材料的磁性能精选文档.ppt

材料的磁性能本讲稿第一页，共二十页ContentsOpticalPropertiesElectricalPropertiesThermalPropertiesMagneticProperties本讲稿第二页，共二十页OtherPropertiesofMaterials(ForSelf-Study)ElectricalProperties：Thermoelectric,PyroelectricandMagnetoelectricProperty (热电、热释电、磁电性能)MagneticProperties：MagnetostrictionandMagnetoresistance (磁致伸缩、磁电阻)OpticalProperties:Electrooptic,PhotoelectricandMagnetoopticProperty (电光、光电、磁光性能)AcousticProperties:Propagation,AbsorptionandElectroacousticProperty (声音的传播、吸收、电声性能)Elasticity:AnelasticityandInternalFriction (滞弹性与内耗)本讲稿第三页，共二十页4.MagneticProperties4.1BASICCONCEPTSMagneticDipoles(磁偶极子):smallbarmagnetscomposedofnorthandsouth.Magneticforces(fields)aregeneratedbymovingelectricallychargedparticles.MagneticfieldlinesofforcearoundacurrentloopandabarmagnetThemagneticmomentasdesignatedbyanarrow本讲稿第四页，共二十页Magneticfieldstrength(磁场强度)H:theexternallyappliedmagneticfield.H=NI/l (4.1)Acylindricalcoil(orsolenoid):Ncloselyspacedturns,lengthl,currentofmagnitudeI.UnitofH:amperespermeter.Magneticfieldstrengthwithinacoildependenceonnumberofturns,appliedcurrent,andcoillength.Magneticinduction(磁感应强度)/magneticfluxdensity(磁通密度)B:themagnitudeoftheinternalfieldstrengthwithinasubstancethatissubjectedtoanHfield.B=H (4.2)Permeability(磁导率):ThepropertyofthespecificmediumthroughwhichtheHfieldpassesandinwhichBismeasured.Thepermeabilityhasdimensionsofwebersperampere-meter(Wb/Am)orhenriespermeter(H/m).BothBandHarefieldvectors.Magneticfluxdensityinamaterialdependenceonpermeabilityandmagneticfieldstrength.本讲稿第五页，共二十页In a vacuum:B0=0H (4.3)Permeabilityofavacuum(真空磁导率)0:auniversalconstant,whichhasavalueof4 x10-7(1.257x10-6)H/m.B0:thefluxdensitywithinavacuum.r=/0 (4.4)Relative permeability(相对磁导率)r:a measure of the degree to which the material can be magnetized,or the ease with which a B field can be induced in the presence of an external H field.AcoilandtheH,B本讲稿第六页，共二十页Magnetization(磁化强度)M:B=0 H+0 M (4.5)Magneticsusceptibility(磁化率)m,(unitless):M=mH (4.6)Magnetizationofamaterialdependenceonsusceptibilityandmagneticfieldstrength.Relationshipbetweenmagneticsusceptibilityandrelativepermeability:m=r -1 (4.7)OriginsofMagneticMomentsEachelectroninanatomhasmagneticmomentsthatoriginatefromtwosources:orbitalmomentsandspinmoments.Ineachindividualatom,orbitalmomentsofsomeelectronpairscanceleachother;thisalsoholdsforthespinmoments.Thenetmagneticmomentforanatomisthesumofthemagneticmomentsofeachoftheconstituentelectrons,includingbothorbitalandspincontributions,andtakingintoaccountmomentcancellation.Foranatomhavingcompletelyfilledelectronshellsorsubshells,thereistotalcancellationofbothorbitalandspinmoments,andisnotcapableofbeingpermanentlymagnetized(He,Ne,Ar,etc.).本讲稿第七页，共二十页4.2DIAMAGNETISMANDPARAMAGNETISMDiamagnetism(抗磁性/逆磁性/反磁性)Veryweakformofmagnetismthatisnonpermanent,andpersistsonlywhileanexternalfieldisbeingapplied.Inducedbyachangeintheorbitalmotionofelectronsduetoanappliedmagneticfield.Themagnitudeoftheinducedmagneticmomentisextremelysmall,andinadirectionoppositetothatoftheappliedfield.r islessthanunity(onlyveryslightly),andm isnegative;B islessthanthatinavacuum,m isontheorderof-10-5.AtomicdipoleconfigurationforadiamagneticmaterialWhenplacedbetweenthepolesofastrong electromagnet,diamagneticmaterialsareattractedtowardregionswherethefieldisweak.This form of magnetism is of nopracticalimportance.