材料的磁性能

材料的磁性能第1頁，共20頁，2023年，2月20日，星期一ContentsOpticalPropertiesElectricalPropertiesThermalPropertiesMagneticProperties第2頁，共20頁，2023年，2月20日，星期一OtherPropertiesofMaterials(ForSelf-Study)ElectricalProperties：Thermoelectric,PyroelectricandMagnetoelectricProperty(熱電、熱釋電、磁電性能)MagneticProperties：Magnetostriction

andMagnetoresistance(磁致伸縮、磁電阻)OpticalProperties:Electrooptic,Photoelectric

andMagnetoopticProperty(電光、光電、磁光性能)Acoustic

Properties:

Propagation,AbsorptionandElectroacousticProperty(聲音的傳播、吸收、電聲性能)Elasticity:Anelasticity

andInternalFriction(滯彈性與內耗)第3頁，共20頁，2023年，2月20日，星期一4.MagneticProperties4.1BASICCONCEPTSMagneticDipoles(磁偶極子):

smallbarmagnetscomposedofnorthandsouth.Magneticforces(fields)aregeneratedbymovingelectricallychargedparticles.MagneticfieldlinesofforcearoundacurrentloopandabarmagnetThemagneticmomentasdesignatedbyanarrow第4頁，共20頁，2023年，2月20日，星期一Magneticfieldstrength(磁場強度)H:theexternallyappliedmagneticfield.H=NI/l(4.1)Acylindricalcoil(orsolenoid):Ncloselyspacedturns,lengthl,currentofmagnitudeI.UnitofH:amperespermeter.Magneticfieldstrengthwithinacoil—dependenceonnumberofturns,appliedcurrent,andcoillength.Magneticinduction(磁感應強度)/magneticfluxdensity(磁通密度)B:themagnitudeoftheinternalfieldstrengthwithinasubstancethatissubjectedtoanHfield.B=μH(4.2)Permeability(磁導率)μ:ThepropertyofthespecificmediumthroughwhichtheHfieldpassesandinwhichBismeasured.Thepermeabilityhasdimensionsofwebersperampere-meter(Wb/Am)orhenriespermeter(H/m).BothBandHarefieldvectors.Magneticfluxdensityinamaterial—dependenceonpermeabilityandmagneticfieldstrength.第5頁，共20頁，2023年，2月20日，星期一Inavacuum:B0=μ0H(4.3)Permeabilityofavacuum(真空磁導率)μ0:

auniversalconstant,whichhasavalueof4π

x10-7(1.257x10-6)H/m.B0:thefluxdensitywithinavacuum.μr=μ/μ0

(4.4)Relativepermeability(相對磁導率)μr:ameasureofthedegreetowhichthematerialcanbemagnetized,ortheeasewithwhichaBfieldcanbeinducedinthepresenceofanexternalHfield.AcoilandtheH,B第6頁，共20頁，2023年，2月20日，星期一Magnetization(磁化強度)M:B=μ0H+μ0M(4.5)Magneticsusceptibility(磁化率)

χm,(unitless):M=

χmH(4.6)Magnetizationofamaterial—dependenceonsusceptibilityandmagneticfieldstrength.Relationshipbetweenmagneticsusceptibilityandrelativepermeability:χm=μr-1

(4.7)OriginsofMagneticMomentsEachelectroninanatomhasmagneticmomentsthatoriginatefromtwosources:orbitalmomentsandspinmoments.Ineachindividualatom,orbitalmomentsofsomeelectronpairscanceleachother;thisalsoholdsforthespinmoments.Thenetmagneticmomentforanatomisthesumofthemagneticmomentsofeachoftheconstituentelectrons,includingbothorbitalandspincontributions,andtakingintoaccountmomentcancellation.Foranatomhavingcompletelyfilledelectronshellsorsubshells,thereistotalcancellationofbothorbitalandspinmoments,andisnotcapableofbeingpermanentlymagnetized(He,Ne,Ar,etc.).第7頁，共20頁，2023年，2月20日，星期一4.2DIAMAGNETISMANDPARAMAGNETISMDiamagnetism(抗磁性/逆磁性/反磁性)Veryweakformofmagnetismthatisnonpermanent,andpersistsonlywhileanexternalfieldisbeingapplied.Inducedbyachangeintheorbitalmotionofelectronsduetoanappliedmagneticfield.Themagnitudeoftheinducedmagneticmomentisextremelysmall,andinadirectionoppositetothatoftheappliedfield.μrislessthanunity(onlyveryslightly),andχm

