighestmagnetizationofso-calledspinelferritesisfoundinmagnetite(Fe3O4)of0.6T.Ferritesaresemiconductormaterialssothattheirresistivityishigherthanmetalalloyatleastbyanorderof5.Abroadrangeofpermeabilitycanbeachieved,butpoorsaturationmagnetizationmakespermeabilityandoperationfrequencyapairofparameterstocompromiseunderSnoek'slimit.MnZnferritehashigherpermeabilityandsaturationmagnetizationandissuitableupto3MHz,whileNiZnferriteisusedatafrequencyhigherthan1MHzduetotheevenhigherresistivity[ 43 ]. Inmicrofabrication,ferritescanbeusedintheformofpowders.Themicron-sizedpowdersmilledfromsinteredmagnetsaremixedwithpolyimideorotherpolymerbinderstoformslurry.Itisdepositedbyscreenprintingandthencured.Theresultantmaterialisactuallyaferrite/polymercomposite.Ithasbeenshownthattheweightratioofferrite 45

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powdercanreach95%,thusthemagneticpropertyofcompositeisprimarilydeterminedbythepowderswith10%lossinthesaturationmagnetization[ 44 ]. 2.3MicrofabricatedPermanentMagnets Permanentmicromagnetscanbefabricatedbydirectdepositionorpowderassembly.Conventionaldirectdepositionmethodsincludesputtering,electroplatingandpulsedlaserdeposition(PLD).Thestrongrareearthpermanentmagnet,suchasNdFeBandSmCo,canbesputteredatelevatedtemperatures.Theirpropertiescanresemblethoseofbulkmaterials.TheelectroplatedpermanentmagnetsincludeCo-richCo-Pt,L10phaseCoPtandFePt,andCoNiPandCoNiMnP.Amongtheseelectroplatedmagnets,L10phaseCoPtandFePtarethestrongestandtheyrequirehightemperatureannealing.Electroplatinggenerallyoersfasterdepositionratesandthickerlmsuptotensofmicrometerscomparedtosputteringandelectroplating. Powders-basedmagnetsprovidealargerangeofchoices.Themagneticpowders,evennanoparticles,canbeseparatelyfabricatedandpackedintotrenchesormoldsonthesubstrates.Usuallyabindermaterialisinvolvedtoxatethepowdersinthetrenches.Duetotheseparateprocessingofthepowders,thesubstratesdonotneedtogothroughhightemperaturetreatment.Becauseofthelimitedllfactorofpowdersandthevolumeofbinder,themagnetizationofpowder-basedmagnetsislessthanhalfofbulkmaterials.Thepowdermagnethaveawidecoverageofthicknessfrommicrometerstomillimeters. ThemagneticpropertiesofsomemicrofabricatedpermanentmagnetsarelistedinTable 2-3 2.4ReviewofFabricationTechniquesofNanocomposite Magneticnanocompositeshavebeenfabricatedinvariousways.Twocommonstructuresofnanocompositesaremultilayerandinclusion-matrix. 2.4.1Multilayer Basedonthinlmdepositiontechnologies,nanometer-thicklmsofmultiplematerialsarealternatelydeposited.Theprocessiseasytocontrol,butthedepositionrate 46

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Table2-3. Microfabricatedpermanentmagnets[ 20 ] MaterialFabricationmethodThickness(m)Hci(kA/m)Br(T)(BH)max(kJ=m3) CoNiPElectroplating1{5255{1050.06{0.11.3{1.8L10CoPt[ 46 ]Electroplating,675C68000.8100Co-PtElectroplating23700.652SmCoSputtering,750C510350.8140NdFeBSputtering,750C512801.4400NdFeBPLD,650C12010000.5577SrFe12O19Powderw/polyimide10{203200.16{0.285{12SmCoDry-packedpowder15{500130{1400.3{0.518{23NdFeBPowderw/wax320720{7400.22{0.348{17 islimitedbecausethealternatedepositionneedtoshifttargetsinthevapordepositionorchangebathintheelectroplating. Asandwichstructure,Ni0:81Fe0:19=(Fe0:7Co0:3)0:95N0:5=Ni0:81Fe0:19,hasbeencreatedforhighsaturationandlowcoercivitysoftmagnetbyShanXiangWang,etal,in2000.5.6%nitrogenwasaddedtoargonowduringthesputteringofFe0:7Co0:3target[ 47 ].Amagneticeldof4kA/mwasappliedtoinducethein-planeanisotropy.Thecoercivityof(Fe0:7Co0:3)0:95N0:5waslowereddownto1.4kA/malongtheeasyaxisand0.4kA/malongthehardaxis,whilethehighsaturationmagnetizationof2.45Twaskeptuntouched.Theresistivityhasbeenquadrupledto55cm.Amazingly,whentwo5nmNi0:81Fe0:19layersweredepositedunderneathandabovethe100nmFe-Co-Nlm,thecoercivitywasfurtherreducedto621A/mand48A/mineasyandhardaxisrespectively.Thesandwichlmwasassoftaspermalloyinhardaxisandhada2.4Tsaturationmagnetization.Themeasuredrelativepermeabilitywas1000,andtheferromagneticresonanceoccurredat1.2GHz[ 48 ].ItwasproposedthattheFe-Co-Nlmsputteredontothepermalloyunderlayerwasexchangecoupledtothepermalloylmduringgrowth[ 49 ].ThehighlyorientedmagneticmomentsinpermalloyinducedthecrystallinestructuresinFe-Co-NtobebetteralignedandreducedtheripplesofmagneticmomentsinFe-Co-N.Thisexchangeinduced 47

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Figure2-9. SEMimagesofthenanolaminatedpermalloycorewithincreasingmagnication.ReprintedfromFigure5inF.Herrault,W.P.Galle,R.H.ShaferandM.G.Allen,"Electroplating-BasedApproachesforVolumetricNanomanufacturing,"2011TechnologiesforFutureMicro-NanoManufacturingWorkshop,Tech.Digestpp.71,withthepermissionoftheauthorsandtheTransducerResearchFoundation. ripplereductionmechanismwasclaimedtoberesponsibleforthesoftmagneticpropertiesofthesandwichlm. Nanolaminatedmagneticalloyshavebeenfabricatedbyelectroplating.Onehundredlayersofpermalloyandcopperwiththicknesseslessthan500nmwereelectroplatedinseparatebathsandthesubstratewasmovedbyaroboticarm[ 50 ].Thecopperlayersactingassacriciallayerswereselectivelyetchedresultingalaminatedpermalloystructure,asshowninFig. 2-9 .Thecoercivityandsaturationmagnetizationoflaminationwasreportedtobecomparabletothepropertiesofelectroplatedpermalloysinglelm.Similarly,70layersof500nm-thickCo-Fe-NilmwereelectroplatedandutilizedasaninductorcoreinaDC-DCconverter[ 51 ].BecauseairgapisolatedCo-Fe-Nilaminates,theeddycurrentlosswassuppressedinthecoreandreportedtobe20timeslessthanhysteresislossat1MHz. 48

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2.4.2Inclusion-Matrix Theinclusion-matrixstructureofnanocompositeisacommonthree-dimensionalstructure.Ithasbeenfabricatedbythecompactionorsinterofcore-shellnanoparticles,thevapordepositionandtheelectrochemicaldeposition.Theanisotropyofinclusionscanexhibitinthenanocompositeproperties.Whenisotropicinclusionsareinashapeclosetosphereandrandomlydistributed,oranisotropicinclusionsarerandomlydistributedandoriented,thenanocompositesexhibitisotropicproperties. 2.4.2.1Dielectric-coatednanoparticles Uponthenotionofreducingeddycurrent,itisstraightforwardtocoatsoftmagneticnanoparticleswithaninsulationlayer.Thecoatingisexpectedtocutothepathofeddycurrentandlimitstheeddycurrentlosswhentheskindepthislargerthantheparticlesizes. Silicondioxide,orsilica,iscommonlyusedforinsulatorcoating.Asol-gelprocessrestedonbase-catalyzedhydrolysisoftetraethylorthosilicate(TEOS)hasbeendevelopedtoformsilicashellwith2{100nmthickness[ 52 ].Ithasbeenappliedforcoatingavarietyofmagneticmaterials,suchas-Fe2O3[ 52 ],Fe[ 7 ]andNi-Fe[ 53 ].Amorphoussilicacoatinghasalsobeenappliedbythehydrolysisofsodiumsilicate[ 54 ].Otherthancore-shellnanomaterials,Co=SiO2nanocompositepowdershavebeenmadeinmixedphasesbyannealingthecolloidsolutionofCoandSiO2[ 55 ]. Thenanocompositelmshavebeenformedbysimplydryingsilica-coatednanoma-terialsinasolution[ 54 55 ],theapplicaitonofstaticpressure[ 7 8 ]andthecombustion-drivencompaction[ 53 ].Thecompactpressureaectsthedensityandpropertiesofcompositelm.Ni-Fe/SiO2compositescompactedby414MPato3.45GPapeakpressureexhibiteda6%increaseinthenanocompositedensityfrom5.81to6.15g=cm3,leadingto33%increaseinthepermeabilityfrom12to16at100MHz[ 53 ]. Theweightfractionofsilicaandmagneticphasecanbetunedbythethicknessofsilicashellandthesizeofmagneticnanoparticles.Ahighweightfractionofsilicacame 49

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Figure2-10. PermeabilityspectraofNi-Fe=SiO2nanocompositeswithdierentsilicaweightfractions.Highweightfractionofsilicainthenancompositesresultedinalowpermeabilityandahighcut-ofrequency.ReprintedfromJournalofAlloysandCompounds,vol.509,X.Lu,G.Liang,Q.Sun,andC.Yang,\High-frequencymagneticpropertiesofFeNi3-SiO2nanocompositesynthesizedbyafacilechemicalmethod,"pp.5082,Copyright2011,withpermissionfromElsevier. fromthicksilicacoating,leadingtoalowpermeabilityandahighcut-ofrequency.ForFe-Co/SiO2nanocomposite[ 54 ]with6%weightpercentageofSiO2,therelativepermeabilitywas9andthecut-ofrequencywas2MHz.For10%SiO2content,therelativepermeabilityremainedat6upto1GHz.Fe-Co=SiO2nanocompositewith10%SiO2hadahighspecicmagnetizationof200emu/g,8kA/mcoercivityand87%ofidealdensity.ThesamerelationshipbetweenthesilicaweightfractionandthepermeabilityspectrumwasalsoobservedinNi-Fe=SiO2,asshowninFig. 2-10 .Thepermealloynanoparticlesagglomeratedintosphericalnanocrystalsinthesizeof100-180nm,coatedbyasilicashellandcompactedintonanocomposites[ 8 ],asshowninFig. 2-11 .Thecompactcompositewith10%SiO2had90emu/gspecicmagnetizationand1.6kA/mcoercivity.Itsrelativepermeabilitywas13,startingtorolloatabout500MHz. Besidesnanoparticles,ironlaminateshavebeencoatedwithsilica[ 7 ].Thelaminatesproducedbymechanicaldeformationofpolymer-coatedFepowdershadsubmicronthicknessandalateraldimensionofafewtohundredsofmicrons.Aftersilicacoatingusingthesol-gelmethod,thelaminateswerecompactedtoformnanocomposites,as 50

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Figure2-11. SEMimagesofthepermalloynanocrystals(a)beforeand(b)aftersilicacoatingandtheFe=SiO2laminates(c)beforecompactionontopviewand(d)aftercompactionatcrosssection.Fig.aandbarereprintedfromJournalofAlloysandCompounds,vol.509,X.Lu,G.Liang,Q.Sun,andC.Yang,\High-frequencymagneticpropertiesofFeNi3-SiO2nanocompositesynthesizedbyafacilechemicalmethod,"pp.5081,copyright2011,withpermissionfromElsevier.Fig.canddarereprintedfromY.Zhao,X.Zhang,andJ.Q.Xiao,\SubmicrometerlaminatedFe=SiO2softmagneticcomposites{aneectiveroutetomaterialsforhigh-frequencyapplications,"AdvancedMaterials,vol.17,no.7,pp.916,2005,withpermissionfromJohnWileyandSons. showninFig. 2-11 .Therelativepermeabilityofthecompositewas40andstartedtorolloatabout100MHz.ApostannealingunderAratmosphereslightlyincreasedthepermeability,butlowereddowntheroll-ofrequency,whichmightbecausedbythefusionofFeintoporosityofSiO2ortheformationofconductiveironsilicide. 2.4.2.2Vapordeposition ClusterdepositionsystemhasbeenbuilttoformtheisotropicFePt=Fenanocomposite[ 56 ].TheFeclusterswereformedwhensputteredFeatomicgascollidedwithArions.ThenanocompositewasmadebyalternatedepositionofalayerofscatteredFeclusters 51

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Figure2-12. Schematicsof(a)thevapordepositionsystemand(b)theeasyaxisdeningprocess,and(c)TEMimageoftheFePt=Fe80Ni20nanocomposite.ReprintedfromX.Liu,S.He,J.-M.Qiu,andJ.-P.Wang,\Nanocompositeexchange-springmagnetsynthesizedbygasphasemethod:Fromisotropictoanisotropic,"AppliedPhysicsLetter,vol.98,no.222507,2011,withpermissionofAIPPublishing. andalayerofsputteredFePt.Themaximumenergydensitywas141kJ=m3forthenanocompositewith8%volumefractionofFecluster,whichishigherthanthemeasurevalue141kJ=m3forthesinglephaseL10FePt,afterannealed600Cfor10min, PermanentmagnetshavethenbeenaddedintovapordepositionsystemfortheanisotropicFePt=Ni80Fe20nanocomposites[ 57 ].AsshowninFig. 2-12 ,the5.8nmFePtnanoparticlesgeneratedfromasputtering-basedsourceandthesputteredNi-Featomsweredepositedonasubstratesimultaneously.Thepermanentmagnetsetsup5kOemagneticeldonthesubstratetodenetheeasyaxisforbothFePtparticlesandNi-Fematrix.ThevolumefractionofFePtparticlesrangedfrom5%to33%andthecompositelmthicknesswas20{30nm.Thecompositewith10%FePtexhibitedthehighestmaximumenergydensityof108kJ=m3,whichwas124%higherthantheisotropiccomposite. 2.4.2.3Electrochemicaldeposition Electrochemicalcodepositionisatechniquetoelectroplateinanelectrolyticbathcontainingnanomaterials.Thistechniquewasoriginallydevelopedforwear-resistantandcorrosion-resistantcoatings[ 58 ].AnexampleperformedbyM.R.Vaezi,etal,wasadding50nmSiCparticlesintomodiedWattsNielectroplatingbathtoimprovecoating 52

