time-domain measurement of ultrafast magne4za4on dynamics

Time-DomainMeasurementofUltrafastMagne4za4onDynamicsinMagne4c

Nanopar4cles

MastersofScienceThesisBrianEgenrietherNovember2015

Agenda

•  Introduc?onandpurpose•  Experimentalobjec?ves•  Theore?calintroduc?ons•  Induc?vetechniquedescrip?on•  Experimentaldetailsandconsidera?ons•  Resultsandinterpreta?ons•  Ques?ons

Introduc?onandPurpose•  Magne?cnanopar?clescomposedofmagne?te(Fe3O4)withadiameterof

10nmarestudied.•  Similarpar?clesareusedininvivomedicalimaging,magne?csensors,

drugdelivery,cancerresearch,andmicroscopicdiffrac?ongra?ngs(CrawfordGroup),etc.

•  Inprac?callyalloftheseapplica?ons,thepar?clesinteractwithfluctua?ngmagne?cfields.

•  Largeamountsoffrequency-domainresearchhasbeendoneonmagne?cnanopar?cles,howeveralmostno?me-domaindataexistsduetothesmallnessofthepar?clesandthehighspeedoftheprocess(~2ns).

•  Havinganideaofhowtheybehaveinthe?medomainyieldsabe[erunderstandingofhowtoemploytheminprac?ce.

•  Italsoservesthefieldofscien?ficinquiry.

ExperimentalObjec?ves•  Todetermineifcollec?onsofmagne?cnanopar?clesundergothe

precessiondynamicspredictedbytheLandau-Lifshitz(LL)theoryandifthese?me-domaindynamicscanbemeasuredbyamagne?cinduc?ontechniquetobedescribed.

•  Tofitthe?me-domaindatatoadampedsinusoidalsolu?ontotheLLequa?on.

•  Tocalculatethefrequency-domainresponsebyemployingaFastFourierTransformtothe?me-domaindata.

•  Tounderstandtheprogressionofthefrequencyasafunc?onoftheappliedmagne?cbiasfield.

•  Toes?matethephenomenologicaldampingparameterofthemagne?tepar?clesasafunc?onofappliedfieldandthespectroscopicspli]ngfactor(org-factor),ofthepar?cles.

LarmorEqua?on

TheLarmorequa?onpredictsmagne?za?onwillprecessaboutanexternalfieldindefinitelyinaplaneorthogonaltothefield.

Landau-LifshitzTheory

Induc?veTechniqueOverviewCoplanarwaveguideWithnanopar?clesample

RapidHfieldreorienta?on

Time-changingmagne?cfieldfrompar?cles

InducedvoltagebyFaraday’sLaw

Superparamagne?sm•  Smallmagne?cpar?cleshavemomentsthatfluctuatetheirorienta?ondueto

thermalexcita?ons.•  Forsinglepar?clesaboveacertaintemperature(the“blocking”temperature)the

netmagne?za?onoveranextended?meiszero.

•  Themagne?za?onisanonlinearfunc?onoftheexternalmagne?cfield.

•  ThiseffectcausestheM-Hcurveofthepar?clesto

close,exhibi?ngnoremanenceorcoercivity.

•  Fordensecollec?onsofinterac?ngpar?cleshowever,theeffec?veblockingtemperaturecanberaised,inducingamixed-stateofferrimagne?smandsuperparamagne?sm.

Superparamagne?sm•  Indensecollec?onsofinterac?ngpar?cles,thecurvemaynotcompletelyclose

norcrossiden?callyatzero.•  Themaximumvalueofmagne?za?onMsisfoundtobe532kA/m,whichisroughly

10%higherthanthetabulatedvalue.ErrorinFe3O4concentra?onorvolumetricmeasurementsofferrofluidcouldbethecause.

•  Par?cleshavealsobeenfoundtohaveahigherMsduetosurfaceeffects,buttheconversehasalsobeenfound.

TwoSampleGeometries

Circularsampleonwaveguide.

Stripsampleonwaveguide.

•  Twotypesofnanopar?clesamplegeometrieswereprepared.•  Ofthese,halfweredriedinadirectedmagne?cfield(2.5kA/m)andhalfwere

not.