isnegative;Bislessthanthatinavacuum,χm

isontheorderof-10-5.AtomicdipoleconfigurationforadiamagneticmaterialWhenplacedbetweenthepolesofastrongelectromagnet,diamagneticmaterialsareattractedtowardregionswherethefieldisweak.Thisformofmagnetismisofnopracticalimportance.第8頁，共20頁，2023年，2月20日，星期一Paramagnetism(順磁性)Eachatompossessesapermanentdipolemomentbyvirtueofincompletecancellationofelectronspinand/ororbitalmagneticmoments.Intheabsenceofanexternalmagneticfield,theorientationsoftheseatomicmagneticmomentsarerandom,suchthatapieceofmaterialpossessesnonetmacroscopicmagnetization.Atomicdipolesarefreetorotate,andactedonindividuallywithnomutualinteractionbetweenadjacentdipoles.μrisgreaterthanunity,andtoarelativelysmallbutpositivemagneticsusceptibility.χmrangefromabout10-5to10-2.Bothdiamagneticandparamagneticmaterialsareconsiderednonmagnetic.AtomicdipoleconfigurationforaparamagneticmaterialSchematicrepresentationofB-Hfordiamagneticandparamagneticmaterials第9頁，共20頁，2023年，2月20日，星期一4.3FERROMAGNETISMFerromagnetism(鐵磁性):

Apermanentmagneticmomentintheabsenceofanexternalfield,andmanifestverylargeandpermanentmagnetizations.Fe(asα-ferrite),Co,Ni,andsomeoftherareearthmetalssuchasGd.χmashighas106,andH<<M:B≈μ0M(4.8)Permanentmagneticmomentsresultfromatomicmagneticmomentsduetouncancelledelectronspinsandasmallerorbitalmagneticmoment.Couplinginteractionscausenetspinmagneticmomentsofadjacentatomstoalignwithoneanother,evenintheabsenceofanexternalfield.MutualalignmentofatomicdipolesforaferromagneticmaterialDomain(磁疇):Mutualspinalignmentexistsoverrelativelylarge-volumeregionsofthecrystal.Saturationmagnetization(飽和磁化強度)Ms:

Themaximumpossiblemagnetization.Thereisalsoacorrespondingsaturationfluxdensity(飽和磁通密度)

Bs.第10頁，共20頁，2023年，2月20日，星期一4.4ANTIFERROMAGNETISMANDFERRIMAGNETISMAntiferromagnetism(反鐵磁性)Thealignmentofthespinmomentsofneighboringatomsorionsinexactlyoppositedirections;Reason:Magneticmomentcouplingbetweenadjacentatomsorionsresultsinanantiparallelalignment;Thesolidasawholepossessesnonetmagneticmoment.AntiparallelalignmentofspinmagneticmomentsforantiferromagneticMnO第11頁，共20頁，2023年，2月20日，星期一Ferrimagnetism(亞鐵磁性/鐵氧體磁性)Thereareantiparallelspin-couplinginteractions,similarincharactertoantiferromagnetism.Thenetferrimagneticmomentarisesfromtheincompletecancellationofspinmoments.Thesaturationmagnetizationsforferrimagneticmaterialsarenotashighasforferromagnets.Ferrite(鐵氧體)inthemagneticsenseshouldnotbeconfusedwiththeferrite(鐵素體)α-iron.Ferrites,beingceramicmaterials,aregoodelectricalinsulators.Forsomemagneticapplications,suchashigh-frequencytransformers,alowelectricalconductivityismostdesirable.SpinmagneticmomentconfigurationforFe2+andFe3+ionsinFe3O4第12頁，共20頁，2023年，2月20日，星期一4.5THEINFLUENCEOFTEMPERATUREONMAGNETICBEHAVIORRaisingthetemperatureresultsinanincreaseinthemagnitudeofthethermalvibrationsofatoms.Withrisingtemperature,theincreasedthermalmotionoftheatomstendstorandomizethedirectionsofanymoments.Thesaturationmagnetizationisamaximumat0K.Withincreasingtemperature,thesaturationmagnetizationdiminishesgraduallyandthenabruptlydropstozeroattheCurietemperature(居里溫度)Tc.FortemperaturesaboveTc,bothferromagneticandferrimagneticmaterialsareparamagnetic.Antiferromagnetismisalsoaffectedbytemperature;thisbehaviorvanishesattheNéeltemperature(尼爾溫度).Attemperaturesabovethispoint,antiferromagneticmaterialsalsobecomeparamagnetic.SaturationmagnetizationasafunctionoftemperatureforFeandFe3O4第13頁，共20頁，2023年，2月20日，星期一4.6DOMAINSANDHYSTERESISEachdomainismagnetizedtoitssaturationmagnetization;Adjacentdomainsareseparatedbydomainboundariesorwalls,acrosswhichthedirectionofmagnetizationgraduallychanges;Normally,domainsaremicroscopicinsize,andforapolycrystallinespecimen,eachgrainmayconsistofmorethanasingledomain;ThemagnitudeoftheMfieldfortheentiresolidisthevectorsumofthemagnetizationsofallthedomains;Foranunmagnetizedspecimen,thevectorsumofthemagnetizationsofallthedomainsiszero.DomainsinaferromagneticorferrimagneticmaterialThegradualchangeinmagneticdipoleorientationacrossadomainwall第14頁，共20頁，2023年，2月20日，星期一FluxdensityBandfieldintensityHarenotproportionalforferromagnetsandferrimagnets.Initialpermeability(初始磁導率)μi:TheslopeoftheB-HcurveatH=0.Hysteresis(磁滯現(xiàn)象):TheBfieldlagsbehindtheappliedHfield.Remanence/remanentfluxdensity(剩磁)Br:ResidualBfieldexistsatzeroHfield.Hysteresisbehaviorandpermanentmagnetizationmaybeexplainedbythemotionofdomainwalls.Coercivity/Coerciveforce(矯頑力)Hc:,A