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hardness[ 59 ].TheweightfractionofSiCparticlesinacompositecoatingwasafunctionoftheparticleconcentrationinthebath,thebathstirringrate,thecurrentdensityandthebathtemperature.Apeakweightfractionof8%wasfoundusing5g/LSiCinthebathand20mA=cm2currentdensity. Electroplatingandelectrolessdepositionofthenanocompositeswithmicrometer-sizedpermanentmagneticparticleshavebeendemonstratedbyShanGuan,etal.ElectrolesscodepositionofNi-PwithNd-Fe-Bparticlesformedacompositeofpermanentmagneticparticlesembeddedinasoftmagneticmatrix[ 60 ].A25g/Lparticleconcentrationinamechanicallystirredbathresultedina9%weightfractionofparticlesincomposite.Patterningphotoresistmoldhasbeenusedtoformmicrostructurearrays.Thethicknessofdepositreached20m.Thecoercivitywas64{191kA=mandtheremanentmagnetizationwas0.24{0.35T.Themaximumenergydensitywas2.2{3.1kJ=m3.Anothertestwasusing1:1mBaFe12O19partlesintheCo-Ni-Ppulsereverseelectroplating[ 61 ].Thearrayofmicrostructureswasdepositedupto50mfortheMEMSmagneticactuators.Theweightfractionoftheembeddedparticleswas11%,whenthe35g/Lparticleconcentration,thecationicsurfactantandthe1.5mA=cm2currentdensitywereused.Magneticstirringwasreportedtobecriticalthatithelpedtodisperseparticlesinbathandtransportthemtothesurfaceofsubstrate.Thecoercityofcompositearrayswas127{175kA/m,theremanentmagnetization0.19{0.25T,andthemaximumenergydensity6.5{8.6kJ=m3. In2012,YoshiakiHayashi,etal[ 62 ],developedaprocessalmostidenticaltotheelectro-inltration.CobaltferritenanoparticlesweredepositedonaCusubstratebytheelectrophoreticdeposition,andthencobaltwaselectroplatedintotheparticlelmtoformCoFe2O4=Conanocomposite.Thevolumeratioofferriteparticleswasexpectedtobe60%inthecompositelm,whichhad690emu=cm3specicmagnetizationand15kA/mcoercivity.Althoughthedesiredexchangecouplingbetweencobaltferriteandcobaltmatrixwasnotdemonstrated,thestructureofnanocompositewasobtained.TheprocesswasthenappliedtotheFe-Co/FePtnanocomposite,whereinthevolumefraction 53

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Figure2-13. CoFe2O4=CoandFe-Co/FePtnanocompositesmadebyelectrophoreticdepositionandelectroplating.SEMimagesshowedCoFe2O4=Colm(a)ontopsurfaceand(b)atcross-section.Cobaltmatrixgrewandcoveredpartoftheparticlesurface.(c)TEMimageofaFe-Co/FePtnanocompositeshowingtheparticlesandthematrixincross-sectionview.Fig.aandbarereprintedfromY.Hayashi,S.Hashi,andK.Ishiyama,\MagneticpropertiesofnanostructuredlmcomposedofCo-ferritenanoparticlesandmetalCopreparedbycombinationofelectrophoreticdepositionandelectroplating,"IEEETransactiononMagnetics,vol.48,no.11,2012.c2012IEEE.Fig.cisreprintedfromY.Hayashi,S.Hashi,H.Kura,T.Yanai,T.Ogawa,K.Ishiyama,M.Nakano,andH.Fukunaga,\Electrochemicalfabricationofnanocompositelmscontainingmagneticmetalnanoparticles,"JapaneseJournalofAppliedPhysics,vol.54,no.075201,2015.Copyright2015theJapanSocietyofAppliedPhysics. ofFe-Coparticlewasupto30%[ 63 ].AlongwiththeincreasingvolumefractionofFe-Conanoparticle,thesaturationmagnetizationofnanocompositeincreasedfrom0.79Tto1.04T,butthecoercivityreducesfrom1,035kA/mto104kA/m,leadingtoalowermaximumenergydensitythansingleFePtlm.TheunevendistributionofFe-Conanoparticleswasreportedtolimitthemagneticproperties. 2.4.3Summary Magneticnanocompositehavebeenfabricatedbythecompactioncore-shellnanomaterials,thevapordeposition,theelectrochemicalcodepositionandelectro-inltration-typeprocess.Forthemultilayerstructure,commonthinlmdepositiontechniquescanbeusedwithslightadaption,butthetransferoftargetsorbathsduringalternatedepositionistime-consuming.Althoughmultiplemetalscanbeelectroplatedinasinglebathusingdierentpotentials,thedepositionrateofmetalusinglowpotentialcan 54

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beslow.Thelaminationofmagneticalloywithairordielectriclayershasbeenaneectiveandcommonlyusedmethodtoreducetheeddycurrentandextendthebandwidthofpermeability. Theassemblyofcore-shellnanomaterialshaswideapplications.Thewelldevelopedtechniquesofmakingnanomaterialscanbeusedandpatternedfordevices.Buttheassemblyhasalowdensitybeforecompaction.Thedependenceoncompactiontoraisecompositedensitymaynotbecompatiblewiththepressure-sensitiveprocesses.Thenanocompositesmadefromsilica-coatednanomaterialscreatesastructureofmagneticinclusionsinasilicamatrix.Thisstructurehasdemonstratedtoextendthebandwidthofpermeability. Vapordepositionhasshowntheversatilityofmakingdensenanocompositethinlmsandthegoodcontrolofnanocompositestructures.Thevacuumenvironmenthelpsmaintaincleaninterface,whichcanbecriticalfortheexchangecouplingbetweenadjacentphases.Butthevacuumenvironmentalsoincreasesthecomplexityofdepositionsystemandtheclusterdepositionsystemrequiresextramaintenance.Thedepositionrateisnotsuitableforthelmsthickerthanafewmicrons. Electrochemicaldepositiontechniqueshavethelimitationofchoosingmetallicmatrix.Otherwisetheyhavetheadvantagesofsimple,low-costequipmentandrapiddepositionfortens-of-micron-thicklms.Intheelectrochemicalcodeposition,thevolumefractionofinclusionsinnanocompositesisaround3%11%,anditis5%29%fortheelectrolesscodeposition[ 64 ].Intheelectro-inltration-typeprocess,thevolumefractionofinclusiosnhasbeenraiseduptomorethan30%.Butassemblingnanomaterialsandmaintainingasucientadhesiontothesubstratecanbechallenging.Includingferriteordielectricnanomaterialsintoametalmatrixisgoingtochangetheelectricalandmagneticpropertiesathighfrequencies.Whetheritwouldextendthebandwidthofpermeabilityremainsunknownanddrivestheinvestigationofelectro-inltratednanocomposites. 55

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CHAPTER3ELECTRO-INFILTRATIONPROCESS Thischapterintroducestheexperimentalstepsoftheelectro-inltrationprocess.AlthoughNi0:5Zn0:5Fe2O4/Fe-Conanocompositehasbeentakenasthemainexampleforintroduction,theelectro-inltrationisutilizedtofabricateavarietyofmagneticnanocomposites,suchas-Fe2O3/Fe-Co,Al2O3/Fe-Co,Ni0:5Zn0:5Fe2O4/Ni-Fe,FePt/Fe-CoandFe-Co/CoPt.Anadditionalstepofannealingissometimesrequiredforproducinghardmagneticmaterials,CoPtforinstance.Table 3-1 liststhefabricationprocessesofthenanocomposites. HavingbeenshowninFig. 1-2 ,thegeneralelectro-inltrationprocesscomprisesthreemajorsteps: 1. Consolidatenanoparticlesintoaporousmicrostructure; 2. Electroplateametalmatrixtoformatwo-phasenanocomposite; 3. Planarizeandcleanthesubstrate. 3.1SubstratePreparation Themicrostructureofananocompositelmisdenedbyaphotoresistmoldonasiliconwafer.Thefabricationprocessstartsfroma(100)siliconwafer,whichiscleanedin70CPRS3000(J.T.Baker)solventfor3minandthensputteredwitha10-nm-thickadhesionlayerofTianda100-nm-thickseedlayerofCu.ThewafercanbeheatedinN2at100Ctodriveoutmoistureandprimedwithhexamethyldisilazane(HMDS,(CH3)3Si-NH-Si(CH3)3)toimproveitsadhesiontothephotoresistthatwillreducethefutureoccurrenceofelectroplatingunderneathphotoresist.Thenaphotoresistmoldwithatypicalthicknessof10mispatternedusingstandardphotolithographictechniques.The ReprintedwithpermissionfromJournalofMicromechanicsandMicroengineering,vol.24,no.107001,2014[ 65 ],IOPPublishing;AIPAdvances,vol.6,no.056111,2016[ 66 ],AIPPublishing;AIPAdvances,vol.6,no.056105,2016[ 67 ],AIPPublishing;andAIPAdvances,vol.7,no.056225,2017[ 68 ],AIPPublishing. 56

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Table3-1. Summaryoffabricationprocessesfornanocomposites Nanocomposite-Fe2O3/Fe-CoNi0:5Zn0:5Fe2O4/Fe-CoAl2O3/Fe-Co SubstratepreparationTi/Cuseedlayer,AZ9260photoresistTi/Cuseedlayer,AZ9260photoresistTi/Cuseedlayer,KMPR2010photoresistNanoparticlefabrication-Fe2O3,purchased,burninairNi0:5Zn0:5Fe2O4,aqueousprecipitationAl2O3,purchasedNanoparticleconsolidationEvaporativeassemblyEvaporativeassemblyElectrophoreticdepositionElectroplatingFe67Co33Fe67Co33Fe67Co33CleaningPolishing,photoresistremovalPolishing,photoresistremovalPolishing,photoresistremovalAnnealing--NanocompositeNi0:5Zn0:5Fe2O4/Ni-FeFePt/Fe-CoFe-Co/CoPt SubstratepreparationTi/Cuseedlayer,AZ9260photoresistTi/Cuseedlayer,AZ9260photoresistTiN/Ti/Ptseedlayer,AZ9260photoresistNanoparticlefabricationNi0:5Zn0:5Fe2O4,aqueousprecipitationFePt,thermaldecompositionCoFe2O4,aqueousprecipitationNanoparticleconsolidationEvaporativeassemblyEvaporativeassemblyEvaporativeassemblyElectroplatingNi80Fe20Fe67Co33CoPtCleaningPolishing,photoresistremovalPolishing,photoresistremovalPolishing,photoresistremovalAnnealing-20minat700Cin4%H210minat675Cin4%H2 moldgoesthrougha90sO2plasmatreatmenttoremovethepossiblephotoresistresidueonCuseedlayerandtoturnthehydrophobicphotoresistsurfaceintoahydrophilicone,whichfacilitatesthefollowingparticleconsolidationandelectroplatingprocesses.A1:10dilutedHCldipisneededtoremovethemonolayeroftriethylsilane()]TJ /F1 11.955 Tf 9.3 0 Td[(Si(CH3)3)andtheoxidizedCusurfaceonseedlayer. Forthechoiceofphotoresist,thepositivetoneAZ9260(Shepley)isgoodforelectroplating,whichcanbeeasilyremovedbyacetone.Ifthechemicalresistancetoorganicsolventsisneeded,suchasforelectrophoresisinethanol,thenegativetoneKMPR 57

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2010(MicroChem)isagoodcandidate,whichdissolvesinN-methyl-2-pyrrolidone(NMP).Whenannealingisinvolved,a25nmTiNinsulationlayerisusuallysputteredonthebaresiliconsubstratetopreventatomdiusionathightemperature.SomeelectroplatingrecipesmayrequirespecialseedlayerotherthanCu.Forexample,CoPtmayneed30nmPtastheseedlayer. Iftheadhesionbetweenseedlayerandphotoresistiscritical,suchaselectroplatingathighcurrentdensities,a10nmTiadhesionlayerissuggestedtosputterontheseedlayer.TheTineedstoberemovedbydippingin1:10dilutedHFtoexposetheCusurface.Anelectroplatingtesthasbeencarriedouttoverifytheadhesionbetweenthephotoresistandseedlayer.Allthesubstratesaresputteredwitha20nmTianda100nmCuon(100)Siwafers.Butsomesubstrateshavetheadditional20nmTioverCuandsomeareHMDSprimed.AZ9260photoresistwitha12mthicknessisspincoatedandpatternedbystandardphotolithographyprocedure.Theelectroplatingistestedata20mA/cm2currentdensityformorethan12mininanacidFe-Cobath[ 69 ]withapHof2.0.Allthesubstratespasstheelectroplatingtestinthenextdayoftheproductionofthesubstrates.After39days,theelectroplatedmetalisobservedunderneathphotoresistonthesubstratewithoutTicoatingandHMDSprime.Table 3-2 summarizesthetestresults.InordertoavoidelectroplatingunderneathphotoresistandTiremoval,onlyHMDSprimeisappliedtothesubstrates. Table3-2. TestofphotoresistadhesiononDay39 ProcedureAddTiadhesionlayerNoTiadhesionlayer ApplyHMDSprimeGooddenitionGooddenitionNoHMDSprimeGooddenitionElectroplatingunderphotoresist 3.2FabricationofNanoparticles Themagneticnanoparticlesaresynthesizedinlaborpurchasedfromvendors.Nomatterhowtheparticlesareprepared,theyshouldhavecleansurfacesandcanformaliquiddispersion. 58

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Thetestedmagneticnanoparticlesincludemaghemite(-Fe2O3),nickelzincferrite(Ni0:5Zn0:5Fe2O4,NiZnferrite),cobaltferrite(CoFe2O4),andironplatinumalloy(FePt).Thenon-magneticnanoparticlesusesaluminumoxide(Al2O3).Thepurchasedmaghemitenanoparticles(<50nm,Sigma-Aldrich#544884)areproducedbyburningironmetalinairandthealuminumoxide(<50nm,Aldrich,#544833)nanoparticlesarepurchasedaswell.TheNiZnferritearepreparedbycolleagueStefanKellyusingaqueousco-precipitationmethod[ 70 71 ].Cobaltferritenanoparticlesarepreparedinasimilarwayusingaqueousco-precipitationmethod.TheironplatinumnanoparticlesaresynthesizedbycolleagueQiBinusingthermaldecomposition. Aqueousco-precipitation .Intheco-precipitationprocesstosynthesize8nmNiZnferriteparticles,1mol/LZn(NO3)2,Ni(NO3)2andFe(NO3)3watersolutionsarerstmixedin1:1:4volumeratiotoformprecursors.35mLprecursorsolutionisdilutedinto150mLwithDIwaterandtransferredintoaask.Thesolutionismildlystirredjusttoallowripplestooccuronsolutionsurface.35mL3mol/LNaOHispouredintoaskandbrownprecipitatesareformed.Thesolutionisthenheatedupto95Covernighttocrystallizetheprecipitatedparticles.Oncetransferredintoabeaker,theparticlescanbecollectedbyamagnetunderthebeakersothatthesupernatantcanbepouredout.AfterwishingparticleswithDIwaterfor3times,30mL2mol/LHNO3addedandstirredfor5min.Thesolutionistransferredtoaask,withadditionof1mL2mol/LFe(NO3)3and29mLDIwater,thenheatedto95Candstirredforahourtopassivateparticles.Afterwashedfor3{5times,theparticlesgothroughdialysisfor3timeswithwaterchangedevery12h.Afterdialysis,thepHcanbeadjustedtoformstablesolutionforstorage. Thesynthesizednanoparticlehasadistributioninsize,giventhe8nmnominalsize.Thehydrodynamicdiameterwasmeasuredusingdynamiclightscattering(DLS)technique.Thesize-dependentBrownianmotionofparticlesiscapturedandthecalculatedhydrodynamicdiameterincludestheactualparticlesizeandthethicknessofathinlayerofchargesthatadheretotheparticlesurfaceinthesolution.Fig. 3-1 showsatypical 59

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Figure3-1. TypicalsizedistributionofthesynthesizedNiZnferritenanoparticles.ReprintedfromS.J.Kelly,X.Wen,D.P.Arnold,andJ.S.Andrew,\Electrophoreticdepositionofnickelzincferritenanoparticlesintomicrostructuredpatterns,"AIPAdvances,vol.6,no.056105,2016,withthepermissionofAIPPublishing. hydrodynamicdiameterdistributionofthesynthesizedNiZnferritenanoparticles,whichhasameandiameterof16.6nmandastandarddeviationof3.6nm. SimilarlyinthesynthesisprocessofCoFe2O4nanoparticles,100mL0.1MFeCl3and100mL0.05MCo(NO3)2precursorsolutionsaremixedandstirred.Then100mL1.8MNaOHsolutionisadded,andtheentiresolutionisheatedat95Cfor12h.Aftersynthesis,theparticlesarecollectedbyanexternalmagnetandwashedwith500mLdeionizedwater3times,stirredwith60mL2MHNO3for5min,andwashedanother4times,followedby3stepsof24hdialysisin4Ldeionizedwaterinordertoremovetheexcesssaltinsolution.ThesynthesizedCoFe2O4nanoparticlesexhibitahydrodynamicdiameterof10:71:4nm,asmeasuredbyDLS. 60

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Theparticlesshouldbestoredinastablesolution.Unstabledispersion,liketheoneinaneutralsolution,maygraduallysettledownandstarttoaggregateandagglomerateovertime.Oncetheaggregationsform,theycannotbeeasilyseparatedbysonication. 3.3ParticleConsolidation Variousmethodscanbeusedforparticleconsolidation,suchasevaporativedeposition,electrophoreticdepositionandmagnetophoreticdeposition.Evaporativedepositionandelectrophoreticdepositionhavebeenstudiedfortheelectro-inltrationprocess.Theformermethodusesasimplesetupbuttakesrelativelylongtime,whilethelatteronedepositsparticlelmintensofminutesbutrequiresabathpreparationandelectronicsetup. Acriticalpointinparticleconsolidationisthatthedepositedparticlesneedtosticktosubstratesduringthesubsequentelectroplatingsteps.ThevanderWaalsforcesarethoughttokeepthenanoparticlesassembledandstucktotheseedlayer.Ithasbeenobservedthatwhenthedepositednanoparticlesaresubmergedintowater,theyarepronetostaywiththesubstrate,comparedtotheparticlesinmicrometersizes.Thesurfacetovolumeratioofparticlesseemstoplayaroleintheparticleadhesion.LookingintothesizedependenceofvanderWaalsforces,theLondondispersionforcebetweentwospheres,whicharisesfromtheinstantaneousinducedelectricdipoles,isexplainedbelow. AdaptedfromH.C.Hamaker[ 72 73 ],theforceFisproportionaltotheparticlediameterDandtheinverseofsquareoftheseparationd,whentheparticlesarecloseasdD. F=HD 12d2(3{1) whereHisHamakerconstantconcerningmaterialpropertiesoftheparticleandmedium,whichcanbeeitherairorbathsolution.Takingtheforcepervolume, F V=2H 9d2D2(3{2) 61

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InthecaseofLondondispersionforcebetweentheseedlayerandasphereparticle, F V=H 9d2D2:(3{3) Sincegoodadhesionoftheparticlesandseedlayeriswanted,smallparticlesizeisfavorableforsucientattractivevanderWaalsforce.Whenasubstratewithdepositedparticlesissubmergedintowater,themediumchangesfromairtowaterandtheHamakerconstantmaydecreasethatresultsinalowerattractiveforce.Whentheattractiveforcebecomessmallerthanthegravityandthedisturbancefromtheuid,theparticlesdetachfromthesubstrate. Amongthemagneticparticles,themagnetostaticforcealsohelpsassembly.Itssizedependencearediscussedintwocongurationswheretheorientationofmagneticmomentsbringaboutattractiveforce.AssumetwosphereswithadiameterofDandauniformmagnetizationMareseparatebyadistancedonanaxisa.Inthecasethattheorientationsofmagnetizationofthespheresareantiparallelperpendiculartotheaxisa,themagnetostaticforcepervolume,adaptedfrom[ 74 ],is F V=30M2 4(d+D)4(3{4) Inanothercasethatbothspheresmagnetizedalongtheaxisa,themagnetostaticforcepervolumeis F V=30M2 2(d+D)4(3{5) Asthemagnetostaticforceisinverselyproportionaltothefourthpowerofparticlediameter,smallparticlesizeiscriticaltogeneratesucientattractiveforce. 3.3.1EvaporativeDeposition Theideaofevaporativedepositionisthatwhenthesolventofaparticledispersionevaporatesandtheparticlesprecipitateout.BecauseofthevanderWaalsforce,theparticlestendtoformadensestructuretominimizeenergy.Whentheparticlesaremonodisperse,theycanevenformalatticestructure.ThevanderWaalsforcecanbe 62

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Figure3-2. Schematicoftheevaporativedepositionprocess. strongenoughtoholdatens-of-microns-thickparticlelmsontheseedlayerduringthesubsequentelectroplatingstep.Formagneticnanoparticles,thedipoleinteractionalsoactsasanattactiveforce. Intheprocedureofevaporativedeposition,theparticlesarerstdispersedinDIwaterusingvortexmixerandsonicationbath.ItneedstoverifythattheparticlesolutionhasapHclosetoneutral.Acidicandalkalinesolutionmaycorrodetheseedlayerduringevaporation.Theparticlesuspensionisdispensedontosubstrates,whichhavebeenplaceinastyrofoamiceboxwithcoldpacks,asshowninFig. 3-2 .Thesolutiondropsslowlyevaporateovernightleavingathinlmofparticles.Theconcentrationofparticlesolutionandthedispensedvolumearetunedtocontrolthelmthickness.Thecoldpackslowerdowntheambienttemperatureandtheiceboxmaintainarelativelyhighhumiditysothattheevaporationratecanbeadjusted.Alowevaporationratewasobservedtomitigatecracksintheparticlelmsandimprovethelmuniformity.Fig. 3-3 showsthelmsurfacesof<50nmmaghemitenanocomparticleand8nmNiZnferritenanoparticles. Whentheparticlesolutionisslowlydispensedontosubstrates,thewaterdropgrowsbiggerandthenexpandsitscoverage.Thethicknessoftheexpandedsolutionlayerisroughlyaconstant,whichisdeterminedbythecontactangleonthephotoresistandthesurfacetensionofsolution.ForthedilutedwatersolutionofNiZnferritenanoparticles, 63

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Figure3-3. OpticalandSEMpicturesof(a,b)thethinlmsofmagnetitenanoparticlesand(c,d)NiZnferritenanoparticles.Fig.aisreprintedfromX.Wen,J.D.Starr,J.S.Andrew,andD.P.Arnold,\Electro-inltration:Amethodtoformnanocompositesoftmagneticcoresforintegratedmagneticdevices,"JournalofMicromechanicsandMicroengineering,vol.24,no.107001,2014,withpermissionfromIOPPublishing. thisthicknessofwaterlayeris4mm.Thenthethicknessofadriedparticlelmisproportionaltoandcontrolledbytheparticleconcentrationinthesolution.Fig. 3-4 showsthemorphologyoflmsurfacespreparedfromthesolutionswithdierentconcentrations.Alinearrelationshipoftheparticleconcentrationandthelmthicknesswasobserved.Whenthelmthicknessgoesoverhalfamicron,smallcracksstarttoappearandbecomebiggerforthickerlms.Themaximumthicknessofacrack-freelmofhalfamicronisobtainedusingthe0.6mg/mLsolution.Whenthecracksareacceptable,theparticlelmthicknesscanreachtensofmicrons. ThevolumefractionofNiZnferriteparticlesinthelmsis33%10%,whichwascalculatedfromthesaturationmagnetizationsoftheparticlelmsandthebulkmaterials.Asareference,thevolumefractionofcloselypackedidenticalspheresis74%.Thelow 64

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Figure3-4. OpticalimagesoftheNiZnferritenanoparticlelms.Theparticleconcentrationandthemeasuredlmthicknessaremarkedonthecornersoftheimages. volumefractioncomesfromthedeviationinparticlesizesandtheresultantrandompackingstructures. 3.3.2ElectrophoreticDeposition Non-magneticaluminananoparticlelmsproducedbyevaporativedepositiondonotstickwelltothesubstrateinwater.Thisstimulatestheinvestigationofelectrophoreticdeposition.Thebasicsetupincludesabathcomprisingsuspendednanoparticles,aDCpowersupply,ananodeandacathode,asshowninFig. 3-5 .Thenanoparticlesaredispersedinthebath,ideallywithamarginalstability.pHofthebathistunedtoassignelectricchargestoparticlesurfacesthatmobilizestheparticlesinanelectriceld.Whenthenanoparticlesarepositivelycharged,theyaremovedtowardsthecathode.Thusthesubstrateisplacedascathodeandtakeanotherelectrodeasanode.Whenthepolarityofparticlechargesischanged,thecathodeandanodeneedtoswap.Amagneticstirbarcanbeusedtoagitatethebathandkeeptheparticlessuspended. Exorbitantaluminananoparticlesaremixedwithethanolusingavortexmixerandasonicationbath.Lettingthesolutionstandstill,thesupernatantisdecantedtouseasthebathsolutionforelectrophoreticdeposition.ThepHisadjustedbyadding0:5vol%of1:30dilutedHClwatersolution.ThesubstratewithexposedCuseedlayerisdippedintodilutedHClacidtoremovetheoxideonsurfaceandthencleanedwithwater.ThesubstrateisconnectedtoaDCpowersupplyascathodeandagraphiterodhasbeenusedasanode.Thepowersupplyprovidesamaximumvoltageof56V,andthecathodeandanodeareseparatedby4mmtoachieveanelectriceldof140V/cmduringthe 65

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Figure3-5. Setupforelectrophoreticdepositionusingthesubstrateascathode Figure3-6. OpticalandSEMimagesoverlookingaluminananoparticlelmpreparedbyelectrophoreticdeposition. deposition.Afterdeposition,thesubstrateistakenoutofthebathandletdrywhilepowerison.A10mindepositioncanachieve10mthickparticlelm.Fig. 3-6 showstheopticalandSEMimagesofthesurfaceofaluminananoparticlelmproducedbyelectrophoreticdeposition. TheelectrophoreticdepositionfornickelzincferritenanoparticleshasbeendevelopedbycolleagueStefanJ.Kelly.Thesynthesizeparticlesarenegativelychargedinanaqueoussolutionwitha11mg/mLconcentrationbyaddingtetraethylammoniumhydroxide(TEAH)topHof11.5.Theaqueoussolutionisdilutedbyisopropanolto5.0vol%toformasolvent-basedbath.Thesubstrateisset-upascathode,whileagraphiteblock 66

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Figure3-7. Opticalimagesofpatternedlmsofnickel-zincferritenanoparticlesdepositedin(A)blanketand(B&C)discretephotoresistmoldswithcircles(A&B)andlines(C).(D)SEMimagesofnickel-zincferritelmdepositedin7mdiametercirclephotoresistmold.ReprintedfromS.J.Kelly,X.Wen,D.P.Arnold,andJ.S.Andrew,\Electrophoreticdepositionofnickelzincferritenanoparticlesintomicrostructuredpatterns,"AIPAdvances,vol.6,no.056105,2016,withpermissionfromAIPPublishing. isusedasanode.Theelectrodesareplacedparalleltooneanotherandseparatedby1cmandanappliedeldof20V/cmwasused.ThenickelzincferritenanoparticleshavebeendepositedontothesubstratewithpatternedKMPRphotoresist.Fig. 3-7 showsthepatternednanoparticlelmswitha7mresolution. 3.4InltrativeElectroplating Theprocessoftheinltrativeelectroplatingissimilartonormalelectroplating.Cathodeandanodeareinsertedintothebathandthenacurrentisapplied.Sincetheseedlayerofcathodeiscoveredbyparticlelm,thebathsolutionneedtoinltrateintothevoidsofparticlelmandcontacttheseedlayer,asshowninFig. 3-8 .Thetransportofmetalionsintheuidchannelinsidetheparticlelmmainlyreliesondiusiondriven 67

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Figure3-8. Schematicsoftheinltrativeelectroplatingprocess.Ontheleftside,thebasicsetupincludesthecathodewithanelectroplatingmoldcoveredbytheparticlelmandtheanodeconnectedbyapowersupply.Ontherightside,themetalionsareplottedtoinltratethroughauidchannelintheparticlelm. bytheconcentrationgradient.Bathagitationhaslittleeectontheimprovementofiontransport.Thisisthemajordierencewiththenormalelectroplating. Softmagneticpermendur(Fe-Co)andpermalloy(Ni-Fe),hardmagneticcobaltplatinum(CoPt)andnon-magneticcopperhavebeenelectroplatedtoformnanocompositelms.Asulfate-basedpermendurbathdevelopedbythegroupofDr.StankoR.Brankovicisusedforhighsaturationmagnetization[ 69 ].Permalloy,withatargetedcompositionofNi80Fe20wasthenelectroplatedusingaconstantcurrentdensityof5mA=cm2inastagnantsulfate-basedbath[ 75 ].Anexternalmagneticeldof50mTcanbeappliedbypermanentmagnetstoinducein-planeanisotropy.Cobaltplatinumiselectroplatedunderaconstantcurrentdensityof100mA=cm2usingtherecipestudiedbycolleagueDr.OloladeD.Onikuasdescribedin[ 46 ].Asulfate-basedacidicbath(250g/LCuSO45H2Oand25mL/LH2SO4)isusedforcopperelectroplating.ThepHofthosebathrangesfrom1to7.Withinthedurationofelectroplating,theacidicbathdoesnotdissolvetheferritenanoparticles. 68