本讲稿第八页，共二十页Paramagnetism(顺磁性)Eachatompossessesapermanentdipolemomentbyvirtueofincompletecancellationofelectronspinand/ororbitalmagneticmoments.Intheabsenceofanexternalmagneticfield,theorientationsoftheseatomicmagneticmoments are random,such that a piece of material possesses no net macroscopicmagnetization.Atomicdipolesarefreetorotate,andactedonindividuallywithnomutualinteractionbetweenadjacentdipoles.r isgreaterthanunity,andtoarelativelysmallbutpositivemagneticsusceptibility.mrangefromabout10-5to10-2.Bothdiamagneticandparamagneticmaterialsareconsiderednonmagnetic.AtomicdipoleconfigurationforaparamagneticmaterialSchematicrepresentationofB-H fordiamagneticandparamagneticmaterials本讲稿第九页，共二十页4.3FERROMAGNETISMFerromagnetism(铁磁性):Apermanentmagneticmomentintheabsenceofanexternalfield,andmanifestverylargeandpermanentmagnetizations.Fe(as-ferrite),Co,Ni,andsomeoftherareearthmetalssuchasGd.m ashighas106,andH M:B 0 M (4.8)Permanent magnetic moments result from atomic magnetic moments due to uncancelledelectronspinsandasmallerorbitalmagneticmoment.Couplinginteractionscausenetspinmagneticmomentsofadjacentatomstoalignwithoneanother,evenintheabsenceofanexternalfield.MutualalignmentofatomicdipolesforaferromagneticmaterialDomain(磁畴):Mutualspinalignmentexistsoverrelativelylarge-volumeregionsofthecrystal.Saturation magnetization(饱 和 磁 化 强 度)Ms:Themaximum possible magnetization.There is also acorrespondingsaturationfluxdensity(饱和磁通密度)Bs.本讲稿第十页，共二十页4.4ANTIFERROMAGNETISMANDFERRIMAGNETISMAntiferromagnetism(反铁磁性)Thealignmentofthespinmomentsofneighboringatomsorionsinexactlyoppositedirections;Reason:Magneticmomentcouplingbetweenadjacentatomsorionsresultsinanantiparallelalignment;Thesolidasawholepossessesnonetmagneticmoment.AntiparallelalignmentofspinmagneticmomentsforantiferromagneticMnO本讲稿第十一页，共二十页Ferrimagnetism(亚铁磁性/铁氧体磁性)Thereareantiparallelspin-couplinginteractions,similarincharactertoantiferromagnetism.Thenetferrimagneticmomentarisesfromtheincompletecancellationofspinmoments.Thesaturationmagnetizationsforferrimagneticmaterialsarenotashighasforferromagnets.Ferrite(铁氧体)inthemagneticsenseshouldnotbeconfusedwiththeferrite(铁素体)-iron.Ferrites,beingceramicmaterials,aregoodelectricalinsulators.Forsomemagneticapplications,suchashigh-frequencytransformers,alowelectricalconductivityismostdesirable.SpinmagneticmomentconfigurationforFe2+andFe3+ionsinFe3O4本讲稿第十二页，共二十页4.5THEINFLUENCEOFTEMPERATUREONMAGNETICBEHAVIORRaising the temperature results in an increase in the magnitude of the thermalvibrationsofatoms.Withrisingtemperature,theincreasedthermalmotionoftheatomstendstorandomizethedirectionsofanymoments.Thesaturationmagnetizationisamaximumat0K.Withincreasingtemperature,thesaturationmagnetizationdiminishesgraduallyandthenabruptlydropstozeroattheCurietemperature(居里温度)Tc.For temperatures aboveTc,bothferromagneticandferrimagneticmaterialsareparamagnetic.Antiferromagnetism is also affected bytemperature;this behavior vanishes attheNeltemperature(尼尔温度).Attemperaturesabovethispoint,antiferromagneticmaterialsalsobecomeparamagnetic.SaturationmagnetizationasafunctionoftemperatureforFeandFe3O4本讲稿第十三页，共二十页4.6DOMAINSANDHYSTERESISEachdomainismagnetizedtoitssaturationmagnetization;Adjacentdomainsareseparatedbydomainboundariesorwalls,acrosswhichthedirectionofmagnetizationgraduallychanges;Normally,domainsaremicroscopicinsize,andforapolycrystallinespecimen,eachgrainmayconsistofmorethanasingledomain;ThemagnitudeoftheM fieldfortheentiresolidisthevectorsumofthemagnetizationsofallthedomains;Foranunmagnetizedspecimen,thevectorsumofthemagnetizationsofallthedomainsiszero.DomainsinaferromagneticorferrimagneticmaterialThegradualchangeinmagneticdipoleorientationacrossadomainwall本讲稿第十四页，共二十页Flux densityB and field intensityH are not proportional for ferromagnets