fieldappliedinadirectionoppositetotheoriginalfieldtoreducetheBfieldtozero.TheB-HbehaviorforaferromagneticorferrimagneticmaterialthatwasinitiallyunmagnetizedTheB-Hbehaviorforaferromagneticmaterialthatissubjectedtoforwardandreversesaturations第15頁，共20頁，2023年，2月20日，星期一ItisnotnecessarytoincreasetheHfieldtosaturationbeforereversingthefielddirection.Itispossibletoreversethedirectionofthefieldatanypointalongthecurveandgenerateotherhysteresisloops.DifferenthysteresiscurvesB-Hbehaviorsforferromagnetic/ferromagneticanddiamagnetic/paramagneticmaterialsThebehaviorofatypicalferromagnetic/ferrimagneticmaterialsisnonlinear,andthatofparamagneticanddiamagneticislinear.Paramagneticsanddiamagneticsarenonmagneticmaterials:thefluxdensityBoftheparamagneticanddiamagneticmaterialsitisontheorderof10-5tesla.第16頁，共20頁，2023年，2月20日，星期一4.7MAGNETICANISOTROPYThemagnetichysteresiscurveshavedifferentshapesdependingonvariousfactors:(1)asinglecrystalorpolycrystalline;(2)ifpolycrystalline,anypreferredorientationofthegrains;(3)poresorsecond-phaseparticles;and(4)otherfactors,suchastemperatureandthestressstate.Magnetic(ormagnetocrystalline)anisotropy(磁/磁晶各向異性):Thedependenceofmagneticbehavioroncrystallographicorientation.Easymagnetizationdirection(易磁化方向):Acrystallographicdirectioninwhichmagnetizationiseasiest—thatis,saturation(ofM)isachievedatthelowestHfield.MagnetizationcurvesforsinglecrystalsofcobaltMagnetizationcurvesforsinglecrystalsofironandnickel第17頁，共20頁，2023年，2月20日，星期一4.8SOFTMAGNETICMATERIALSSoftmagneticmaterials(軟磁材料)：Smallhysteresisloop(thinandnarrow)HighinitialpermeabilityandalowcoercivityReachsaturationmagnetizationwitharelativelylowappliedfield(i.e.,iseasilymagnetizedanddemagnetized)LowhysteresisenergylossesSchematicmagnetizationcurvesforsoftandhardmagneticmaterialsThesaturationfieldormagnetizationisdeterminedonlybythecompositionofthematerial.Thesusceptibilityandcoercivity,whichinfluencetheshapeofthehysteresiscurve,aresensitivetostructuralvariablesratherthantocomposition.Eddycurrents(渦流):

Electricalcurrentsinducedinamagneticmaterialbyamagneticfieldthatvariesinmagnitudeanddirect