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Inltrativeelectroplatingusestheconstantcurrentmode,followingtheoperationofnormalelectroplating.Whenanelectriccurrentisapplied,themetalgrowsfromtheseedlayer,inltratesintotheparticlelmandsubmergestheparticles.Theparticlesareboundbytheelectroplatedmetalmatrixformingadensenanocompositemicrostructure. OpticalimageshavebeentakenonthesurfaceofaNiZnferrite/Fe-Cosample,whichhasanon-uniformFe-Comatrix,thickerthantheparticlelmononeside,butthinnerontheotherside.TheseimageshowninFig. 3-9 explainshowtheelectroplatedmetalapproachesandsubmergesthesurfaceofparticlelm.Insteadofheavinguptheparticles,theFe-Coalloyinsilvercolorinltratesoutofparticlelmsurfaceandencapsulatestheparticles.TheSEMimagestakenatthecleavedcross-sectionsconrmtheobservationthatFe-Comatrixgrowsupformingananocompositeanddepositsametallmonthesurface. Thestructureoftheelectro-inltratednanocompositematerialcanbeclearlyseeninthehighresolutionSEMandTEMimages.Fig. 3-10 ashowsanSEMimageofthecross-sectionofacleaved-Fe2O3/Fe-Cosample,illustratingtheprogressionoftheelectro-inltrationgrowthfrontmovingupthroughthenanoparticles.ThisimagingandEDSelementalmappingverifythatFe-Cometalgrowsupfromthesubstratesurfaceandencapsulatesthe-Fe2O3nanoparticlesintheelectroplatinggrowthfront.Fig. 3-10 bshowsTEMimagesofanothersamplecross-section(sectionedbyfocusedionbeammilling)indicatingadensecomposite,withnoapparentporesinthe2-m-thickelectro-inltratedlayer.Theinsetconrmsanintimatephaseboundarybetweenasinglebig-Fe2O3particleandthesurroundingFe-Comatrix.Fig. 3-10 cdepictsthegrowthfrontofNiZnferrite/Fe-Cosample. Fig. 3-10 dexhibitsthecross-sectionofaNiZnferrite/Fe-Cosample,wheretheparticlelmisthickandcracked.Duringelectroplating,thecopperseedlayerinthecracksaredirectlyexposedtothebathsolution.Intheparticlelm,thecurvedgrowthfrontofmetalmatrixmovesfasterthanthatinthecracks,indicatingapreferenceof 69

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Figure3-9. OpticalimagesofthesurfaceofaNiZnferrite/Fe-ColmandSEMimagesatthecross-sections.ThegrowingprocessofanFe-Cometalmatrixisillustratedformingalayerofnanocompositeandametallmonthesurface. 70

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Figure3-10. SEMandTEMimagesshowingthecross-sectionsoftheelectro-inltrated-Fe2O3/Fe-CoandNi0:5Zn0:5Fe2O4/Fe-Cosamples.Fig.aandbarereprintedfromX.Wen,J.D.Starr,J.S.Andrew,andD.P.Arnold,\Electro-inltration:Amethodtoformnanocompositesoftmagneticcoresforintegratedmagneticdevices,"JournalofMicromechanicsandMicroengineering,vol.24,no.107001,2014,withpermissionforIOPPublishing. electroplatingintheparticlelm.Thisphenomenonissimilartothecapillaryattraction,andtherapidelectroplatingintheparticlelmmaybecausedbythegeometryofuidchannels.Thisphenomenonisbenecialforthedepositionofnanocomposite,becauseitprovidessomedegreesofrejectiontothepuremetalphase. Inordertoevaluatethevolumefractionofparticlesinsideananocomposite,NiZnferriteisinltratedwithanon-magneticCumatrix.ComparingthesaturationmagnetizationofthenanocompoistetothebulkNiZnferrite,thevolumefractionof 71

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particleswasmeasuredtobe37%11%,whichisclosetothevalueofconsolidatedparticlelm.Thisprovidesanotherevidencethattheelectroplatedmetalmatrixllsthevoidsintheparticlelmwithoutmovingparticlesapart. 3.5PlanarizationandCleaning Itwouldbepossibletocontroltheparticlelmthicknessandelectroplateametalmatrixtomatchthethicknesses.However,itwilltakeaconsiderableeorttocharacterizeandstandardizetheprocess,whichisnotpreferredatthecurrentstageofresearch.Thusitistypicaltoseeamismatchbetweenthethicknessofparticlelmandthatofthemetalmatrix.Eitherparticlesorapuremetalappearsonthesurfaceasexcessresidues.Neverthelessitiseasytoexcessivelydepositathickerparticleormetallmsothatthetypeofresiduecanbechosen.Polishingisappliedtoremovetheresiduesandpossiblypolarizethelmsurface. Thephotoresistmoldcanberemovedbyacetoneandcleanedwithisopropanol.Whennanoparticlesaretheexcessresiduesandaatsurfaceisnotrequired,thephotoresistisremovedandthentheparticlescanbepolishedawayusingapolishingpadwithsoftbers.Ifthepolarizationisneeded,thesampleshouldbepolishedrstusingaSiCordiamondlappingdiscandthephotoresistisremovedinthenextstep.Inthepreparationofpolishing,thesubstrateismountedontheatsurfaceofasteelcarrierusingCrystalbondadhesive.Polishingisperformedonarotatinglappingdiscallowingthegravityofsteelcarriertobetheonlysourceofverticalpressure.Whenthepolishingisdown,thesamplesaredismountedaftermeltingCrystalbondandcleanedwithacetone. Becausetheseedlayerisstillonthewafer,themagneticnanocompositesareelectricallyconnected.TheCuseedlayercanbeetchedbytheblueetchant(asaturatedsolutionofcoppersulfateinammoniumhydroxide)andtheTiadhesionlayerisremovedbydippingina1:10water-dilutedhydrouoricacid. 72

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Figure3-11. SEMimagesatcleavedcross-sections:(a)theconsolidatedCoFe2O4nanoparticlelm(b)theannealednanoparticlelm(Fe-ColmwithoutCoPt),(c)theannealedCoPtlm(withoutparticles)and(d)theannealedFe-Co/CoPtnanocompositelm.ReprintedfromX.Wen,J.S.Andrew,andD.P.Arnold,\Exchange-coupledhardmagneticFe-Co/CoPtnanocompositelmsfabricatedbyelectro-inltration,"AIPAdvances,vol.7,no.056225,2017,withpermissionfromAIPPublishing. 3.6Annealing Annealingcanreducematerialsandchangematerialstructuresforelectro-inltratedmaterials.InthecaseofFe-Co/CoPtnanocomposites,samplesareannealedtoreducetheCoFe2O4nanoparticlestohighmagnetizationFe-CoandalsoconvertCoPtfromas-depositedlowcoercivityA1phasetoahighcoercivityL10phase.Afterremovingphotoresist,thesubstratewithCoFe2O4=CoPtlmisannealedinforminggas(4%H2,96%N2)usingatubefurnace(Lindberg/BlueMHTF55322C)at675Cfor30minwitha22Cminrampupratefollowedbynaturalcooldown. TheSEMpicturesatthecleavedcross-sectionsoftheCoFe2O4particlelmbeforeandaftertheannealingareshowninFig. 3-11 alongwiththeannealedCoPtandnanocompositelms.TheannealedCoFe2O4particlesconverttoFe-Colmwitha 73

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dierentmorphologyanddensify.ThenanoparticlesarenotvisibleintheannealedFe-Co/CoPtnanocompositelm. 3.7ThicknessMeasurement Thethicknessmeasurementiscriticalincharacterizingthinlms,includingparticlelms,metalsandnanocomposites.Themostconvenientmethodwidelyusedinthisdissertationisbyopticalmicroscope(OlympusBX51)andabuilt-inpositioner.Thepositionerreadsthestageheightwhilethefocalplaneisadjustedtofocusonlmsurfaceandtheexposedsubstratesurface.Thenthechangeinthestageheightismeasuredasthelmthickness.Theaccuracyishalfamicronandthismethodcanbeusedtomeasurethelmsthickerthan3m. Opticalprolometer(BrukerContourGT-I)providesthicknessmeasurementwithanominalresolutionof0.1nm.Byimagingthethinlmsurfaceandthesurroundingexposedsubstrateandttingthesubstratecurvature,theaccuratelmthicknesscanbemeasuredforopaquethinlms. SEMisthethirdmethodthathasbeencommonlyusedinthisdissertation.Atthesamplecross-sections,thelmthicknessmeasurementisaccurateandstraight-forward.Howeveronlyonespotatatimeisthethicknessmeasuredat.Thusthreetovespotsareusuallytakentoaveragethethicknessresults. 3.8Summary Acompletefabricationprocesshasbeendevelopedfor-Fe2O3/Fe-Co,Ni0:5Zn0:5Fe2O4/Fe-Co,Ni0:5Zn0:5Fe2O4/Ni-Fe,andFe-Co/CoPtnanocomposites.Thechoiceofsolvent,particleconcentrationandevaporationratearenon-trivialintheevaporativeprocessandneedtoadjustfortheparticlesindierentmaterialsandsizes.Thesynthesisortop-downfabricationofparticlesshouldensureacleanparticlesurfaceforthefollowinginltrativeelectroplating.DCcurrentmodeelectroplatinghasbeenfoundsuccessfulfordepositingdenseFe-Co,Ni-FeandCoPtmatrices.Maghemite,NiZnferriteandcobaltferritenanoparticlesareallabletosustaintheacidicpHofelectroplatingbath.Duetotheeasy 74

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removalofexcessiveparticles,theelectroplatedmetalmatrixcanbekeptthinnerthantheparticlelmsothataatnanocompositesurfaceisobtained.Thevolumefractionofnanoparticlesinnanocompositesisaround37%,whichcannotbedirectlycontrolledinthecurrentprocess.Coatingparticlewithashellofthematrixmaterialwouldbeacandidatemethodfortuningparticlespacingandvolumefraction. 75

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CHAPTER4SOFTNANOCOMPOSITEMAGNETS Softnanocompositemagneticmaterialsaredevelopedprimarilyforpowerinductorsascorematerials.Theinductorcoresareexpectedtooperateatthefrequencyrangeoftenstohundredsofmegahertzwhereintheeddycurrentturnsintoasignicantlosscontributor.Thegoalofdevelopingsoftnanocompositemagneticmaterialsistomaintainthehighpermeabilityatthehighoperationfrequencies. Inordertosubduetheeddycurrentlossinamagneticthinlm,theskindepthshouldbekeptatleastcomparabletothelmthickness.RewriteEquationEq. 2{32 =1 p f(4{1) Theskindepthdecreaseswithfrequencyfandconductivitythatrequiresthematerialtomaintainahighelectricalresistivity.Otherthanlamination,theelectro-inltratedlmincorporatenanoparticleswithhighresistivitytoreducetheeddycurrentloss. Commonsoftmagneticferritesandalloys,suchas-Fe2O3,Ni0:5Zn0:5Fe2O4,Fe67Co33andNi80Fe20,aswellasnon-magneticAl2O3havebeenchosentostudythepropertiesofnanocomposites. 4.1CharacterizationMethod -Fe2O3/Fe-CoandNi0:5Zn0:5Fe2O4/Ni-Fenanocompositethinlmshavebeenfabricatedintwodimensions,3:6mm3:6mmand5mm12mmforcharacterization.Thesmaller3.6mmsquaresamplesareintendedforhysteresisloopmeasurementusingaVSM(ADETechnologies,ModelEV9).ThelargerrectangularsamplecanbeusedforVSMmeasurementaswell,butthesesamplesareprimarilymadeforradiofrequency(RF)permeabilitymeasurementusingthemicrostriplinepermeametertestxtureintroduced ReprintedwithpermissionfromJournalofMicromechanicsandMicroengineering,vol.24,no.107001,2014[ 65 ],IOPPublishing;andAIPAdvances,vol.6,no.056111,2016[ 66 ],AIPPublishing. 76

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inSection 4.1 andavectornetworkanalyzer(AgilentE5071C).Filmresistivitiesaremeasuredonlyonthe5mm12mmsamplesusinga4-pointprobestation(VeecoFPP5000). Permeabilitymeasurement .ThecomplexRFpermeabilityoverarangeof3MHzto3GHzismeasuredusingamicrostriplinepermeametertestxtureandavectornetworkanalyzer. ThemethodofRFpermeabilitymeasurementisbasedonthepropagationofelectromagneticwaveinamicrostripline[ 76 ].Thefabricatedtestxtureisashortedmicrostripline,asshowninFig. 4-1 ,whichisconnectedtothenetworkanalyzer.Thereectioncoecient(S11)isdeterminedbythepropagationconstantinthemicrostripline,whichisaectedbytheeectivepermittivity(e)andeectivepermeability(e)ofthespacewherethewavetravelsthrough. =j! c0p ee(4{2) Whenthextureisloadedwithasample, Ssmpl11=eemtylemty+smpllsmpl=ej! c0lemtyp emtyeemtye+lsmplp smplesmple(4{3) wherelsmplisthelengthofthinlm,lstrpisthelengthofmicrostripline,andlemty=lstrp)]TJ /F3 11.955 Tf 11.95 0 Td[(lsmplisthelengthofemptypart. Duetothesmallthicknessofmagneticthinlmscomparedtothegapbetweenthemicrostripandthegroundplane,itisapproximatedthatthepermenanceofthinlmhasalinearrelationshipwiththeeectivepermeability.Therelativepermeabilityofthinlmiscalculatedby film=K te(4{4) whereKisageometricfactorwhichcanbeobtainedbymeasuringcalibrationsamples,andtisthethicknessofthinlm. 77

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Figure4-1. Pictureofthemicrostriplinepermeametertestxtureloadedwitharectangularsample. Duringmeasurement,atransversemagneticDCbiaseldisappliedtosaturatethesamplesothattheeectivepermeabilityequalsto1whiletheeectivepermittivityremainstheunchanged.Inordertocompensatetheparasiticmagneticpropertyinthextureandconnector,theeectivepermeabilityofemptyxtureisalsomeasured.Thereectioncoecientisactuallymeasuredin4conditions:emptyxture(Semty11),emptyxturewithasaturatingeld(Semty;satr11),loadedxture(Ssmpl11)andloadedxturewithasaturatingeld(Ssmpl;satr11).Thethinlmpermeabilityiscalculatedbyemtye=jc0ln[)]TJ /F1 11.955 Tf 9.3 0 Td[(Semty;satr11] 4flstrp2 (4{5)emtye=0@jc0ln[)]TJ /F1 11.955 Tf 9.3 0 Td[(Semty11] 4flstrpq emtye1A2 (4{6)smple=0@jc0ln[)]TJ /F1 11.955 Tf 9.3 0 Td[(Ssmpl;satr11] 4flsmpl)]TJ /F3 11.955 Tf 13.15 11.27 Td[(lemtyq emtye lsmpl1A2 (4{7)smple=0@jc0ln[)]TJ /F1 11.955 Tf 9.3 0 Td[(Ssmpl11] 4flsmplq smple)]TJ /F3 11.955 Tf 13.15 11.27 Td[(lemtyq emtye lsmplq smple1A2 (4{8)film=K t(smple)]TJ /F3 11.955 Tf 11.96 0 Td[(emtye)+1 (4{9) 78

