大学物理实验报告英文版.docx

大学物理实验报告英文版.docx

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大学物理实验报告英文版

大学物理实验报告

FerroelectricControlofSpinPolarization

ABSTRACT

Acurrentdrawbackofspintronicsisthelargepowerthatisusuallyrequiredformagneticwriting,incontrastwithnanoelectronics,whichrelieson“zero-current,”gate-controlledoperations.Effortshavebeenmadetocontrolthespin-relaxationrate,theCurietemperature,orthemagneticanisotropywithagatevoltage,buttheseeffectsareusuallysmallandvolatile.Weusedferroelectrictunneljunctionswithferromagneticelectrodestodemonstratelocal,large,andnonvolatilecontrolofcarrierspinpolarizationbyelectricallyswitchingferroelectricpolarization.Ourresultsrepresentagianttypeofinterfacialmagnetoelectriccouplingandsuggestalow-powerapproachforspin-basedinformationcontrol.

Controllingthespindegreeoffreedombypurelyelectricalmeansiscurrentlyanimportantchallengeinspintronics（1, 2）.Approachesbasedonspin-transfertorque（3）haveprovenverysuccessfulincontrollingthedirectionofmagnetizationinaferromagneticlayer,buttheyrequiretheinjectionofhighcurrentdensities.Anidealsolutionwouldrelyontheapplicationofanelectricfieldacrossaninsulator,asinexistingnanoelectronics.Earlyexperimentshavedemonstratedthevolatilemodulationofspin-basedpropertieswithagatevoltageappliedthroughadielectric.Notableexamplesincludethegatecontrolofthespin-orbitinteractioninIII-Vquantumwells（4）,theCurietemperature TC （5）,orthemagneticanisotropy（6）inmagneticsemiconductorswithcarrier-mediatedexchangeinteractions;forexample,（Ga,Mn）Asor（In,Mn）As.Electricfield–inducedmodificationsofmagneticanisotropyatroomtemperaturehavealsobeenreportedrecentlyinultrathinFe-basedlayers（7, 8）.

Anonvolatileextensionofthisapproachinvolvesreplacingthegatedielectricbyaferroelectricandtakingadvantageofthehystereticresponseofitsorderparameter（polarization）withanelectricfield.Whencombinedwith（Ga,Mn）Aschannels,forinstance,aremanentcontrolof TC overafewkelvinwasachievedthroughpolarization-drivenchargedepletion/accumulation（9, 10）,andthemagneticanisotropywasmodifiedbythecouplingofpiezoelectricityandmagnetostriction（11, 12）.Indicationsofanelectricalcontrolofmagnetizationhavealsobeenprovidedinmagnetoelectricheterostructuresatroomtemperature（13–17）.

Recently,severaltheoreticalstudieshavepredictedthatlargevariationsofmagneticpropertiesmayoccuratinterfacesbetweenferroelectricsandhigh-TC ferromagnetssuchasFe（18–20）,Co2MnSi（21）,orFe3O4 （22）.Changingthedirectionoftheferroelectricpolarizationhasbeenpredictedtoinfluencenotonlytheinterfacialanisotropyandmagnetization,butalsothespinpolarization.Spinpolarization[.,thenormalizeddifferenceinthedensityofstates（DOS）ofmajorityandminorityspincarriersattheFermilevel（EF）]istypicallythekeyparametercontrollingtheresponseofspintronicssystems,epitomizedbymagnetictunneljunctionsinwhichthetunnelmagnetoresistance（TMR）isrelatedtotheelectrodespinpolarizationbytheJullièreformula（23）.Thesepredictionssuggestthatthenonvolatilecharacterofferroelectricsattheheartofferroelectricrandomaccessmemorytechnology（24）maybeexploitedinspintronicsdevicessuchasmagneticrandomaccessmemoriesorspinfield-effecttransistors

（2）.However,thenonvolatileelectricalcontrolofspinpolarizationhasnotyetbeendemonstrated.

