具有铁电和铁磁双重性能的新材料Word文档格式.docx

具有铁电和铁磁双重性能的新材料Word文档格式.docx

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[url]

西班牙马德里材料科学研究所

Topic:

Improving 

the 

properties 

of 

FeMo 

double 

perovskites 

for 

applications 

in 

spintronics:

“ 

Can 

we 

go 

future?

”

题目：

铁钼双钙钛矿体系的性质改善在自旋电子学中的应用

Dr. 

M. 

Sagawa日本、

　　Topic:

Research 

and 

Development 

Sintered 

Nd-Fe-B 

Magnet

　　题目：

烧结Nd-Fe-B磁体的研究现状与发展趋势

J.Phys.:

Condens.Matter11No40（11October1999）L445-L450

PII:

S0953-8984（99）06738-7

LETTERTOTHEEDITOR

Magneticpropertiesofthedoubleperovskites

RPBorges

RMThomas

CCullinan

JMDCoey

RSuryanarayanan

LBen-Dor

LPinsard-Gaudart

andARevcolevschi

PhysicsDepartment,TrinityCollege,Dublin2,Ireland

LaboratoiredePhysicochimiedesSolidesduCNRS,Université

deParis-Sud,91405Orsay,France

Received6August1999,infinalform24August1999

Abstract:

Thedouble-perovskiteoxides

and

showevidenceofferromagneticmetallicbehaviour,withCurietemperaturesof345-426Kandsaturationmagnetizationof3.5-3.9µ

B/formulaunit.Ceramicsshowlittleanisotropicmagnetoresistance,butthelow-fieldisotropicnegativemagnetoresistanceof

is7%in1Tatroomtemperatureandreaches15%at77K.Aneffectofabout1%isfoundin20mTatroomtemperatureinbothceramicsandpressedpowdersofthiscompound.

Doubleperovskites

withtwoorderedB-sitecationsareknowntobeferromagneticfor

Feand

W,Re[1]-[5]whereAisanalkalineearthcation.Recentinterestinthefamily[6]-[12]derivesfromareportbyKobayashietal[6]that

isahalf-metallicferrimagnet,withanelectronicstructurewhereonlyminorityspinsarepresentattheFermilevel.ThecalculatedelectronicstructureisanalogoustothatfortheB-sitesof

exceptthattheminorityspinelectronhas

character,ratherthan

character.Aneutrondiffractionstudyof

byGarcia-Landaetalindicatedthattherewasanironmomentof

butnolocalizedmomentonMo.Astudyof

bythepresentauthorsusingneutrondiffractionandMö

ssbauerspectroscopyhasshownthatthereisaformal

chargeconfiguration,andanironmomentof

with

ontheMo[10].Themolybdenumelectronsaredelocalized,andtheycontributetotheferromagnetic

couplingbyoverlapwiththeempty

orbitals.

Muchoftheinterestindoubleperovskitesarisesfromtheprospectsoffindinghalf-metallicoxideswithhighCurietemperatures.ItisnowplainthatCurietemperaturesofmixed-valencemanganitesofthetype

cannotbeincreasedabove400Kbyanymanipulationofthecompositionorstoichiometry[13].TheCurietemperaturefor

issimilar.SubstantiallyhigherCuriepointsarerequiredifdeviceswhichrelyonspin-polarizedtransportinhalf-metallicoxidesaretooperateinapractically-usefultemperaturerangearoundroomtemperature.Thesedeviceswillrelyonlow-fieldswitchingofthemagnetizationofadjacentdecoupledferromagneticregionswhichareseparatedbysomesortofbarrier,acrosswhichelectronsmaypasswithoutcompletelylosingtheirspinpolarization.Examplesareplanartunneljunctionswithaninsulatingbarrier[14],polycrystallinematerialwithgrainboundaries[15]andpressedpowderwithinterparticulatecontacts[16].

Herewereportonthemagneticpropertiesandmagnetoresistanceofthree

ceramics.Compoundswerepreparedfromamixtureofoxidesorcarbonates,calcinedat

andthenregroundandrefired,withafinalfiringfor5-10hoursat

inanatmosphereof1%HinAr.AllcompoundsweresinglephaseandexhibitedaseriesofsuperstructurereflectionsduetocationorderofFeandMoonalternateB-sitesoftheperovskitestructure.TheorderedNaCl-typesuperstructureof

isillustratedinfigure1.Latticeparametersandx-raydensitiesarelistedintable1.Theceramicsallhaveadensityofabout90%ofthex-raydensity.Microstructuresofthesinteredpolycrystallineceramicsareillustratedinfigure2.

Figure1.Crystalstructureof

.

Table1.Propertiesof

;

Sr,Ba.

Figure2.Microstructureofpolycrystalline

（a）,polycrystalline

（b）andapressed

powder（c）.

Curietemperaturesweredeterminedbyacsusceptibilitymeasurementsonceramicbars.Theexcitationfieldwas

at1kHz.MagnetizationwasmeasuredusingaSQUIDmagnetometerinfieldsupto5T.Resistivitywasmeasuredusingthestandardfour-probetechnique,andmagnetoresistancewasdeterminedinfieldsofupto1.2TusingaMULTIMAGMM1200-27.5permanentmagnetfluxsource[17],wherethemagneticfieldisvariable,bothinmagnitudeanddirectionintheplaneofthesample.