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Table4-1. Magneticpropertiesofparticlesandthinlms MaterialBulkmaterialParticlelm Saturationmagneti-zationJs(mT)Particlesize(nm)Saturationmagneti-zationJs(mT)CoercivityHc(kA/m)Relativepermeabil-ityrat0A/m -Fe2O3500<5013014:12283Ni0:5Zn0:5Fe2O42388-20691:03071 4.2NanoparticleFilms Ferritesarethecommoncorematerialforpowerinductors,becauseoftheirdecentmagneticpropertyandlowcost.Ferritesaresemiconductormaterials,whichsuerlessfromtheeddycurrentlosscomparedtometals.Theirhighfrequencyperformancesarelimitedbythesaturationmagnetization,asdescribedbySnoek'sequationinSection 2.1.5 Theparticlelmsofmaghemiteandnickelzincferritehavebeendepositedbytheevaporativeassembly.ThemagneticpropertiesofthebulkmaterialsandtheirparticlethinlmsarecomparedinTable 4-1 .TherelativepermeabilityrisextractedfromthehysteresisloopatH=0A=m. ThesaturationmagnetizationofbulkmaghemitematerialiscitedfromDr.J.M.Coey'sbook[ 34 ].TheaveragespecicmagnetizationofNiZnferriteparticlesis36.3emu/gbasedonthemeasurementofsaturatedmagneticmomentandweight.Usingadensityof5:2g=cm3,thesaturationmagnetizationofbulkNiZnferritewascalculatedtobe238mT.ThemagneticpropertiesofparticlelmsarecollectedfromtwotypicalhysteresisloopsshowninFig. 4-2 .Maghemiteparticlelmhasahighersaturationmagnetization,andNiZnferritehasamuchlowercoercivity.Thecurvedhysteresisloopsexhibitanisotropicpropertyoftheparticlelmsasexpected.Thevolumefractionofparticlesinthelmsareestimatedtobe26%formaghemiteand29%forNiZnferrite,basedontheratioofsaturationmagnetizationsoftheparticlelmstothebulkmaterial. Theoperationbandwidthofmaghemiteparticlelmislargethattherealpartofpermeabilitystartstorolloat1.4GHz,asshowninFig. 4-3 .Buttherelative 79

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Figure4-2. HysteresisloopsofmaghemiteandNiZnferriteparticlelms. permeabilityhasalowvalueof2atMHzfrequencyrange.Fortheapplicationsofpowerelectronics,thefrequencyrangeofinterestisfromtenstohundredsofMHz.Becauseferriteparticlelmhasasteadypermeabilityatthisfrequencyrange,thenanoparticlescanbeutilizedtoreduceeddycurrentlossinnanocompositeandextendtheoperationbandwidth. 4.3ElectroplatedAlloy Fe-Coelectroplating .TheelectroplatedFe-ColmsfollowingS.R.Brankovic'srecipe[ 69 ]hasasaturationmagnetizationashighas2.4T.TypicalpropertiesoftheelectroplatedlmsarelistedinTable 4-2 .TheFe-Colmistypicallyelectroplatedinaconstantcurrentwithadensityof20mA=cm2ina400mLstagnantbath.A5cmby5cmcobaltplateisusedastheanode,separatedapartfromthesubstrateby6cm.10m=hisatypicaldepositionrate. Goodmagneticpropertiesistheadvantageofthisrecipe,butthebaduniformitybecomesthedrawback.Hydrogenbubblesgenerateduringelectroplatingandleavedentsonthemetallm.Ifthebathisagitatedusingamagneticstirbar,thebubbledents 80

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Figure4-3. Permeabilityspectrumofamaghemiteparticlelm becomelesssevere,butathickerlmwouldbeelectroplatedononesideofthepattern.Afewtestsonpulseelectroplatingandpulsereverseelectroplatinghavebeentried,buttheimprovementonthicknessuniformityisnotsignicant. Table4-2. Magneticpropertiesofelectroplatedlms MaterialSaturationmagnetizationJs(T)RemanentmagnetizationJr(T)CoercivityHc(kA/m)Relativepermeabilityrat0A/m Fe-Co1:8)]TJ /F1 11.955 Tf 11.96 0 Td[(2:40:3)]TJ /F1 11.955 Tf 11.95 0 Td[(1:0300)]TJ /F1 11.955 Tf 11.96 0 Td[(1200180)]TJ /F1 11.955 Tf 11.96 0 Td[(1800Ni-Fe1:1)]TJ /F1 11.955 Tf 11.96 0 Td[(1:80:03)]TJ /F1 11.955 Tf 11.95 0 Td[(0:530)]TJ /F1 11.955 Tf 11.95 0 Td[(130830)]TJ /F1 11.955 Tf 11.96 0 Td[(4500 Ni-Feelectroplating .Asulfate-basedrecipe[ 75 ]usedbyMEMSresearchersischosenforNi-Feelectroplating.TheNi-Felmistypicallyelectroplatedinaconstantcurrentwithadensityof5)]TJ /F1 11.955 Tf 12.37 0 Td[(20mA=cm2ina400mLbathwithagitation.A5cmby5cmnickelplateisusedastheanode,separatedapartfromthesubstrateby6cm. ThethicknessuniformityoftheNi-Felmselectroplatedinagitatedbathsisgoodoverthe3.6mmby3.6mmdepositionarea.Fig. 4-4 showstheprolesofNi-Felmselectroplatedunderdierentagitationconditionsmeasuredbyanopticalprolometer.Thelmsareelectroplatedat5mA=cm2for10min,13minand10minrespectively.In 81

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Figure4-4. ThicknessprolesofNi-Felmselectroplatedinbathswith(a)noagitation,(b)60rpmagitationofa6.3cmlongstirbarand(c)180rpmagitation.TheXandYprolesshowthethicknessdataatthescanlinesoverthethicknesscolormaps. a600mLbeaker,60rpmrotationofthe6.3cmlongstirbarcangenerateavortexonthesurfaceofthebathandmildlyagitatethebath.180rpmrotationagitatesthebathseverely.Accordingtothedataoflinescans,thethicknessvariationsare1.1movera2.0maveragethicknessfornoagitation,0.6mfor60rpm,and0.2movera1.2maveragethickness.ThehysteresisloopsoftheNi-Felmselectroplatedwithnoagitationandsevereagitationshowsimilarmagneticproperties. ThetypicalmagneticpropertiesoftheNi-FelmsarelistedinTable 4-2 .ComparedtoFe-Co,Ni-Fehasalowersaturationmagnetization,buttheremanentmagnetizationislower,coercivityismuchlower,andrelativepermeabilityismuchhigher.Therelationshipsofmagneticpropertiesandcurrentdensityisstudiedintheelectroplatingtestswith5)]TJ /F1 11.955 Tf 10.39 0 Td[(20mA=cm2currentdensitiesand5min-2hdurations.Thebathisagitatedbythestirbaratthe120rpm.ThehysteresisloopsoftheNi-Fesamplesintheformofmagneticmoment(m)versusappliedeld(H)areshowninFig. 4-5 .Thesamplesaremeasuredinplane.Generallylargecurrentdensityandlongdurationgivelargemagneticmoment,thicklmsandsmallcoercivity.Whenthehysteresisloopsareplottedinthe 82

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formofmagnetization(J)versusappliedeld(H),theycanbegroupedbyloopshape,revealingacorrelationwithlmthickness,asshowninFig. 4-6 .Thesquarenessoftheloopsdecreaseswiththethicknessandthemagnetizationtendstoincreaselinearlywiththeappliedeldforthicklms.Thedepositionrateisstable,2.0m=hfor5mA=cm2,4.6m=hfor10mA=cm2,and8.3m=hfor20mA=cm2,fromthedatatinFig. 4-7 a.ThenominalcompositionoftheelectroplatedNi-FelmsfromthisbathisNi80Fe20.TheEDSdataindicatesalargerpercentageofFeatomsintheelectroplatedalloythanthenominalvalue,asshowninFig. 4-7 b.TheFeatomicpercentageincreasesfrom26%{33%at5mA=cm2to41%to43%at20mA=cm2.Thesaturationmagnetizationandthecoercityexhibitcomplicatedrelationshipswiththeelectroplatingparameters,butgenerallydecreasewiththelmthickness,asshowninFig. 4-7 cand 4-7 d.Thereisaweakcorrelationamonglowsaturationmagnetization,lowcoercivityandahigherpercentageofNi.Thecoercivityofthelmsthinnerthanhalfamicrometerisapparentlylargerthanthosethickerlms.AsNi80Fe20isthetargetcompositionforpermalloy,5mA=cm2currentdensityistypicallyused. Ni-Feelectroplatinginanexternalmagneticeldcaninduceanisotropicmagneticproperties.TwoNdFeBpermanentmagnetsareplacedonbothsidesofthebeakertoapplya90mTeldintheplaneofthesubstrate.TheeasyaxesoftheelectroplatedNi-Fecrystalspartlyaligninthedirectionofmagneticeld.Thelmiselectroplatedat5mA=cm2.ThehysteresisloopsinFig. 4-8 aremeasuredinplanealongwithandperpendiculartothedirectionoftheappliedmagneticeld.Theloopinthedirectionoftheappliedmagneticeldismoresquaredthantheoneinperpendiculardirection,moreresemblingtheshapeofanisotropicNi-Fehysteresisloopineasyaxis. 4.4SoftMagneticNanocompositeMagnets Avarietyofcombinationofnanoparticlesandmetalmatrix,aslistedinTable 4-3 ,havebeentestedtoexplorethepropertiesofelectro-inltratedsoftmagneticnanocomposites.Thenanocompositesincorporatinganon-magneticphaseareformedto 83

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Figure4-5. m-HhysteresisloopsofelectroplatedNi-Felms Figure4-6. HysteresisloopsofelectroplatedNi-Felmsgroupedbyloopshapes 84

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Figure4-7. PlotsofNi-Fepropertiesandelectroplatingparameters:(a)lmthicknessversuselectroplatingduration,and(b)Featomicpercentage,(c)saturationmagnetizationand(d)coercivityversuslmthicknessatvariouscurrentdensities. studythepropertiesofthestructuredsinglemagneticphasewithoutmagneticinteraction.TheNi0:5Zn0:5Fe2O4=Cunanocompositeisusedtoevaluatetheparticlevolumefractionbymeasuringthesaturationmagnetizationandcomparingitwiththebulkvalues.TheAl2O3=Fe67Co33nanocompositeisstudiedforthemagneticpropertiesofelectro-inltratedFe67Co33matrix.The-Fe2O3=Fe67Co33nanocompositeisrststudiedassoftmagneticcompositewithhighsaturationmagnetization.ThenNi0:5Zn0:5Fe2O4=Fe67Co33istestedutilizingthesoftNi0:5Zn0:5Fe2O4nanoparticleswithacoercitylowerthantheFe67Co33matrix.FinallytheNi0:5Zn0:5Fe2O4=Ni80Fe20nanocompositesareinvestigatedforlowercoercivityandhighcoercivity. 85

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Figure4-8. HysteresisloopsofNi-Felmelectroplatedina90mTmagneticeld Table4-3. Choiceofmaterials(inclusion/matrix)inthesoftmagneticnanocomposites InclusionMatrix Non-magneticHighsaturationmagnetizationLowcoercivity Non-magneticAl2O3=Fe67Co33Highsaturationmagnetizationferrite-Fe2O3=Fe67Co33LowcoercivityferriteNi0:5Zn0:5Fe2O4/CuNi0:5Zn0:5Fe2O4=Fe67Co33Ni0:5Zn0:5Fe2O4=Ni80Fe20 4.4.1-Fe2O3=Fe67Co33nanocomposites ThemaghemiteparticlesandFe-Coalloyswererststudiedfortheirhighsaturationmagnetizations.Thehighsaturationmagnetizationisadesirablepropertyforbothsoftmagneticandhardmagneticapplicationsandtheelectro-inltrationofthosematerialswouldhaveabroadapplicationarea.Theparticlesandalloyswithbettercoercityandpermeabilitywillbeutilizedforabettersoftmagneticnanocomposite. Maghemitenanoparticleshavealargesaturationmagnetization,whichismeasuredtobe0.40T,amongferrites.PermendurwithacompositionofFe67Co33hasthehighest 86

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room-temperaturesaturationmagnetizationof2.4Tinallbulkmaterials[ 34 ].Itcanbeelectroplatedinanacidicbathatroomtemperature[ 69 ].ThetypicalconditionofelectroplatingisusingaCoanodeanda400mLstagnantoragitatedbathsolutionina600mLbeakerandapplyingaDCcurrentwithadensityof5)]TJ /F1 11.955 Tf 11.96 0 Td[(20mA=cm2. The-Fe2O3/Fe-Conanocompositesamplesandthesinglephasesamplesarefabricatedtocomparethemagneticpropertiesinthehysteresisloopsandthepermeabilityspectra.Themaghemiteparticlelmsaredepositedbytheevaporativeassembly.Thealloylmwaselectroplatedinastagnantbathat20mA=cm2for30min,andthecompositelmiselectroplatedat5mA=cm2for30min.Thelmthicknessesare7:5mforFe-Colm,7mformaghemiteparticlelm,and1:7mfornanocompositelm. ThehysteresisloopsareshowninFig. 4-9 .Thecompositeexhibitsasaturationof1.0T,halfthatoftheFe-Co(2.0T).Thissaturationvalueismuchhigherthanabulkferrite(0.3{0.45T)andonparwithpermalloy(1.0T).Thecoercivityofthecompositeis1.5kA/m,whichisanorderofmagnitudelowerthantheparticles(14.1kA/m)butlargerthantheFe-Co(0.3kA/m). Fig. 4-9 bcomparesthenormalizedhysteresisloopsofthe-Fe2O3/Fe-Codoublelayerandthenanocomposite.Thedoublelayercurvecanbeconceptualizedasthesuperpositionoftheindividualloopsofthe-Fe2O3nanoparticlesandtheFe-Cometal.Akinkisobservedonthedoublelayercurve,indicatedbythearrowinthegure,indicativeofamixed,two-phasemagneticresponse.Thehysteresisloopofdoublelayerisbasicallyasuperpositionoftheloopsof-Fe2O3andFe-Co.Incontrast,thenanocompositeexhibitsasmooth,kink-freecurve.Thesemeasurementsindicateexchangecouplingbetweenthenanoparticleinclusionphaseandelectroplatedmatrixphase. Fig. 4-10 showspermeabilityspectraofthesamesamplesinthehystersisloops.Below20MHz,therelativepermeabilityofthenanocompositeisaround80,halfthevalueofFe-Coalloyof180,andoneorderofmagnitudehigherthanthe-Fe2O3particles.TheFe-Coalloyhasthelowestcut-ofrequencyatabout20MHz,whilethefrequencyfor 87