WeaddressthisissueexperimentallybyprobingthespinpolarizationofelectronstunnelingfromanFeelectrodethroughultrathinferroelectricBaTiO3 （BTO）tunnelbarriers（Fig.1A）.TheBTOpolarizationcanbeelectricallyswitchedtopointtowardorawayfromtheFeelectrode.Weusedahalf-metallic（25）bottomelectrodeasaspindetectorintheseartificialmultiferroictunneljunctions（26, 27）.Magnetotransportexperimentsprovideevidenceforalargeandreversibledependenceofthe TMR onferroelectricpolarizationdirection.

Fig.1

（A）Sketchofthenanojunctiondefinedbyelectricallycontrollednanoindentation.Athinresistisspin-coatedontheBTO（1nm）/LSMO（30nm）bilayer.Thenanoindentationisperformedwithaconductive-tipatomicforcemicroscope,andtheresultingnano-holeisfilledbysputter-depositingAu/CoO/Co/Fe.（B）（Top）PFMphaseimageofaBTO（1nm）/LSMO（30nm）bilayerafterpolingtheBTOalong1-by-4–μmstripeswitheitheranegativeorpositive（tip-LSMO）voltage.（Bottom）CTAFMimageofanunpoledareaofaBTO（1nm）/LSMO（30nm）bilayer.Ω,ohms.（C）X-rayabsorptionspectracollectedatroomtemperatureclosetotheFeL3,2 （top）,BaM5,4 （middle）,andTiL3,2 （bottom）edgesonanAlOxnm）/Alnm）/Fe（2nm）/BTO（1nm）/LSMO（30nm）（D）HRTEMand（E）HAADFimagesoftheFe/BTOinterfaceinaTa（5nm）/Fe（18nm）/BTO（50nm）/LSMO（30nm）Thewhitearrowheadsin（D）indicatethelatticefringesof{011}planesintheironlayer.[110]and[001]indicatepseudotetragonalcrystallographicaxesoftheBTOperovskite.

ThetunneljunctionsthatweusedinthisstudyarebasedonBTO（1nm）/LSMO（30nm）bilayersgrownepitaxiallyonto（001）-orientedNdGaO3 （NGO）single-crystalsubstrates（28）.Thelarge（~180°）andstablepiezoresponseforcemicroscopy（PFM）phasecontrast（28）betweennegativelyandpositivelypoledareas（Fig.1B,top）indicatesthattheultrathinBTOfilmsareferroelectricatroomtemperature（29）.ThepersistenceofferroelectricityforsuchultrathinfilmsofBTOarisesfromthelargelatticemismatchwiththeNGOsubstrate（–%）,whichisexpectedtodramaticallyenhanceferroelectricpropertiesinthishighlystrainedBTO（30）.ThelocaltopographicalandtransportpropertiesoftheBTO（1nm）/LSMO（30nm）bilayerswerecharacterizedbyconductive-tipatomicforcemicroscopy（CTAFM）（28）.Thesurfaceisverysmoothwithterracesseparatedbyone-unit-cell–highsteps,visibleinboththetopography（29）andresistancemappings（Fig.1B,bottom）.NoanomaliesintheCTAFMdatawereobservedoverlateraldistancesonthemicrometerscale.

WedefinedtunneljunctionsfromthesebilayersbyalithographictechniquebasedonCTAFM（28, 31）.Topelectricalcontactsofdiameter~10to30nmcanbepatternedbythisnanofabricationprocess.Thesubsequentsputterdepositionofa5-nm-thickFelayer,cappedbyaAu（100nm）/CoOnm）/Conm）stacktoincreasecoercivity,definedasetofnanojunctions（Fig.1A）.ThesameAu/CoO/Co/FestackwasdepositedonanotherBTO（1nm）/LSMO（30nm）sampleformagneticmeasurements.Additionally,aTa（5nm）/Fe（18nm）/BTO（50nm）/LSMO（30nm）sampleandaAlOxnm）/Alnm）/Fe（2nm）/BTO（1nm）/LSMO（30nm）samplewererealizedforstructuralandspectroscopiccharacterizations.

