JouleheatinginducedcoexistedspinSeebeckeffectandspinHallmagnetoresistanceintheplatinumYWXWangSHWangLKZouJWCaiZGSunandJRSunStateKeyLaboratoryofAdvanceTechnologyforMaterialSynthesisan ID: 483493
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- R X O H K H D W L Q J L Q G X F H G F R H [ L V W H G V S L Q 6 H H E H F N H I I H F W D Q G V S L Q + D O O P D J Q H W R U H V L V W D Q F H L Q W K H S O D W L Q X P ) & L W D W L R Q \f G R L 9 L H Z R Q O L Q H 9 L H Z 7 D E O H R I & R Q W H Q W V 3 X E O L V K H G E \ W K H $ U W L F O H V \ R X P D \ E H L Q W H U H V W H G L Q $ S S O 3 K \ V / H W W \f - $ S S O 3 K \ V & \f - $ S S O 3 K \ V & \f - $ S S O 3 K \ V \f $ S S O 3 K \ V / H W W \f Jouleheating-inducedcoexistedspinSeebeckeffectandspinHallmagnetoresistanceintheplatinum/YW.X.Wang,S.H.Wang,L.K.Zou,J.W.Cai,Z.G.Sun,andJ.R.SunStateKeyLaboratoryofAdvanceTechnologyforMaterialSynthesisandProcessing,WuhanUniversityofTechnology,Wuhan430070,ChinaBeijingNationalLaboratoryforCondensedMatterPhysicsandInstituteofPhysics,ChineseAcademyofSciences,Beijing100190,PeoplesRepublicofChina(Received10September2014;accepted24October2014;publishedonline4November2014)SpinSeebeckeffect(SSE)andspinHallmagnetoresistance(SMR)areobservedsimultaneouslyinthePt/YhybridstructurewhenthermalgradientisproducedbyJouleheating.Accordingtotheirdependencesonappliedcurrent,thesetwoeffectscanbeseparated.TheirdependenceonheatingpowerandmagneticÞeldissystematicallystudied.Withtheincreaseofheatingpower,theSSEenhanceslinearly,whereastheSMRdecreasesslowly.Theoriginofthespincurrentsisfurtheranalyzed.TheheatingpowerdependencesofthespincurrentsassociatedwiththeSSEandtheSMRarefoundtobedifferent.2014AIPPublishingLLCLLChttp://dx.doi.org/10.1063/1.4901101Theexplorationforeffectiveapproachesforthegenera-tion,detection,andmanipulationofspincurrenthasbeenafocusofspintronics.ItwasrecentlyreportedthatthespinSeebeckeffect(SSE)canprovideafeasibleapproachtogen-eratestabilizedspincurrent:Whenatemperaturegradientisestablishedalongtheferromagnet(FM),aspincurrentwillbeformedalongtheFM.Thisspincurrentcanbedetectedbyanoblemetal(NM)andconvertedintoavoltageviainversespinHalleffect(ISHE).Sinceplatinumhasastrongspin-orbitcoupling,thusastrongISHE,itisusuallyutilizedasadetectorofthespincurrent.TheSSEhasbeenobservedinawidevarietyofmaterials,includingmagneticsemiconductors,andmagneticinsulators.BasedontheSSE,aconceptofspincaloritronicshasbeenpresented,andithasreceivedaworldwideattentioninrecentLongitudinalgeometryisthesimplestexperimentalsetupfortheSSEstudy,forwhichthethermalgradientwasestablishedbyeitherattachingaheatsourceandaheatsink,respectively,tothetwoendsofthesample,locallyheat-ingthesamplebyalaserbeam,orapplyingacurrentthroughtheNM.IntheÞrstcase,agoodthermalcontactbetweentheheatreservoirsandthesamplesisrequired.Whileinthecaseoflaserheating,thetemperaturegradientexistsonlyinaverylocalregion.BasedonJouleheating,etal.suggestedanewmethodtogeneratethermalgradient.Becauseofitsfeasibilityandcontrollability,thismethodshowsobviousadvantagesovertheothertwoones.Asreported,magneticproximityeffectandchargeandspincurrentinteractionexistinthePt/Yhybridstructure.Therefore,onemustbecarefulwhendeal-ingwiththeSSEinthiskindofstructure.AccordingtoMiaoetal.themagnetoresistance(MR)ofPt/YIGcouldbemainlyascribedtothespincurrentreßectionatthePt/YIGinterfaceinlowmagneticÞeldsandtotheproximityeffectunderhighÞelds.TheformeriscalledspinHallmagnetoresistance(SMR),anewMRmechanismproposedbyNakayamaetal.AccordingtoNakayamaetal.SMRisinducedbyanonequilibriumproximityeffect.DuetothecoexistedspinHalleffectandISHE,themomen-tumexchangeofthespincurrentatthePt/YIGinterfaceaffectsinreturntheresistanceofthePtstrip.Asaconse-quence,MRappearsinPtasthedirectionofthemagnetiza-tionofYIGrotates.Accordingtotheabovediscussions,theSSEandSMRmayappearsimultaneouslywhenestablishingthethermalgradientbyJouleheating.However,howtoseparatethesetwoeffectsandtheirmutualinßuenceremainunaddresseduptonowdespitetheintensivestudiesontherespectiveSSEandSMR.Inthisletter,theaccompaniedSSEandSMRwerereportedfortheJoule-heatedPt/YIGhybridstructure,andtheirrespectivedependencesonheatingpowerandmagneticÞeldsareestablished.Differencesofthespinaccumulationsassociatedwiththesetwoeffectsarediscussed.(111)-orientedGd(GGG)(50.5mmwasadoptedassubstratesanda21-mthicksinglecrystalYIGslabwasgrownabovetheGGGbyliquidphaseepitaxy.Hall-bar-shapedPtthinlayersweredepositedabovethesub-stratebymagnetronsputtering.Twotypicallayerthick-7nmand30nm,wereadoptedforthepresentstudies.Theresistivity,,ofthePtÞlmswasmfor7nmandmfor30nmattheroomtemperature,measuredbythestandardfour-probetechnique.APt(7nm)/Cu(3nm)/YIGsamplewasalsopre-paredforcomparisonstudy,wheretheCubufferlayerwasintroducedtodepressthemagneticproximateeffect.Thesamplesweremountedonaheatsink(copper)withther-mallyconductiveadhesive,andsealedinanelectromagneticshieldingbox.Theelectricmeasurementswereperformedundertheambientconditions.showsaschematicillustrationoftheexperi-mentalsetup.AlargecurrentwasappliedalongthelongdimensionofthePtÞlmtoestablishathermalgradientalongdirectionofthesample,andamagneticÞeldwasappliedinthesampleplaneatanangleofwithrespectto Authortowhomcorrespondenceshouldbeaddressed.Electronicaddresses:jrsun@iphy.ac.cnandsun_zg@whut.edu.cn0003-6951/2014/105(18)/182403/5/$30.002014AIPPublishingLLC,182403-1APPLIEDPHYSICSLETTERS,182403(2014) toorientthemagneticmomentoftheYIG.Atransversewasthenproducedandrecorded.Ingeneral,thethermalgradientinducedbyJouleheatingwillgenerateaspincurrentinjectionintoPt,thustheSSE.InadditiontotheSSE,accordingtoNakayamaetal.,thespincurrentreßec-tionatthePt/YIGinterfacemayalsoproduceaSMRinPt,yieldingavoltagechangealongthe-directionofthePtstip.InadditiontoSMR,theconventionalanisotropicmagnetore-sistance(AMR)isalsopossiblewhentheproximateeffectisstrong.Obviously,theSSEwillremainunaffectedwhereastheMReffect,eithertheSMRortheAMR,changessignwhilecurrentdirectionreverses.Sinceissimplypro-portionaltoheatingpower(),wehavethefollow-ingrelations)andandVxy(I)þVxy(I)]/2andandVxy(I)Vxy(I)]/2,i.e.,fromtheirdifferentdependencesonoppositelydirectedcurrents,theandtheMRcanbedistinguishedfromeachother.TheSSEandMReffectofthePt/YIGstructurewerestudiedbycyclingmagneticÞeldalongtheroute0.01T0.01T0.01T.Themeasurementbeginsabout20minaftertheapplicationoftheheatingcurrent,whenasteadythermalgradientisestablished.Figs.showthemagneticÞelddependenceof(blacklines),(bluelines),andMR(redlines),obtainedbyapplyingmagneticÞeldatanangleof,here,theMRisdeÞnedasasVMR(H)ÐVMR(0)]tPt/Iqxx(H).Tocomparetheeffectofcur-rentreversion,bothdataobtainedunderarepre-sented.Notably,theexhibitsastrongÞelddependenceinthelowÞeldregion,whichisthesignatureoftheMR,asÞrstreportedbyRef.peakappearsaround0,withasmallhysteresisfortheascending-descendingÞeldopera-tions.AccordingtoFig.,thisisexactlytheÞeldrangeforthemagneticreorientationofYIG.Thecurvesareupsidedownwhenthecurrentdirectionisreversed,indicat-ingthattheMRgovernsthe.AbovethesaturationÞeld,isnearlyconstant.ThisisunderstandablesincetheYIGhasbeenmagneticallysaturated.ThereisamisalignmentforthebaselinesofthecurvesunderthepositiveandnegativeÞelds.ThisisasignatureoftheSSE,whichcontrib-utesawhichchangessignasthemagneticalignmentoftheYIGreverses.TogetaquantitativecharacterizationoftheSSE,in1(c)1(d),weshowtherelations(bluecurves).Asexpected,isnearlyconstantwhenhigh,andundergoesasuddendrop/jumpasthroughthesaturationÞeldofYIG;themagneticreversionofYIGcausesthechange.AsimpleanalysisindicatesthattheVforPt(7nm)/YIGandVforPt(30nm)/YIG.Therearetwopossibleexplanationsfortheinthelattersample,i.e.,thethickPthasshuntedtheorthethermalgradientinthesetwosam-plesaredifferent.Itispossiblethatthethermalgradientpro-ducedbyJouleheatinginYIGisinhomogeneous,concentratingnearthePt/YIGinterface.ThesmallloopcanbeascribedtothemagnetichysteresisoftheYIG.TheMRisalsoshowninFigs.curves).ItgrowsrapidlywithwhentheÞeldislow,and FIG.1.(a)Aschematicillustrationoftheexperimentalsetup.AcurrentappliedtothePtstripandtheJouleheatinducedtemperaturegradientisestablishedinthedirectionoftheYIG.(b)NormalizedmagnetizationofYIG,presentedasafunctionofmag-neticÞeld.(c),(d),and(e)MagneticÞelddependencesofthetransverse(blackcurve),curve),andMR(redcurve)forthesamplesofPt(7nm)/YIG,Pt(30nm)/YIG,andPt(7nm)/Cu(3nm)/YIG,respectively.Themeasurementswereperformedwiththeappliedcurrentsof19mAforPt(7nm)/YIGandPt(7nm)/Cu(3nm)/YIG,and50mAforPt(30nm)/YIG.Here,themagneticÞeldisappliedinthesampleplaneatanangleofwithrespecttothecurrentdirection.182403-2Wangetal.Appl.Phys.Lett.,182403(2014) saturates,abovetheÞeldof50Oe,atthevalueof0.014%for7nmand0.0048%for30nm.ThedecreaseoftheMRwiththeincreaseofisconsistentwiththepreviouslyreportedresults.Asmentionedabove,eithertheSMRortheAMRmaycontributetoMR.AccordingtoNakayamaetal.,theSMRcouldbethemainmechanismdeterminingtheMRforthePt/YIGhybridstructure.AccordingtoMiaoetal.magneticproximityeffect,thustheAMRprevailsunderhighÞelds.TorevealtheoriginoftheMRobservedhere,theMRofthePt(7nm)/Cu(3nm