Sometypicalmagnetizationandsusceptibilitydataareshowninfigure3.Thesusceptibilityischaracteristicofaferromagnet.Thesaturationmagnetizationreachesavalueof3.5-3.9µ

B/formula,asshowninthetable.

Figure3.Magnetizationcurveof

at5K（

squares;

fullcircles;

emptycircles）.Theinsertshowstherealpartofthesusceptibilitymeasuredin

at1kHz.

Resistivityoftheceramicsamplesisquitevariable,butallhavethepositivetemperaturecoefficientofresistivitycharacteristicofametal.The

samplesarethemostconducting,with

m,comparabletothevaluesforthemoreconductingferromagneticmanganites.Theresistivityof

dependsonthethermaltreatment,withvaluesintherange1-

m.Muchoftheresistivityarisesatgrainboundaries.ThemicrostructuresofthesinteredCaandSrcompoundsappearquitedifferent（figure2（b））.Anisotropicmagnetoresistanceissmall,lessthan0.3%,buttheSrandBasamplesexhibitasubstantialisotropicmagnetoresistanceinquitelowfields.Resultsshowninfigure4aretypical.Valuesof

in1Tat290Kandat77Kare7%and15%,respectively,for

.Thereisnosignificanthysteresisbutasubstantiallow-fieldresponseisseen.Forexample,a1%magnetoresistanceeffectatroomtemperaturerequiresafieldofjust20mT.

Figure4.Magnetoresistanceof（a）polycrystalline

measuredat290Kandat77Kand（b）pressedpowderof

measuredat77K.

NomagnetoresistancewasmeasuredonaceramicsampleoftheCacompound,perhapsbecausethelowresistivityisalmostentirelyofintrinsicorigin,withlittlegrain-boundarycontribution.Howeveronapressedpowdersample（figure2（c））with80%ofthex-raydensity,valuesin1Tat290Kandat77Kwerelessthan0.5%and4%,respectively.ApressedpowderoftheSrcompoundshows2%magnetoresistanceat290Kand6%at77Kinafieldof1T（figure4（b））.

Themagneticorderingtemperaturewefindfor

iscomparabletovaluesreportedintheliterature[4]-[6],althoughfor

isslightlyhigherthanthatreportedbyMaignanetal[8].Energy-dispersivex-rayanalysisshowsthatoursampleshaveclosetotheexpectedcationratio,buttheoxygenstoichiometryandtheextentofNaCl-typecationorderwerenotaccuratelydetermined.Thesaturationmagnetizationisalsolowerthanthe（5-1）=4µ

B/formulaexpectedfromtheionicferrimagneticconfiguration

.An

chargestatefortheironhasbeenclearlyconfirmedbytheisomershiftandhyperfinefielddeterminedby

Mö

ssbauerspectroscopy[10].ThemagnetizationoftheSrcompoundcomesclosesttotheexpectedvalue.Possibleexplanationsforthediscrepancyincludecovalentmixingwiththeunpolarized2p（O）orbitals,spincantingorimperfectcationorderonalternatesites,togetherwithantiferromagneticcouplingofironionsinadjacentB-sites.Astoichiometric,orderedhalf-metallicferromagnetmusthaveasaturationmagnetizationthatisanintegralnumberofBohrmagnetonsbecausethenumberofelectronsperformulaisaninteger,andthenumberofspinup（orspindown）electronsisalsointegralbecauseofthegapinoneorotherdensityofstates.

Themostinterestingresultisthatthemagnetoresistance,whichiscomparableinallthreedoubleperovskitesinvestigated,ishigherinbothceramicsandpressedpowdersthanitisinmanganiteswithasimilarCurietemperature.Theextendedcharacterofthe

orbitalscontainingtheminorityspinelectrons,comparedtothemorelocalized

bandelectronsinthemanganitesmaybethereason.Themanganitesarethoughttohaveafewlayersofcantedspinsatthesurfacewhichspoilthepolarizationoftheemittedelectrons[18].Inthedoubleperovskites,thereislesstendencyfortheMoelectronstobelocalizedandcolinearferromagneticordermaybesustainedrightuptoasharpmagneticinterfaceatthesurface.TheresultsencouragefurtherworktotunetheCurietemperaturetohighervaluesbyalteringtheoxidecompositionorstoichiometry,andalsotofabricateplanartunneljunctionsusingthesematerials.

ThisworkformspartoftheOxideSpinElectronicsNetwork.OXSENisaTMRnetworksupportedbytheEuropeanCommission.

References

[1]

LongoJMandWardR1961J.Am.Chem.Soc.831088

CrossRefLink|OrderfromInfotrieve

[2]

SleightAW,LongoJMandWardR1962Inorg.Chem.1245

[3]

PattersonF,MoellerCandWardR1963Inorg.Chem.2196

[4]

GalassoFS1969Structure,PropertiesandPreparationofPerovskiteTypeCompounds（Elmsford,NewYork:

Pergamon）

[5]

SleightAWandWeiherJF1972J.Phys.Chem.Solids33679

CrossRefLink|InspecAbstract|ChemPortAbstract|OrderfromInfotr