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Figure4-9. Hysteresisloopsof-Fe2O3lm,Fe-Colmandtheircomposites.Normalizedloopsof-Fe2O3/Fe-Cocompositeanddoublelayerlmsarecomparedin(b).ReprintedfromX.Wen,J.D.Starr,J.S.Andrew,andD.P.Arnold,\Electro-inltration:Amethodtoformnanocompositesoftmagneticcoresforintegratedmagneticdevices,"JournalofMicromechanicsandMicroengineering,vol.24,no.107001,2014,withpermissionfromIOPPublishing. 88

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Figure4-10. Permeabilityspectraof-Fe2O3particlelm,Fe-Colmandtheircomposites the-Fe2O3nanoparticlesishigherthan1.4GHz.Thenanocompositeexhibitsacut-ofrequencyinbetween,atabout200MHz,whichisattributedtoexchangecouplingbetweenthe-Fe2O3nanoparticlesandFe-Comatrixaswellasthesuppressionofeddycurrentbytheelectricallyinsulatingmaghemitenanoparticles. Tosuccessfullyinhibiteddycurrents,theelectro-inltratedregionsofFe-Coshouldbesmallerthantheskindepth,whichisabout531nmat1GHz(assumingr=180and=20cm).ThepermeabilitydatainFig. 4-10 andtheimagesinFig. 3-10 bothgiveevidencethatthe-Fe2O3particlesgranulatetheFe-Cointosmallersizedregions(<531nm)andhenceinhibiteddycurrentlosses.Lastly,theproductofthelow-frequencypermeabilityandresonantfrequencycanbeusedasagureofmeritforperformance.Thisproductinthenanocompositeis5timeslargerthanthatintheFe-Coalloy. 4.4.2Ni0:5Zn0:5Fe2O4=Fe67Co33andAl2O3=Fe67Co33Nanocomposites Sincemagneticcorematerialsrequirelowhysteresisloss,Ni0:5Zn0:5Fe2O4nanoparti-cleswithalowercoercivityandahigherpermeabilityareutilizedtotestnanocompositeproperty.Non-magneticalumina(Al2O3)nanoparticlesarealsousedtoinvestigatepropertiesofnanocompositewithsinglemagneticphase. 89

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TheNi0:5Zn0:5Fe2O4andAl2O3nanoparticlesaredepositedviadielectrophoreticdeposition.TheFe-Colmiselectroplatedat20mA=cm2for5minandthebathisagitatedusing6cmlongstirbarat180rpm.Thecompositelmiselectroplatedat5mA=cm2for15minwith100rpmagitation.Thecurrentdensityofcompositelmhasbeenlowereddownwiththeconsiderationthattheiondiusionmaybeslowintheparticlelm.Therotatingspeedofstirbarisadjustedtoremovegeneratedbubbles. ThehysteresisloopsofNi0:5Zn0:5Fe2O4/Fe-Conanocomposite,Al2O3/Fe-ConanocompositeandFe-ColmarecomparedinFig. 4-11 TheelectroplatedFe-ColmandAl2O3/Fe-Cocompositelmbothhavesinglemagneticphaseandthenanocompositelmisactuallyaporousmagneticmaterial.Thegeometrycanaectproperty.ComparedtoFe-Colm,thecompositehasalargercoercivityandalowersaturationmagnetizationduetonon-magneticaluminaparticles.Therearethreepossiblereasonstoexplaintheincreasedcoercivity.First,thecompositeandcrystalstructureofFe-Coelectro-inltratedinaluminaparticlelmhavechangedthatleadstopropertychange.Second,thelowcoercivityofelectroplatedFe-ColmbenetsfromHerzer'srandomanisotropymodeldescribedinSection 2.1.4 .Intheporousmedium,thenumberofneighborcrystalsdecreases,andsoisthenumberofinteractingcrystals.Thereductionofeectiveanisotropyconstantismitigatedthatresultsinahighcoercivity.Third,attheinterfacebetweenaluminaandFe-Co,thedefectsandinterfacialeectcanpindownthemovementofmagneticdomainwallandincreasecoercivity. ThepermeabilityspectraareplottedinFig. 4-12 .ThethicknessofNi0:5Zn0:5Fe2O4/Fe-Co,Al2O3/Fe-CoandFe-Colmsaresimilar,2:1m;2:3mand2mrespectively.At100{200MHzfrequencyrange,Fe-Cohasthehighestpermeability.Butatthefrequencybeyond,Ni0:5Zn0:5Fe2O4/Fe-ConanocompositelmdeclinesslowerthanpureFe-Colm.ThereisanobviouspeakintheimaginarypartofpermeabilityinFe-Co.Howeverforthenanocomposite,thepeakofimaginarypermeabilityissubdued. 90

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Figure4-11. HysteresisloopsofNi0:5Zn0:5Fe2O4/Fe-Conanocomposite,Al2O3/Fe-ConanocompositeandFe-Colm. Figure4-12. PermeabilityspectraofNi0:5Zn0:5Fe2O4/Fe-Co,Al2O3/Fe-CocompositelmsandFe-Colm. ThepermeabilityofAl2O3/Fe-Conanocompositeislow,butitsrealpartofpermeabilityofcompositerollsoat1.5GHz,muchhigherthantheotherlms.Interestingly,thepermeabilityofAl2O3/Fe-CocompositeislowerthanthatofFe-Countiltheyoverlapbeyond1.5GHz.Theporousgeometryofcompositelmcanreduceeddycurrentloss,buttheperformanceofcompositelmwouldbesimilartoathinnerFe-Colm. 91

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4.4.3Ni0:5Zn0:5Fe2O4=Ni-FeNanocomposites Electroplatedpermalloyisapracticalchoiceformatrixmaterial,asitiswidelyusedasmagneticcorematerialsandpossesseshighpermeabilityandlowcoercivity.TheresultantNi0:5Zn0:5Fe2O4/Ni-Fenanocompositeobtainsthelargestpermeabilityandsmallestcoercivityamongthetestedcompositematerials. Fig. 4-13 showsin-planehysteresisloopsfortheNi0:5Zn0:5Fe2O4/Ni-Fenanocomposite,apermalloysampleelectroplatedatthesamecurrentdensityasthecomposite,andaconsolidatedparticlelayer.Thesampleswithpermalloyexhibitanisotropy,whichisinducedbytheexternalmagneticeldduringelectroplating.Thecoercivitiesofthenanocompositealongthehardaxisandeasyaxisare250A/mand268A/mrespectively,whicharehigherthanthecoercivitiesofpermalloylm(102A/mand145A/m,respectively).TheincreaseofcoercivityinthenanocompositeispresumedtobecausedbyexchangecouplingtotheNiZnferriteparticleswithahighercoercivity(1003A/m). ThecomplexpermeabilitiesofaNi0:5Zn0:5Fe2O4/Ni-Fenanocomposite(68136nmthick)andapermalloylm(98543nm)arethenmeasuredalongthehardaxisupto3GHz,asshowninFig. 4-14 .Thelow-frequencyrelativepermeabilitythenanocomposite,0r320,isaroundonefourththepermeabilityofNi-Fe,0r1200,presumablyduetothedilutionofhigh-permeabilityNi-Febythelower-permeabilityNiZnferriteandthehighercoercivityofthecompositelm.Thesmall-signalpermeabilitysensedbyACmagneticeldcorrespondstotheslopeofthehysteresisloopatthepointofthebiasDCeld.Inthiscase,themeasuredpermeabilityofnanocompositewithoutbiaseld,correspondingtotheslopeatthepointofH=0,isreducedbythebroadenedloopwithincreasedcoercivity.Fortheapplicationofmagneticcore,cutofrequencies(denedasthefrequencieswhererealpermeabilitydropsbyhalf)ofnanocompositeandpermalloylmsarearound93MHzand60MHz,respectively.However,the50%increaseofoperatingfrequencyrangeinthenanocompositeiscompromisedbythelowerpermeability,relativetothepermalloy. 92

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Figure4-13. Hysteresisloopsof(a)aNiZnferrite/Ni-Fenanocomposite,(b)apermalloylm,and(c)aNiZnferritenanoparticlelayer.ReprintedfromX.Wen,S.J.Kelly,J.S.Andrew,andD.P.Arnold,\Nickel-zincferrite/permalloysoftmagneticnanocompositesfabricatedbyelectro-inltration,"AIPAdvances,vol.6,no.056111,2016,withpermissionfromAIPPublishing. 93

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Figure4-14. ComplexpermeabilityspectrumofNiZnferrite/Ni-Fenanocompositeandpermalloylm.ReprintedfromX.Wen,S.J.Kelly,J.S.Andrew,andD.P.Arnold,\Nickel-zincferrite/permalloysoftmagneticnanocompositesfabricatedbyelectro-inltration,"AIPAdvances,vol.6,no.056111,2016,withpermissionfromAIPPublishing. Ananalysisofpotentialeddycurrentsuppressioncanalsobeconducted.Aroughrst-orderapproximationofthiseddy-currentcornerfrequency(whereskindepthequalslmthickness)canbecalculatedbyarrangingEq. 2{32 into, feddy= id2(4{10) TheDCresistivityoftheNiZnferrite/Ni-Fecompositeismeasuredtobe43mcm,amodestenhancementof39%overtheNi-Fepermalloylm(31mcm).Theestimatedcornerfrequencyforthepermalloysampleiscalculatedtobefeddy=67MHz,whichmatchesreasonablywellwiththe60MHzcutofrequencyobservedinthepermeabilityspectrum.Combiningthehigherresistivity,lowerinitialpermeability,andlowerlmthickness,theeddy-currentcornerfrequencyofthenanocompositeshouldshowan11xincrease,feddy=730MHz.However,thepermeabilityspectrumdoesnotshowsuchasubstantialincrease,indicatingthateddycurrentlossmaynotbethedominantfactorinthepermeabilitydeclineofthenanocomposite. 94

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Interestingly,althoughNiZnferriteparticleshavealowpermeability(0r<5),theirpresenceinthepermalloymatrixdoesseemtoaectthehigh-frequencymagneticbehaviorofthecomposite.First,thehigh-frequencyrollobehaviorisdierent{therealpermeabilityofthenanocompositedecreasesataslowerrateof13dB/decade,comparedto22dB/decadeinthepermalloy.Second,thepermeabilityspectrumofpermalloylmshowsatypicalpatternofrelaxation,.However,inthenanocomposite,theshapeoftheimaginarypermeabilityofthenanocompositedoesnotfollowthisnormaltrend;thepeakismoreattened,asindicatedbythedashedcurveinFig. 4-14 ,implyingalowerenergylosscomparedtoatraditionalsingle-phasematerial. 4.5ComparisonofHighFrequencyMagneticProperties Thegoodperformanceandadvantageofmicroinductorsrelyonbothhighinductancedensityandhighoperationfrequency.Forthemagneticcores,theytranslateintohighpermeabilityandhighcutofrequency.Amongthefabricatedmagneticlms,itshowsatrendoftrade-obetweenpermeabilityandcutofrequency.RoughlyspeakingtheFe-CobasednanocompositeshaveahighercutofrequencythantheNi-Febasednanocomposites,anditistheoppositecaseforpermeability.Astocomparedierentmaterials,theproductofpermeabilityandcutofrequency(-fproduct)canbeselectedasthegureofmerit.Morespecically,thecutofrequencyisdenedasthefrequencywherethepermeabilitystartstosignicantlydecrease,andthevalueofpermeabilityistakenatthecutofrequency. Theperformancesofmorethan10samplesareplottedintheplaneofpermeabilityvsfrequency,asshowninFig. 4-15 ,includingsinglephasematerialsandnanocompositelms.Thecontoursofconstant-fproductsareshownasthedashedlinesintheplot.ThemagneticparticlelmsandtheAl2O3/Fe-Conanocomposite,whichhavelowpermeabilityandhighcutofrequency,locateonthebottomrightcorneroftheplot,whilethehighestpermeabilitymarkedbyapointrepresentingsinglephaseNi-Felmpeaksonthetopleftcorner.OntheareainbetweenscattertherestofNi-FeandFe-Colmsandtheir 95

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Figure4-15. Graphicrepresentationofcutofrequencyandpermeabilityforelectro-inltratedsoftmagneticnanocompositesamples. nanocomposites.Twopairsofslimblackarrowspointattwonanocompositesamplesfromtheirsinglephasecomponents.ThemarkedNi0:5Zn0:5Fe2O4/Ni-Fesampleexhibitsapermeability,acoercivityanda-fproductbetweenthatoftheNi0:5Zn0:5Fe2O4particlelmandtheNi-Felm.Intheothercase,themarked-Fe2O3/Fe-Cosampleshowsapermeabilityandacoercivitybetweenthatofthe-Fe2O3particlelmandtheFe-Colm,butits-fproductishigher,5timesashighasthatoftheFe-Colm,achievingthehighestvalueamongalloftheplottedsamples.The-Fe2O3/Fe-Cosamplehasdemonstratedtheadvantagesoftheelectro-inltrationprocess,bywhichtheformedsoftmagnetnanocompositescanextendtheoperationfrequencyandachievelarger-fproductthansinglephasemetallms. 4.6Summary Themagneticpropertiesofthe-Fe2O3/Fe-Co,Ni0:5Zn0:5Fe2O4/Fe-Co,Al2O3/Fe-Co,andNi0:5Zn0:5Fe2O4/Ni-Fenanocomposites,alongwithcorrespondingparticlelmsandelectroplatedmetallmshavebeencharacterizedandcompared.Generallyspeaking,theDCmagneticpropertiesofnanocompositematerialsresidein-betweenparticleslmsandpuremetallms.Incorporatingmagneticnanoparticles,thecompositelms 96