)/YIGstructureisstudiedunderthesameconditionasthatofPt(7nm)/YIG.Fig.showsthattheMRremainswithouttheproximateeffect,thoughitsmagnitudereducesduetotheshort-circuiteffectofthehighlyconductiveCulayer.SinceCuhasalongspindiffusionlengthandaveryweakspin-orbitcoupling,itcancarryaspincurrentoveralongdistancebutpreventsthemagnetizationofthePtlayerbyproximateeffect.Basedonasimplecalculation,weshowthattheshort-circuiteffectofa3-nm-thickCulayerreducestheMRbyafactorofwhiletheexperimentallyobservedreductionisbyafactorof90.ConsideringthepossibleinßuenceoftheCulayeronspincurrent,thisagreementissatisfactory.CombiningtheresultsinFig.withthoseinRefs.,weconcludedthattheMRobservedinoursamplesisdominatedbySMR,i.e.,theSSEandSMRcoexistintheJoule-heatedPt/YIGhybridstructure.TocomparewiththeeffectofJouleheating,wealsomeasuretheSSEforaverticalstructurewhenthethermalgradientisestablishedbyaheaterunderneaththeGGG.Fig.showstheexperimentalsetup,andFig.2(b)istheexper-imentalresults.TwofeaturescanbeidentiÞedfromthedatainFig..First,thesignoftheproducedbyunderneathheatingisoppositetothatofthecausedbyJouleheating,indicatingthatthetemperaturegradientinthesetwocasesisopposite.Second,thevalue,whichVforPt(7nm)/YIGandVforPt(30nm)/YIG,ismuchlowerthanthatproducedbyJouleheating.ItmeansthattheJouleheatingismoreeffectiveinproducingthermalgradient.OnethingdeservingspecialattentionisthatthedifferenceoftheforthePt(7nm)/YIGandPt(30nm)/YIGissmallerthanthatobservedinthecaseofJouleheating(Figs.).Providedthatthethermalgradientinthecaseofunderneathheatingissimilarfortwosamples,thethermalgradientcausedbyJouleheatingcouldbedifferent,thoughtheheatingpowerissimilar.Torevealthephysicaloriginofthethermoelectricsignals,westudiedtherelationswhenmagneticÞeldisappliedatdifferentangles.InFigs.3(a),weshowvaluesdeducedfromthe,45,90,135,and180,respectively.Obviously,exhibitsastrongdependenceon.Itismaximaland180andminimalfor,varyingintheformof,whereVforPt(7nm)/YIG,0.76forPt(30nm)/YIG,and0.39VforPt(7nm)/Cu(3nm)/YIG(Fig.3(c)).Incontrast,theSMRtakesthemaximalvaluesatand135andtheminimalvaluesat,and.AfurtheranalysisindicatesthatSMRexhibitsavariationwith),similartothatobservedbyNakayamaetal.TheÞttingparametersareforPt(7nm)/YIG,5forPt(30nm)/YIG,and1.2forPt(7nm)/Cu(3nm)/YIG.Obviously,relationsownthetypicalfeaturesoftheSSEandSMR,respectively.TheaboveexperimentresultsindicatethatwhenthethermalgradientisestablishedbyJouleheating,theSSEandSMRcoexist,andcanbeseparatedandanalyzed FIG.2.(a)Aschematicillustrationoftheexperimentalsetup.(b)nalsasfunctionsofmagneticÞeld. FIG.3.(a)and(b)MagneticÞelddependenceoftheforthesampleofPt(7nm)/YIG,measuredatdifferentangles.LabelsintheÞguremarkthevaluesof.