time-domain measurement of ultrafast magne4za4on dynamics
TRANSCRIPT
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Time-DomainMeasurementofUltrafastMagne4za4onDynamicsinMagne4c
Nanopar4cles
MastersofScienceThesisBrianEgenrietherNovember2015
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Agenda
• Introduc?onandpurpose• Experimentalobjec?ves• Theore?calintroduc?ons• Induc?vetechniquedescrip?on• Experimentaldetailsandconsidera?ons• Resultsandinterpreta?ons• Ques?ons
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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.
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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.
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LarmorEqua?on
TheLarmorequa?onpredictsmagne?za?onwillprecessaboutanexternalfieldindefinitelyinaplaneorthogonaltothefield.
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Landau-LifshitzTheory
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Induc?veTechniqueOverviewCoplanarwaveguideWithnanopar?clesample
RapidHfieldreorienta?on
Time-changingmagne?cfieldfrompar?cles
InducedvoltagebyFaraday’sLaw
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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.
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Superparamagne?sm• Indensecollec?onsofinterac?ngpar?cles,thecurvemaynotcompletelyclose
norcrossiden?callyatzero.• Themaximumvalueofmagne?za?onMsisfoundtobe532kA/m,whichisroughly
10%higherthanthetabulatedvalue.ErrorinFe3O4concentra?onorvolumetricmeasurementsofferrofluidcouldbethecause.
• Par?cleshavealsobeenfoundtohaveahigherMsduetosurfaceeffects,buttheconversehasalsobeenfound.
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TwoSampleGeometries
Circularsampleonwaveguide.
Stripsampleonwaveguide.
• Twotypesofnanopar?clesamplegeometrieswereprepared.• Ofthese,halfweredriedinadirectedmagne?cfield(2.5kA/m)andhalfwere
not.
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ExperimentLayout
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TemporalDricErrorCorrec?onToextracttheinduc?vesignalfromthestepvoltagewaveformsubtrac?vesynthesisisemployed.Astepsignalwithoutprecessionissubtractedfromonethathasprecession,leavingonlythedesiredinduc?vesignal.
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TemporalDricErrorCorrec?on
“Zeroing”stepsignalisnotalignedin?mewith“Precession”stepsignalduetoaslightdricintriggersignal.
Thisintroducesrela?velylargevoltagespikesandanapparentlynoisysignal(red).Signalsmustbe?meshicedtocorrelatethembetween0Vand-2.5V.Thisyieldstheactualsignal(blue).
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Time-DomainResults
Exampleofatypicalcorrectedmeasurement.Notethemeasuredvoltagesignaliss?llnotexactlyadampedsinewaveaspredicted.Why?
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FFTResults
TwomainresonancepeaksareseenwhenanFFTisdoneon?me-domainresults.Theymustbothbeaccountedforina?medomaindatafit.Thelowfrequencymodeistheknownresonantfrequencyofmagne?tefromFMRexperiments.Thedefiniteoriginofthehighermodeispresentlyunknown.
FFT
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Time-DomainDataFit
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Time-DomainDataFit
Circularfield-drieddata,resultstypical.
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Time-DomainDataFits
Circularfield-drieddata,resultstypical.
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LowFrequencyMode
Circularfield-drieddata,resultstypical.
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HighFrequencyMode
Circularfield-drieddata,resultstypical.
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FrequencyDomainAnalysis
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FrequencyDomainFitCircularField-DriedData
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FrequencyDomainFitStripField-DriedData
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FrequencyDomainFitCircularNonField-DriedData
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FrequencyDomainFitStripNonField-DriedData
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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.
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Field-DriedDamping
Circularfield-drieddamping. Stripfield-drieddamping.
• Thelowmodedampinghasanearlymonotonicdecreaseasbiasfieldincreases.• Thehighmodereachesaminimumatdifferentpointsandincreases
drama?cally.
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NonField-DriedDamping
• Thelowmodedampingforthestriphasanearlymonotonicdecreaseasbiasfieldincreasesbutthecircleincreases(asdoestheerror).
• Thehighmodereachesaminimumatdifferentpointsandincreasesmuchlessthanforthefield-driedcase.
Circularnonfield-drieddamping. Stripnonfield-drieddamping.
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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?