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1、NanoStructuredMaterials,Vol.11,No.8,pp.987±993,1999ElsevierScienceLtdPergamonCopyright©2000ActaMetallurgicaInc.PrintedintheUSA.Allrightsreserved.0965-9773/99/$±seefrontmatterPIIS0965-9773(00)00430-XSOFTMAGNETICPROPERTIESOFNANOCRYSTALLINENi3FeANDFe75Al12.5Ge12.5H.N.Frase,R.D.Shu
2、ll,*L.-B.Hong,T.A.Stephens,Z.-Q.Gao,andB.FultzDivisionofEngineeringandAppliedScience,MailCode138-78CaliforniaInstituteofTechnology,Pasadena,CA91125(ReceivedApril14,1999)(AcceptedDecember2,1999)AbstractÐMagnetizationcurvesweremeasuredonNi3FeandFe75Al12.5Ge12.5nanocrystalsofdiffe
3、rentgrainsizes.Thesematerialswerepreparedbyhigh-energyballmilling,followedbyannealingatvarioustemperatures.Thealloycompositionswerechosenbecausetheyhavelowmagnetostrictioninbulkform,implyingthatstraininthesamplesshouldhavelittleeffectontheirmagneticproperties.TheM-Hmagnetizatio
4、ncurveswereusedtoobtainthecoercivity,themaximumpermeability,andthesaturationmagnetization.DifferencesinthesemagneticpropertieswererelatedtochangesingrainsizeandinternalRMSstrain.Inspiteofthelowbulkmagnetostrictionofthesematerials,theinternalstresscontrolledthecoercivity.Thechan
5、gesinpermeability,however,werenotasexpectedfromthetrendingrainsize.Wesuggestthatthepowdermorphology,playsanimportantroleindeterminingthesoftmagneticpropertiesofthesenanocrystallinealloys.©2000ActaMetallurgicaInc.1.IntroductionMuchinterestinthemagneticpropertiesofnanocrystalsfol
6、lowedthereportbyYoshizawa,etal.,ofaFe-Si-basednanocrystallinematerialthathadlowcoercivity,Hc,andhighpermeability,m(1).Thesoftmagneticpropertiesfoundinthisnanocrystallinemagneticmaterialhavebeenexplained,atleastinpart,bytheªrandomanisotropymodel.º(2,3)Thismodelpredictsthattheexc
7、hangeinteraction,A,willdominatethemagnetocrystallineanisotropy,K1,formaterialshavinggrainsizessmallerthanLex5=(A/K),theferromagneticexchangelength(typicallytensofnm).Whentheferromagneticexchange1lengthexceedsthegrainsize,theeffectivemagneticanisotropyisaveragedoveranumberofgrai
8、ns3626containedwithinthevolumeV5L.Forsmallgrainsizes,D