(c)and(d)angulardependenceofandSMRforPt(7nm)/YIG,Pt(30nm)/YIG,andPt(7nm)/Cu(3nm)/YIG.Solidlinesareresultsoftheoreticalcalculationbasedonthecosorsin2dependence,andsymbolsareexperimentdata.182403-3Wangetal.Appl.Phys.Lett.,182403(2014) independently.Togetanideaabouttheeffectofheating,inFigs.,weshowSMRasfunctionsof.Asexpected,theexhibitsalin-eargrowthwith:Heatingpowerenhancesthespincurrentbyincreasingthermalgradient.Fascinatingly,displaysamuchmorerapidincreasewiththeincreaseofforthesampleof7nmthanfor30nm,indicatingthatthethinnersampleismoresensitivetoJouleheating.Different,SMRundergoesaslightdecreasewith.ThistemperaturedependenceoftheSMRcanbetheoreticallyAccordingtoChenetal.themaximumSMRcanbedescribedby ktPt 2kGtanh2 tPt2kþ2kGtcoth isthespindiffusionlength,isthespinHallisthespinmixingconductance,andisthecon-ductivityofPt.Notingthatisafunctionoftemperature,accordingtotheElliot-Yafetassumption,oneÞndsthattheSMRwilldecreasewiththeincreaseoftempera-Therefore,theslightdecreaseoftheMRwithtemper-atureisthefurtherevidenceoftheSMReffect.Itsuggeststhatthiseffectiscloselyrelatedtothebulkspin-orbitalcou-plinginPt,stemmingfromthereductionofthespindiffu-sionlengthathightemperatures.ThisobservationisparticularlyinterestingsinceitishelpfulindistinguishingtheSMRfromtheeffectcausedbytheRashbaspin-orbitalcouplingatthePt/YIGinterface,asshownrecentlybyetal.Finally,wewouldliketopresentabriefdiscussionaboutthespincurrentsthatareresponsiblefortheSSEandtheSMR.AlthoughtheISHEplaysanimportantroleincausingboththeSSEandtheSMR,themechanismsforthespinaccumulationinthesetwocasesaredifferent.Asformu-lated,thespincurrentdensitythatcausestheSSEhasthe DVISHECg (2)whereistheopen-circuitspinHallconversionefÞ-isacorrectionfactor,isthewidthofthePtstrip.canbefurtherexpressedas 2ehhshktanh tPt2k (3) e2hcoth (4)Incontrast,thespincurrentassociatedwiththeSMRhastheSMR hshekwtanh (5)AdoptingtherepresentativeparametersofthePt/YIGhybrid0.11,and1.5nm,theSMRcanbedirectlycalculated.WeshowtherelationinFig.andtheSMRrelationinFig..BothSMRhavethevaluessimilartothoseaspreviouslyreported.exhibitsaquadraticdependenceonwhichisunderstandablesinceisproportionaltoHowever,theSMRessentiallylinearlygrowswith,indicat-ingthattheeffectoftemperatureisnotstrong.ThisresultisconsistentwiththedatainFig..Accordingtotheaboveanalyses,obviously,thespinaccumulationstemsfromthermalgradientfortheSSE,whereasitiscausedbythespintransfertorqueofdccurrentatthePt/YIGinterfacefortheSMR.Insummary,wedemonstratethatthecurrentheatingisaconvenientandeffectivemethodtoproducetheSSE.DuetothenonequilibriumproximityeffectofthePtÞlm,more-over,theheatingcurrentinducessimultaneouslyaSMR,aneffectjointlyproducedbytheSpinHalleffectandtheinversespinHalleffect.Withtheincreaseofheatingpower,theSSEenhanceslinearlywhereastheSMRdecreasesslowly.Theoriginofthespincurrentsisfurtheranalyzed,anddifferentdependencesonheatingcurrentareobservedforthespincurrentsassociatedwiththeSSEandSMR,indicatingthatthespinaccumulationaredifferentforthesetwoeffects.ThisworkhasbeensupportedbytheNationalBasicResearchofChina(GrantNos.2011CB921801and2012CB921403)andtheNationalNaturalScienceFoundationofChina(GrantNos.11174231,11074285,51372064,and11134007).H.Ohno,Nat.Mater.,952(2010).J.C.LeBreton,S.Sharma,H.Saito,S.Yuasa,andR.Jansen,82(2011). 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