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obtainlowlossathundredsofMHzrange,asindicatedbythereduced00rspectra.Intermsofmagneticcoreapplication,Ni0:5Zn0:5Fe2O4/Ni-Fepossessesthebestcoercivityandpermeabilityamongthecompositesat250A/mand320.-Fe2O3/Fe-Coexhibitsanimprovementonthetrade-obetweenpermeabilityandoperationfrequencyoverthesinglephaseFe-Co.Withtheaidofnon-magneticaluminananoparticles,theelectroplatedporousmetalisdiscoveredtoexhibitanincreasedcoercivity.Theideathatthenanocompositeofferriteinclusionsandametalmatrixwouldincreaseresistivityandsuppresstheeddycurrentlosshasbeenexperimentallysupport.However,thesignicantimprovementwouldrequiredenserparticleinclusionsandmoresophisticatedcontrolofcompositematerialstructures. 97

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CHAPTER5HARDNANOCOMPOSITEMAGNETS Sincetheconceptofexchange-springmagnetwasproposedbyKnellerandHawigin1991[ 77 ],nanocompositematerialscomprisingsoftandhardmagneticphaseshavebeenextensivelystudiedthroughtheoreticalmodeling(e.g.[ 6 78 ])andexperimentsinthepursuitofstrongpermanentmagnetsthatoutperformanysinglephasematerials.Two-dimensionalbilayerthinlmstructureshaveexperimentallyconrmedtheexchange-coupling-basedenhancementofenergydensity[ 79 80 ].Torealizethicker,andthree-dimensionalexchange-coupledmaterials,sputteringsystemshavebeenmodiedforgas-phasenanoparticledeposition(clusterdeposition)tocreateaninclusion-matrixstructurefornanocomposites[ 57 81 82 ].Incomparisontothatapproach,thisstudyutilizesmagneticnanoparticlebuildingblocksandanelectro-inltrationprocess[ 63 66 ]tofabricatethree-dimensionalexchange-coupledFe-Co/CoPtnanocompositelmswithrelativelysimpleexperimentalinfrastructure. Takingtheadvantagesofexchangecoupling,bothsoftmagnetandhardmagnetcanbeusedfortheinclusionphaseormatrixofthenanocomposite.Inoptimalcaseoftheoreticcalculation,thestructureusinghardmagneticinclusionmayevenhaveabetterproperty.HardmagneticNdFeBandFePtnanoparticleshavebeenutilizedwithelectroplatedFe-Cotoformexchange-coupledmagnet.However,thesynthesizedNdFeBnanoparticlesexhibitalowercoercitycomparedtobulkNdFeB.ThestorageandcleaningofsynthesizedNdFeBandFePtnanoparticlesareprovedtobetroublesome.FinallythelessaggressiveapproachofutilizinghardmagneticCoPtformatrixmaterialhasbeentaken. ReprintedwithpermissionfromAIPAdvances,vol.7,no.056225,2017[ 68 ],AIPPublishing. 98

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5.1TheoreticalModel Thehighcoercivityofpermanentmagnetsstemsfromthelargemagnetocrystallineanisotropy.Theexchangecouplingamongthemagneticmoments,thecrystaldimensionsandarrangement,andthedefectsandimpuritiesalsoaectthecoercivityofmaterials.Thenucleationeld,asatheoreticallowerboundforcoercivity,foraparticleisdimensiondependent,andthereisanoptimalsizetoachievethehighnucleationeld,asshowninFig. 2-8 .Consideringthinlmandbulkmaterialsasassembliesofparticleswithinterparticleinteraction,thestructureandsizeofgrainsarecriticaltotheirmagneticproperties.Forexample,theprocessingofmeltingspinningandhotpressingformanufacturingNdFeBmagnetsisallaboutgettingtherightstructuresandtherightsizeofthegrains[ 83 ].Asinglephasehardmagnetcanpossessacomplicatedmaterialstructure. Whenasoftmagneticphaseisintroducedintoahardmagneticphasetoformanexchange-coupledmagnet,thestructureofnanocompositebecomesevenmorecomplicated.Thestructuresandsizesofbothphasescontributethemagneticpropertiesofthenanocomposites.Thestudyoftheoptimalnanocompositestructuretypicallyneglectsthestructuraldetailsofeachphasetokeeptheproblemtangible.ThemagneticpropertiesofsinglephasematerialsaresummarizedbythesaturationmagnetizationMs,theanisotropyconstantK1andtheexchangestinessconstantA. Thehardmagneticphaseandthesoftmagneticphasearecoupledbyexchangethroughthephaseboundaries(interfaces)inexchange-couplednanocomposites.Theexperimentaldataoftheexchangeenergyoftheadjacentmagneticmomentsacrosstheboundariesisnotwidelyavailablefordierentcombinationofmaterials.Thusaperfectalignmentofthemagneticmomentsattheinterfacetypicallyassumedoranaverageexchangestinessconstantofbothphasesistaken. Thephaseboundaryinnanocompositescanbemodeledasabilayerstructure.AschematicofmagnetizationprocessisshowninFig. 5-1 .Thehardmagneticlayer 99

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Figure5-1. Schematicofmagnetizationprocessofahard-softbilayer andthesoftmagneticlayerareinitiallymagnetizedinonedirection.Whenareversemagneticeldisappliedthebilayer,thesoftmagneticlayerisrstdemagnetized.Adomainwallappearsinthesoftphaseasthemagneticmomentsatthephaseboundaryisexchange-coupledtothemagneticmomentsinthehardphase.Asthemagneticeldincreases,thesoftmagneticphaseispartiallyremagnetizedandtherotationofmagneticmomentspropagatestowardsthephaseboundary.Asthedomainwallsinthesoftphaseissqueezedinwidthbyastrongermagneticeld,itsenergydensityincreases.Whentheenergydensityofthedomainwallbecomescomparabletothatofthehardphase,therotationofmagneticmomentsmovesacrosstheboundaryandthehardphasestartstobedemagnetized.Whentheappliedeldissucientlystrong,thehardphaseisdemagnetizedandthenremagnetized. Duetotheexchangecouplingacrossthephaseboundary,thosemagneticmomentsinthesoftphaseclosetotheboundaryrequireahigherdemagnetizingeldthanthesinglephasesoftmagnet.Thematerialis\hardened"bytheexchangecoupling.Theideaofexchange-coupledmagnetistoachievesuchananocompositestructurethatthewholesoftmagneticphaseishardened.Withthebenetofhighmagnetizationofthesoftphase,thereisapromiseofhighenergydensitiesfortheexchange-couplednanocomposites. 100

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Inordertodeterminetheeectivethicknesswithinwhichthemagneticmomentsinthesoftphaseareeectivelycoupledtothehardphase,KnellerandHawig[ 77 ]suggestedacriticaldimensionbcm,whichistheBlochdomainwallthicknessinthesoftphasethatmakestheenergydensityofthedomainwallequaltotheenergydensityinthehardphaseatitsequilibriumstate. Em=m bcm=Eok=ok ok(5{1) whereEistheenergydensityofadomainwall,isthedomainwallenergyperarea,oisthedomainwallenergyperareaatequilibrium,andoisthedomainwallwidthatequilibrium.ThenotationfollowingKnellerandHawigusesthesubscriptkforahardmagneticphaseasithasahighanisotropyandthesubscriptmforasoftmagneticphaseasitpossesseslargemagnetization. Assumingbcmomyields bcm=r Am 2Kk(5{2) whereAistheexchangestinessconstantandKistheanisotropyconstant.Themagneticmomentswithinbcmawayfromthephaseboundaryisconsideredeectivelycoupledtothehardphase. KnellerandHawigalsoarguedthatitseemsreasonableforpracticalpurposestotakethecriticalthicknessofthehardphasebckaboutequaltotheequilibriumdomainwallwidthofthehardphase. bck=ok=r Ak Kk(5{3) GenerallyAk
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fcclatticegiveavolumefractionof=24p 29%.Inabcclatticethevolumefractionofthehardphaseisalsop 3=649%. SkomskiandCoey[ 6 ]calculatedthenucleationeldofastructurewhichhasidealsoftinclusions(Km=0)inahardmatrixusingmicromagneticmodels.ThedierentialequationforsolvingtheorientationofthemagneticmomentsandthenucleationeldresemblesSchrodinger'sequation. )]TJ /F1 11.955 Tf 19.01 8.09 Td[(2A(r) 0Ms(r)r2m+2K1(r) 0Ms(r)m=Hnm(5{4) wherem=mx(r)^x+my(m)^ywiththemagnetizationM(r)=Ms(r)(mx(r)^x+my(r)^y+p 1)]TJ /F1 11.955 Tf 11.96 0 Td[(mx(r)2)]TJ /F1 11.955 Tf 11.95 0 Td[(my(r)2^z). ItwasfoundthatwhenthedimensionofthesoftinclusionisbelowtheBlochdomainwallwidthofthehardphaseok,thenucleationeldkeepsitsmaximumvalue.Whenthethicknessofthehardphasebetweenthesoftinclusionsismorethanok,thereductionofnucleationledduetotheexchangecouplingisnotconsiderable.Thustheok=p Kk=Akcanbeusedforthesizeofsoftinclusionsandtheseparationamongtheinclusions.Thesedimensionsarechosenforkeepinghighnucleationeldandensuringtheeectiveexchangecouplingbetweenhardandsoftphases,thustheycanbeconsideredaslowerbounds.Theoptimalstructureforthehighestmaximumenergydensityofnanocompositemagnetsrequiresalargervolumefractionofthesoftphase. Ghidiniandhiscolleagues[ 78 ]calculatedtheoptimaldimensionsformultilayerstructuresbasedonthemicromagneticmodel.Theyfoundthereisanupperboundforthesoftlayerthicknesstmtoachievehighmaximumenergydensityforcompositemagnetsevenwhenthehardlayerthicknesstktm.Theanalyticalexpressionoftheupperboundis tm=4s Am 0MsmMsk)]TJ /F1 11.955 Tf 11.95 0 Td[(4Kmtan)]TJ /F4 7.97 Tf 6.58 0 Td[(1s (4Kk)]TJ /F3 11.955 Tf 11.96 0 Td[(0Msk2)Ak (0MsmMsk)]TJ /F1 11.955 Tf 11.96 0 Td[(4Km)Am(5{5) 102

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Table5-1. Roomtemperaturemicromagneticparametersformagneticmaterials[ 34 ] MaterialMs(MA=m)Js(T)K(MJ=m3)A(pJ=m) Nd2Fe14B1.281.614.98Sm12Co171.001.254.216FePt1.141.436.610[ 84 ]CoPt0.801.014.910 -Fe1.712.150.04822Fe65Co351.952.450.0210.7Ni80Fe200.831.04-0.0027 Table5-2. Criticaldimensionsforexchange-couplednanocomposites[tm=tk(nm=nm)] Model-Fe/Nd2Fe14BFe65Co35/Nd2Fe14BFe65Co35=FePtFe65Co35=CoPt Kneller&Hawig9.4/8.06.6/8.05.7/7.76.6/9.0Skomski&Coey>2.5/>2.5>2.5/>2.5>2.6/>2.6>2.2/>2.2Ghidini,etal8.8/3.6y<11.7/-<12.5/-<15.1/Table 5-1 liststhemicromagneticparametersforthecommonmagneticmaterials.Basedonthesethreemodels,thesuggesteddimensionsofthesoftandhardphasesarecalculatedandlistedinTable 5-2 .Notethatalthoughthemodelsarebuiltfordierentnanocompositestructures,suchasmultilayerandsoftinclusionsinahardmatrix,thecalculateddimensionsprovidethegeneralguidanceforarbitrarystructures.Thecriticaldimensionsofbothphasesturnouttobeintherangeofafewnanometers. Thesaturationmagnetizationofnanocompositesarethevolumetricaverageofincludedphases. Ms=fkMsk+fmMsm(5{6) wherefisthevolumefractionofaphaseinthecompositeandfk+fs=1.Assumingoptimaldimensionsareknown,iftheactualdimensionofthehardphaseislargertheoptimumorthesoftphaseissmallerthanitsoptimum,thecoercityofthecompositeincreaseswiththeexpenseofalowersaturationmagnetization.Intheotherway,ifthe yOptimaldimensionscalculatedin[ 78 ]usingslightlydierentparameters. 103

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actualdimensionofthehardphaseissmallerthanitsoptimumandthesoftphaseislarger,thesaturationmagnetizationofthecompositeincreases,butthecoercivitymaygreatlydecrease,duetotheuncoupledsoftphase. Followingthepreviousanalysisthatthedimensionandvolumefractionofeachphasearecriticaltothemagneticpropertiesofthenanocomposites,thesoftandhardmagneticmaterialshavebothbeenusedfortheinclusionandmatrix.TheexploredcombinationofnanocmpositematerialsarelistedinTable 5-3 .RareearthNd2Fe17Bmagneticnanoparticlesarersttestedasthehardmagneticphaseforitssuprememagneticproperties.However,Nd2Fe17Bisnotchemicallystableandthefabricatedparticlesinlabdidnotexhibitsucientlyhighcoercity.TheNd2Fe17B=Fe67Co33nanocompositeonlydemonstratedthefeasibilityoffabricationprocess.Withtheconcernofchemicalstability,L10FePtnanoparticlesandelectroplatedCoPtmagnetswerechosenforthehardphase.Fe67Co33wasselectedforitshighest2.4Tsaturationmagnetization. Table5-3. Choiceofmaterials(inclusion/matrix)fortheexchange-coupledhardmagneticnanocomposites InclusionMatrix HighsaturationmagnetizationsoftmagnetMetalalloyhardmagnet RareearthhardmagnetNd2Fe17B=Fe67Co33MetalalloyhardmagnetFePt=Fe67Co33HighsaturationmagnetizationsoftmagnetFe67Co33=CoPt 5.2FePt/Fe-CoNanocomposites HardmagneticFePtnanoparticlesandelectroplatedsoftmagneticFe-Coarechosentoformexchange-couplednanocompositesforthehighcoercivityofFePtandthehighsaturationmagnetizationofFe-Co.Thetargetstructureishardmagneticnanoparticlesembeddedinasoftmatrix.Thevolumefractionofnanoparticlesisabout30%inan 104