andferrimagnets.Initialpermeability(初始磁导率)i:TheslopeoftheB-H curveatH=0.Hysteresis(磁滞现象):TheB fieldlagsbehindtheappliedH field.Remanence/remanentfluxdensity(剩磁)Br:ResidualB fieldexistsatzeroH field.Hysteresis behavior and permanent magnetization may be explained by themotionofdomainwalls.Coercivity/Coerciveforce(矫顽力)Hc:,A fieldappliedinadirectionoppositetotheoriginalfieldtoreducetheB fieldtozero.TheB-H behaviorforaferromagneticorferrimagneticmaterialthatwasinitiallyunmagnetizedTheB-H behaviorforaferromagneticmaterialthatissubjectedtoforwardandreversesaturations本讲稿第十五页，共二十页ItisnotnecessarytoincreasetheHfield to saturation before reversingthefielddirection.It is possible to reverse thedirection of the field at anypoint along the curve andgenerateotherhysteresisloops.DifferenthysteresiscurvesB-H behaviorsforferromagnetic/ferromagneticanddiamagnetic/paramagneticmaterialsThebehaviorofatypicalferromagnetic/ferrimagnetic materials is nonlinear,and that ofparamagneticanddiamagneticislinear.Paramagnetics and diamagnetics are nonmagnetic materials:the flux density B of theparamagneticanddiamagneticmaterialsitisontheorderof10-5tesla.本讲稿第十六页，共二十页4.7MAGNETICANISOTROPYThemagnetichysteresiscurveshavedifferentshapesdependingonvariousfactors:(1)asinglecrystalorpolycrystalline;(2)ifpolycrystalline,anypreferredorientationofthegrains;(3)poresorsecond-phaseparticles;and(4)otherfactors,suchastemperatureandthestressstate.Magnetic(ormagnetocrystalline)anisotropy(磁/磁晶各向异性):Thedependenceofmagneticbehavioroncrystallographicorientation.Easy magnetization direction(易 磁 化 方 向):A crystallographic direction in whichmagnetizationiseasiestthatis,saturation(ofM)isachievedatthelowestH field.MagnetizationcurvesforsinglecrystalsofcobaltMagnetizationcurvesforsinglecrystalsofironandnickel本讲稿第十七页，共二十页4.8SOFTMAGNETICMATERIALSSoftmagneticmaterials(软磁材料)：Smallhysteresisloop(thinandnarrow)HighinitialpermeabilityandalowcoercivityReachsaturationmagnetizationwitharelativelylowappliedfield(i.e.,iseasilymagnetizedanddemagnetized)LowhysteresisenergylossesSchematicmagnetizationcurvesforsoftandhardmagneticmaterialsThesaturationfieldormagnetizationisdeterminedonlybythecompositionofthematerial.Thesusceptibilityandcoercivity,whichinfluencetheshapeofthehysteresis curve,aresensitive tostructural variablesratherthantocomposition.Eddycurrents(涡流):Electricalcurrentsinducedinamagnetic material by a magnetic field that varies inmagnitudeanddirectionwithtime.Increasing the electrical resistivity to minimizeenergylossinsoftmagneticmaterials.本讲稿第十八页，共二十页4.9HARDMAGNETICMATERIALSHardmagneticmaterials(硬磁材料):highremanence,coercivityandsaturationfluxdensitylowinitialpermeabilityhighhysteresisenergylosses.Energyproduct(磁能积):(BH)maxThevalueoftheenergyproductisrepresentativeoftheenergyrequiredtodemagnetizeapermanentmagnet.Schematic magnetization curve that displays hysteresis.WithinthesecondquadrantaredrawntwoBH energyproductrectangles;theareaoftherectanglelabeled(BH)maxisthelargestConventional Hard Magnetic Materials：Magnetsteels,CuNiFealloys,AlNiCoalloys,Hexagonalferrites(BaO-6Fe2O3)High-EnergyHardMagneticMaterials：Sm-CoMagnets:SmCo5Nd-Fe-BMagnets:Nd2Fe14B本讲稿第十九页，共二十页4.10MAGNETICSTORAGESchematic diagram of a hard disk drive thatemploys the perpendicular magnetic recordingmedium;also shown are inductive write andmagnetoresistive-readheadsTransmissionelectronmicrographshowingthe microstructure of the perpendicularmagneticrecordingmediumusedinhard-diskdrivesScanning electron micrographsshowingparticulatemediausedintapememorystorage本讲稿第二十页，共二十页