ExperimentLayout

TemporalDricErrorCorrec?onToextracttheinduc?vesignalfromthestepvoltagewaveformsubtrac?vesynthesisisemployed.Astepsignalwithoutprecessionissubtractedfromonethathasprecession,leavingonlythedesiredinduc?vesignal.

TemporalDricErrorCorrec?on

“Zeroing”stepsignalisnotalignedin?mewith“Precession”stepsignalduetoaslightdricintriggersignal.

Thisintroducesrela?velylargevoltagespikesandanapparentlynoisysignal(red).Signalsmustbe?meshicedtocorrelatethembetween0Vand-2.5V.Thisyieldstheactualsignal(blue).

Time-DomainResults

Exampleofatypicalcorrectedmeasurement.Notethemeasuredvoltagesignaliss?llnotexactlyadampedsinewaveaspredicted.Why?

FFTResults

TwomainresonancepeaksareseenwhenanFFTisdoneon?me-domainresults.Theymustbothbeaccountedforina?medomaindatafit.Thelowfrequencymodeistheknownresonantfrequencyofmagne?tefromFMRexperiments.Thedefiniteoriginofthehighermodeispresentlyunknown.

FFT

Time-DomainDataFit

Time-DomainDataFit

Circularfield-drieddata,resultstypical.

Time-DomainDataFits

Circularfield-drieddata,resultstypical.

LowFrequencyMode

Circularfield-drieddata,resultstypical.

HighFrequencyMode

Circularfield-drieddata,resultstypical.

FrequencyDomainAnalysis

FrequencyDomainFitCircularField-DriedData

FrequencyDomainFitStripField-DriedData

FrequencyDomainFitCircularNonField-DriedData

FrequencyDomainFitStripNonField-DriedData

FrequencyDomainDiscussion•  Thedataarewellfi[edtotheKi[leequa?onofferromagne?cresonancewhere

thedemagne?zingfactorsareseentodescribespheres,nottheoverallsample.•  ThescalefactorKisfoundtobepropor?onaltotheHfield,notthe

magne?za?onM(H)aswasexpectedforademagne?za?onfield.Thisisnotpresentlyunderstood.

•  Thequan?tyHAmaybea[ributedtotheslightremanentfieldfoundearlierorpossiblytothemagnetocrystallineanisotropyfieldgivenby

•  Pu]ngthevaluesK1=13kJ/m3andMs=480kA/myieldsHA=43kA/m,whichiswithin10%ofallvaluesofHAfoundfromprecessiondataatHb=0.

•  IfthelargestvalueofMs=532kA/mfoundfromtheVSMisused,theerrorislessthan1%,howeverthismeasurementhas2poten?alvolumetricerrorsmen?onedearlier.

Field-DriedDamping

Circularfield-drieddamping. Stripfield-drieddamping.

•  Thelowmodedampinghasanearlymonotonicdecreaseasbiasfieldincreases.•  Thehighmodereachesaminimumatdifferentpointsandincreases

drama?cally.

NonField-DriedDamping

•  Thelowmodedampingforthestriphasanearlymonotonicdecreaseasbiasfieldincreasesbutthecircleincreases(asdoestheerror).

•  Thehighmodereachesaminimumatdifferentpointsandincreasesmuchlessthanforthefield-driedcase.

Circularnonfield-drieddamping. Stripnonfield-drieddamping.

Conclusions•  The?me-domainsignalsarewellfi[edtotwoexponen?allydampedsinusoids.•  Thelowfrequencyistheresonantfrequencyofmagne?te,theoriginofthehigher

modehasnotbeendefini?velyiden?fied.•  Thesamplesalldisplaybehaviorofnearlysphericalobjectsforbothfrequencies.•  Fielddryingversusnonfield-dryingthesampleshasli[leeffectontheprecession

frequenciesasdoessampleshape.•  Theg-factorswerefoundtoagreewellwiththepreviouslyreportedvalue.•  Thebiasfieldreduc?onwasnotfoundtobeafunc?onofthemagne?za?on.•  Lowfrequencydampinggenerallydecreaseswithincreasingbiasfield,aresult

similar(qualita?vely)toimpulseinduc?onexperimentsonthinfilms.•  Highfrequencydampingislargelyaffectedbyfielddrying.Itincreases

drama?callyathighbiasfields.•  Duetodampingalmostallofthedynamicshavedissipatedwithin2ns.

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Ques?ons?