Weusedbothaconventionalhigh-resolutiontransmissionelectronmicroscope（HRTEM）andtheNIONUltraSTEM100scanningtransmissionelectronmicroscope（STEM）toinvestigatetheFe/BTOinterfacepropertiesoftheTa/Fe/BTO/LSMOsample.TheepitaxialgrowthoftheBTO/LSMObilayerontheNGOsubstratewasconfirmedbyHRTEMandhigh-resolutionSTEMimages.Thelow-resolution,high-angleannulardarkfield（HAADF）imageoftheentireheterostructureshowsthesharpnessoftheLSMO/BTOinterfaceoverthestudiedarea（Fig.1E,top）. Figure1D revealsasmoothinterfacebetweentheBTOandtheFelayers.WhereastheBTOfilmisepitaxiallygrownontopofLSMO,theFelayerconsistsoftexturednanocrystallites.Fromthein-plane（a）andout-of-plane（c）latticeparametersinthetetragonalBTOlayer,weinferthat c/a =±,ingoodagreementwiththevalueoffoundwiththeuseofx-raydiffraction（29）.TheinterplanardistancesforselectedcrystallitesintheFelayer[.,~Å（Fig.1D,whitearrowheads）]areconsistentwiththe{011}planesofbody-centeredcubic（bcc）Fe.

WeinvestigatedtheBTO/FeinterfaceregionmorecloselyintheHAADFmodeoftheSTEM（Fig.1E,bottom）.OntheBTOside,theatomicallyresolvedHAADFimageallowsthedistinctionofatomiccolumnswheretheperovskiteA-siteatoms（Ba）appearasbrighterspots.Latticefringeswiththecharacteristic{100}interplanardistancesofbccFe（~Å）canbedistinguishedontheoppositeside.Subtlestructural,chemical,and/orelectronicmodificationsmaybeexpectedtooccurattheinterfacialboundarybetweentheBTOperovskite-typestructureandtheFelayer.TheseeffectsmayleadtointerdiffusionofFe,Ba,andOatomsoverlessthan1nm,orthelocalmodificationoftheFeDOScloseto EF,consistentwithabinitiocalculationsoftheBTO/Feinterface（18–20）.

TocharacterizetheoxidationstateofFe,weperformedx-rayabsorptionspectroscopy（XAS）measurementsonaAlOxnm）/Alnm）/Fe（2nm）/BTO（1nm）/LSMO（30nm）sample（28）.Theprobedepthwasatleast7nm,asindicatedbythefiniteXASintensityattheLaM4,5 edge（28）,sothattheentireFethicknesscontributedsubstantiallytothesignal.Asshownin Fig.1C （top）,thespectrumattheFeL2,3 edgecorrespondstothatofmetallicFe（32）.TheXASspectrumobtainedattheBaM4,5 edge（Fig.1C,middle）issimilartothatreportedforBa2+ in（33）.Despitethepoorsignal-to-noiseratio,theTiL2,3 edgespectrum（Fig.C,bottom）showsthetypicalsignatureexpectedforavalencecloseto4+（34）.FromtheXAS,HRTEM,andSTEManalyses,weconcludethattheFe/BTOinterfaceissmoothwithnodetectableoxidationoftheFelayerwithinalimitoflessthan1nm.

Aftercoolinginamagneticfieldof5kOealignedalongthe[110]easyaxisofpseudocubicLSMO（whichisparalleltotheorthorhombic[100]axisofNGO）,wecharacterizedthetransportpropertiesofthejunctionsatlowtemperatureK）. Figure2A （middle）showsatypicalresistance–versus–magneticfield R（H）cyclerecordedatabiasvoltageof–2mV（positivebiascorrespo