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Figure5-2. OpticalimagesatthesurfaceofannealedFePt/Fe-Conanocomposites electro-inltratednanocomposite,thusthenanocompositeisexpectedtohaveahighmagnetization. TheFePtnanoparticlesaresynthesizedbycolleagueBinQiusingthermaldecompo-sitionmethodandstoredinhexane.thevolatilityofhexane,theparticlescanberapidlydepositedontosubstratesbyevaporationofsolvent.Thediameteroftheparticlesis6nm.Afterannealedinforminggasat700Cfor20minwithCaakescoveringtheparticlelm,FePtconvertstoL10phaseanditscoercivitycanachieve1010kA/m. Fe-CoiselectroplatedthroughtheFePtparticlelayersusingBrankovic'srecipe[ 69 ]underconstantcurrentwithadensityof5{10mA=cm2.ThesubstrateshaveTi/Cuseedlayersanda5cmby5cmCoplateisusedforanode.Theelectro-inltratedsamplsareannealedinthesamewayastheFePtparticles.Fig. 5-2 showtheopticalimagesofsurfaceofthecompositelmafterannealingina140mby140msquarearray.ThecracksformduringtheevaporativedepositionofFePtnanoparticlesandtheannealingasthesamplevolumeshrink. Them-HhysteresisloopsoftheFePt/Fe-ConanocompositesandthesinglephasesamplesofFePtandFe-ColmsareshowninFig. 5-3 .ForthetwonanocompositesamplesinFig.a,theelectroplatedFe-CoincreasesthesaturatedmagneticmomentsandtheremanentmagneticmomentscomparedtheFePtsampleandthecoercivitiesremainaround800kA/m.Inthesecondquadrant,thekinkswithgraduallychangingslopesappearindicatingpartiallycoupledsoftphases,aspointedoutbythearrow.Fig.bshows 105

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Figure5-3. Them-HhysteresisloopsofFePt/Fe-ConanocompositescomparedwithFePtandFe-Colms ananocompositesamplewithalargeFe-CovolumefractionandanFe-Colm.DuetothelargeportionofuncoupledFe-Co,thecoercivityofthenancompositedropsto16kA/m.Followingthedemagnetizationcurve,theslopeofpartofthecurveislesssteepthanthatoftheFe-Co,alsoindicatingapartiallycoupledFe-Cophase. Theroughmorphologyofthenanocompositeandparticlesamplesmakesitdiculttoevaluatethesamplevolumes,thusthemagnetizationsandthemaximumenergydensitycannotbecompared.Nonetheless,thelargeportionofuncoupledsoftphaseisobservedanditcannotbenettheenergydensity. Inordertoavoidtheuncoupledsoftphase,thestructuresofthesoftphaseandthehardphaseareswapped.ThesoftmagneticnanoparticlesembeddedinahardmagneticmatrixleadstothedevelopmentofFe-Co/CoPtnanocomposites.TheFePtnanoparticlessynthesizedbythermaldecompositionneedtobereplacedbyaqueouslyco-precipitatednanoparticles.TheoleicacidattachedtoFePtnanoparticlesurfaceduringthesynthesisprocessprovesdiculttoremoveevenafter10timesofDIwaterwash.Theoleicacidmakestheparticlesurfacehydrophobicthatpotentialpreventstheinltrativeelectroplatingandmayobstructtheexchangecouplingbetweentheparticleandmatrix. 106

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Table5-4. Elementalatomiccompositionsformagneticlms(at%)asmeasuredbyEDS MaterialOFeCoPt ConsolidatedCoFe2O467.621.011.4-ReducedCoFe2O4(AnnealedFe-Co)49.732.417.9-AnnealedCoPt--38.961.1AnnealedFe-Co/CoPtnanocomposite-14.441.444.5 5.3Fe-Co/CoPtNanocomposites Aqueouslyco-precipitatedCoFe2O4nanoparticlesembeddedintheelectroplatedCoPtformtheCoFe2O4=CoPtnanocomposites.TheatomicratioofFeandCointheparticlesmatchestheratiointhehighmagnetizationFe-Co.Inasingleannealingsession,theferriteparticlesarereducedtoFe-CoalloywhiletheCoPtmatrixisconvertedtotheL10phase. Theelementalcompositionsforthefabricatedsingle-phasematerialsandthenanocomposite,basedontheEDSdata,areshowninTable 5-4 .ThesynthesizedCoFe2O4nanoparticlesexhibita1.8:1atomicratiobetweenFeandCo,closetothe2:1ratiointheferritecomposition,andtheratioremainsaftertheparticlesareannealed.TheoxygenobservedintheannealedparticlelayermaybeattributedtotheoxidizationofFe-Coalloyafterannealing.ThecompositionratioofCotoPtismeasuredtobearound39:61,whichdeviatesfromtheidealratioof50:50requiredfortheL10phase.However,theTiseedlayercontributesadditionalsignalinthemeasuredcompositionratio. AsshowninFig. 5-4 a,theresultantFe-Co/CoPtnanocompositeexhibitsarelativelysmoothhysteresiscurveindicativeofexchangecoupledmediumwithasmallkink(asopposedtoamixtureoftwophases,whichmayexhibitapronouncedkink).Thenanocompositeexhibitsahighsaturationmagnetizationof0.97Tandaremanentmagnetizationof0.70T,muchhigherthanthe0.45Tand0.42T,respectively,fortheCoPtlm.Thecoercivityof127kA/mis20xlargerthanthecoercivityofthereduced/annealedFe-Coparticlelm(6.6kA/m),butaboutone-sixththatoftheCoPtlm(784kA/m).Consequently,thenanocompositemaximumenergydensityof21.8 107

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Figure5-4. (a)HysteresisloopsoftheannealedCoPtlmandtheFe-Co/CoPtnanocompositelmmeasuredinplane.(b)Energydensityofthemagnetsinthesecondquadrant.ReprintedfromX.Wen,J.S.Andrew,andD.P.Arnold,\Exchange-coupledhardmagneticFe-Co/CoPtnanocompositelmsfabricatedbyelectro-inltration,"AIPAdvances,vol.7,no.056225,2017,withpermissionfromAIPPublishing. kJ=m3isslightlylowerthantheCoPtlm(26.8kJ=m3).InFig. 5-5 a,theFORCdiagram[ 85 ]ofthenanocompositeshowstwomagneticphases:thedominantexchange-coupledphaseexhibitsswitchingeldsrangingfrom50to120kA/m,andasmallerphaseappearswith5kA/mswitchingeld,whichispresumablyuncoupledFe-Co,consistentwiththesmallkinkobservedinthehysteresiscurve.TheMplot[ 86 ]oftheFe-Co/CoPtnanocompositeshowninFig. 5-5 balsoconrmstheexistenceofexchange-couplingbetweenthesoftandhardmagneticphases.ThepositiveMvaluesat20{90kA/mindicateexchangecouplingandmatchwiththedominantphaseintheFORCdiagram.Themaximumenergydensityofthenanocompositecanbefurtherimprovedbyavoidingtheuncoupledsoftphase. 5.4Summary FePt/Fe-CoandFe-Co/CoPtnanocompositelmsarefabricatedusinganelectro-inltrationprocess,whichutilizesmagneticnanoparticlesandelectroplatingtoformthree-dimensionalcompositestructures.TheFe-Co/CoPtnanocompositelmexhibitsa 108

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Figure5-5. (a)FORCdiagramand(b)MplotoftheFe-Co/CoPtnanocompositelm.ReprintedfromX.Wen,J.S.Andrew,andD.P.Arnold,\Exchange-coupledhardmagneticFe-Co/CoPtnanocompositelmsfabricatedbyelectro-inltration,"AIPAdvances,vol.7,no.056225,2017,withpermissionfromAIPPublishing. saturationmagnetizationof0.97T,aremanentmagnetizationof0.70Tandacoercivityof127kA/m,resultinginamaximumenergydensityof21.8kJ=m3.TheexchangecouplingbetweensoftmagneticFe-CophaseandhardmagneticCoPtphasewasconrmedinFORCdiagramandMplot.Themaximumenergydensityisexpectedtoimproveinafullycouplednanocomposite.Additionallytheseresultsmotivatefollow-oneortstomorethoroughlystudytheFe-Co/CoPtnanocompositestructure(e.g.grainsize,grainboundaries,volumefractions,diusioneects)throughfurtherx-raydiraction(XRD)andtransmissionelectronmicroscopy(TEM)measurementsandanalysis. 109

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CHAPTER6CONCLUSION Theelectro-inltrationprocesshasbeenexploredtomakemagneticnanocompositematerials.Ithasbeendemonstratedthattheprocesscanformadensecompositematerialinaparticle-matrixstructure.Theprocessisconductedatroomtemperatureusingsimplesetupwithouttherequirementofvacuumenvironment.Itisageneralprocesscapableofusingsortsofnanomaterialsandmetalstodepositnanocompositesforvariousfunctionalapplications. 6.1Summary Intheexplorationoftheelectro-inltrationprocesstowardsthecontrolofnano-compositematerialstructureandthebettermagneticproperties,theresearchresultshavedemonstratedtheencouragingoutcomeinthreeaspects. 1.Thecompleteprocesshasbeendevelopedforthefabricationofnanocompositematerials,includingthenanoparticleconsolidationandtheinltrativeelectroplating. Themaghemitenanoparticles,nickelzincferritenanoparticles,cobaltferritenanoparticlesandironplatinumnanoparticleshavebeendepositedintheformofthinlmsbyevaporativeassembly.Withouttheaidofbindingmaterial,theparticlelmsshowthegoodadhesiontosubstratesandmaintainthemorphologyinaqueoussolutions.Thethicknessofcrack-freelmisaroundhalfamicron,andmorethan10-m-thicklmshavebeenmadewiththetoleranceofcracks.Thevolumefractionofparticlelmisaround30%. Theinltrativeelectroplatingprocessesforpermendur,permalloyandcobaltplatinumalloyhavebeendeveloped.Itisconrmedthatthedepositedmetalmatricesllthevoidsamongparticlesandformdensenanocompositematerials.Themetalmatricesshowapreferencetodepositintheparticlelmratherthanformingapuremetallayer.Thevolumefractionofparticlesinananocompositelmalsomaintainsatthelevelaround 110

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30%.Theferritenanoparticlescansustainthecorrosivechemicalenvironmentduringelectroplating. Whenannealingstepisnotneeded,theelectro-inltrationprocessutilizesroomtemperatureprocedures,whicharecompatiblewiththestandardphotolithographyandthepost-CMOSprocesses. 2.Magneticpropertiesofsoftmagneticnanocompositehavebeeninvestigated.The-Fe2O3/Fe-CoandNi0:5Zn0:5Fe2O4/Ni-Fenanocompositeswerefabricatedandcharac-terized.Thesaturationmagnetization,coercivityandpermeabilityofnanocompositesarebetweentheperformanceofparticlelmandpuremetallm.Thenanocompositeshowsahighoperationbandwidthcomparedtothemetallmanditsproductofpermeabilityandcutofrequencyislargerthanboththeparticlelmandthemetallm.Thepeakoftheimaginarypartofpermeabilityofthenanocompositeisreducedinamplitude,indicatinglowerenergyloss.Thepropertiesofnanocompositearepromisingforpotentialhighfrequencyapplications. 3.Hardmagneticnanocompositematerialsfabricatedbyelectro-inltrationhasbeenexplored.Theannealingstepaftertheelectro-inltrationhassuccessfullyreducedferritetoalloyandinducedphasetransition.TheFe-Co/CoPtnanocompositeexhibitsthedesiredexchangecouplingeectbetweensoftandhardphasesthatencouragesfurtheroptimization. 6.2FutureWork Duringtheinvestigation,morequestionsaregeneratedthanthosearesolved.Futureresearchwillbeguidedbythefollowingquestions.Howdoestheelectro-inltraionprocessaectthematerialproperties?Cantheelectro-inltratednanocompositefabricatenewfunctionalmaterials? Inordertoobtainandcreatetheanswerstothesequestions,thefutureworkissuggestedinfourthrusts. 111

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6.2.1FirstDirection:StructureStudy Thersttopicistostudythestructureandthemagneticpropertiesoftheelectro-inltratedmetalmatrix.Eventhoughtheelectro-inltratednanocompositeswithnon-magneticnanoparticlesshowlargercoercivitythennormallyelectroplatedmetals,itisnotclearhowtheinltrativeelectroplatingaectthecrystalstructureofthemetalmatrix.ThemorphologyofmatricescanbestudiedbySEM,andthecrystalstructuretestedbyX-raydiraction(XRD), 6.2.2SecondDirection:SoftMagneticNanocomposites Apermeameterhasbeenmadepreviouslytocharacterizehighfrequencycomplexpermeabilityforthinlms.Thispermeameteriscapableofmeasuringalloylmswhichhavearelativelylargepermeability,butnotsensitiveenoughtomeasureparticlelms.Themethodofmeasuringthepropagationconstantforpermeabilityhasahighsensitivityoverthefrequencyrangeofhundredsofmegahertztogigahertz.Sinceabroaderfrequencyrangedownto1MHzandahighersensitivityispreferred,apick-upcoiltypepermeametercanbedesignedandfabricated.HFSSsoftwareishelpfulforthedevicedesignandthemillingtoolsareaccessibleforfabrication. Afterthenanocompositedemonstratesgoodsoftmagneticproperties,prototypeinductorsutilizingcompositecorecanbedesignedandfabricatedtoshowtheenhance-mentoninductance.Fullloopcoresaresuggested,eitherfortoroidinductors,ortowraparoundthespiralinductors. 6.2.3ThirdDirection:HardMagneticNanocomposites Thethirdtopicistonetunethematerialstructureoftheelectro-inltratednanocompositeforpermanentmagnets.IntheFe-Co/CoPtnanocompositemagnet,thegeometryanddimensionsoftheparticlesandmetalmatrixarethekeytohardmagneticproperties.Thesynthesisorlteringofhighlymonodispersedmagneticnanoparticlesmaybetherststeptotry.Ashightemperatureannealingistypicallyrequiredforhardmagnets,theeectofinterdiusionneedtobeevaluated. 112

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6.2.4FourthDirection:SuperlatticeMetaconductor Laminatedsuperlatticestructureincorporatingcopperandmagneticthinlmshasdemonstratedthecancelingofeddycurrenteectthatleadstolowlossconductoratradiofrequency[ 87 ].Utilizingelectro-inltrationprocesstobuildnanocompositeswithmagneticinclusionsembeddedincoppermatrixcanbeafastandcheapprocesstofabricatelowlossconductor.However,thenanocompositestructureoftheparticlesinmatrixmaynotbeidealforthemetaconductor.Thenanobersornanowiresinamatrixthataligntothemagneticuxlinesofthecurrentinmetaconductormaygiveabetterperformance. 113

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BIOGRAPHICALSKETCHXiaowasbornandraisedinLuzhoubyYangtzeRiver,andhegrewupinHainanIsland.HegraduatedwithaB.S.ininformationengnieeringin2011fromShanghaiJiaoTongUniversity.XiaodecidedtostudyMEMSandattendedthegraduateschoolintheelectricalandcomputerengineeringdepartmentattheUnivesityofFlorida.HejoinedInterdisciplinaryMicrosystemsGroupandstudiedmicrofabricationandmagneticsundertheguidanceofDr.DavidP.Arnold.HeearnedhisM.S.inelectricalengineeringin2013andhisPh.D.degreeinelectricalengineeringin2017. 121