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1、JournalofMagnetismandMagneticMaterials200(1999)24}43Spin-polarizedtransportandmagnetoresistanceinmagneticoxidesA.Gupta*,J.Z.SunIBMT.J.WatsonResearchCenter,M.S.2-240,P.O.Box218,YorktownHeights,NY10598,USAReceived27January1999;receivedinrevisedform17March1999AbstractMagneticoxidemateria
2、lspossessingahighdegreeofspinpolarizationhavebeenfoundtoexhibitenhancedspin-dependenttransportproperties.Forexample,interfacescreatedbynaturallyoccurringorarti"cialgrainboundariesintheperovskitemanganites,LaAMnO,resultinalargemagnetoresistance(MR)atlow"elds.Extrinsicgrain1~xx3boundary
3、MRhasalsobeenreportedinotherconductingmagneticoxides,suchaschromiumdioxide(CrO),pyrochlore2TlMnO,magnetite(FeO),andtheordereddouble-perovskiteSrFeMoO.Evenlargerchangesinresistancesatlow2273426"eldsareobtainedinthelayeredmanganite,LaSrMnOforx"0.3,whichpossessesanaturaltunneljunction2~2
4、x1`2x27structureinthec-axisdirection.But,byfar,thelargestMRe!ecttodatehasbeenobservedinthin"lmferromag-netic/insulating/ferromagnetictunneljunctionstructures,whereresistancechangesashighasafactorof10atlow"eldshasbeenreportedforthestrontium-dopedperovskitemanganites.Inmostcases,theenha
5、ncedMRe!ectinthesemagneticoxidesislimitedtoratherlowtemperaturesanddecreasesrapidlywithincreasingtemperature.Withabetterunderstandingofthenatureoftheboundariesinthesejunctions,itmightbepossibletoenhancetheMRathighertemperatures.(1999ElsevierScienceB.V.Allrightsreserved.Keywords:Spinpo
6、larization;Magnetoresistance;Grainboundary;Tunneljunction;Manganite;Magnetite;Pyrochlore;Chromiumdioxide1.Introductionmajorityandminoritycarrierbands,resultinginaspinimbalanceattheFermilevel.ThevalueofOverthepastfewyears,theemerging"eldofthespinpolarization,P,dependsontheextenttomagne
7、toelectronicshasspurredupinterestinwhichtheconductionbandscrosstheFermisur-studyingthespin-polarizedtransportpropertiesofface,andisde"nedasmetallicferromagneticmaterials[1].Inafer-romagneticmetal,suchasFe,Co,orNi,theex-N!NchangeenergysplitstheconductionbandintoP"ts,N#NtswhereN(p"C,B)a
8、rethe
