大学物理实验报告FerroelectricControlofSpinPolarizationABSTRACTAcurrentdrawbackofspintronicsisthelargepowerthatisusuallyrequiredformagneticwriting,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),theCurietemperatureTC(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,aremanentcontrolofTCoverafewkelvinwasachievedthroughpolarization-drivenchargedepletion/accumulation(9,10),andthemagneticanisotropywasmodifiedbythecouplingofpiezoelectricityandmagnetostriction(11,12).Indicationsofanelectricalcontrolofmagnetizationhavealsobeenprovidedinmagnetoelectricheterostructuresatroomtemperature(13–17).Recently,severaltheoreticalstudieshavepredictedthatlargevariationsofmagneticpropertiesmayoccuratinterfacesbetweenferroelectricsandhigh-TCferromagnetssuchasFe(18–20),Co2MnSi(21),orFe3O4(22).Changingthedirectionoftheferroelectricpolarizationhasbeenpredictedtoinfluencenotonlytheinterfacialanisotropyandmagnetization,butalsothespinpolarization.Spinpolarization[i.e.,thenormalizeddifferenceinthedensityofstates(DOS)ofmajorityandminorityspincarriersattheFermilevel(EF)]istypicallythekeyparametercontrollingtheresponseofspintronicssystems,epitomizedbymagnetictunneljunctionsinwhichthetunnelmagnetoresistance(TMR)isrelatedtotheelectrodespinpolarizationbytheJullièreformula(23).Thesepredictionssuggestthatthenonvolatilecharacterofferroelectricsattheheartofferroelectricrandomaccessmemorytechnology(24)maybeexploitedinspintronicsdevicessuchasmagneticrandomaccessmemoriesorspinfield-effecttransistors(2).However,thenonvolatileelectricalcontrolofspinpolarizationhasnotyetbeendemonstrated.WeaddressthisissueexperimentallybyprobingthespinpolarizationofelectronstunnelingfromanFeelectrodethroughultrathinferroelectricBaTiO3(BTO)tunnelbarriers(Fig.1A).TheBTOpolarizationcanbeelectricallyswitchedtopointtowardorawayfromtheFeelectrode.Weuse...
