Lecture-1-磁性物理和磁学基本概念

磁性材料：原理、工艺与应用

MagneticMaterials:Basictheory,ProcessingandApplicationsLecture1磁学根本概念与磁性物理根底1OutlineIntroductiontothiscourseOriginofMagnetismVarioustypesofmagnetismVariousmagneticmaterialsSummary2Beforestart,somequestions…Whatmetalsaremagnetic?SinceFearemagneticmetal,whydoesitnotattractasmallpieceofiron?Pureironismagneticandsomesteelisnot,Why?Fe,W,Mo,V,Nbareallstructuredandhaveunpairedelectrons,whyisonlyFemagnetic?WhydoNdFeBmagnetspackedwithFefoilwhenposted?3ASurveyHaveyoueverstudied-Ferromagnetism《铁磁学》?Magnetism《磁学》?Electromagnetism《电磁学》?SolidPhysics《固体物理》?MaterialsPhysics《材料物理》?GeneralPhysics《普通物理》〔《大学物理》〕?4AboutThisCourseItisnot-Electromagnetism《电磁学》!Magnetism《磁学》!MagneticPhysics《磁性物理》！Itis–MagneticMaterials！Weemphasizetheory,processingandapplication!5CourseStructure共32学时，含：论文报告4学时学术报告4学时讨论2学时讲授24学时内容：磁性根底、软磁材料、硬磁材料、磁记录、磁致伸缩材料、磁性-物性相互作用、巨磁阻材料、吸波材料、磁性薄膜、磁性纳米结构、磁性材料最新进展6Whatisyouropinionaboutthiscourse?Whatdoyouwanttolearn?Howtoteach?Letmeknowbyemail!7AllAboutExam论文报告，70%；平时，30%。Topicsforyourreport:Recentprogressinadvanced-1.NanocompositeRare-earthpermanentmagneticmaterials;2.Nanocompositesoftmagneticmaterials;3.perpendicularmagneticrecording;4.Magnetoelectricmaterials;5.Magneticthinfilmsformicrowaveabsorber;6.GMRmaterials;7.One-dimensionmagneticnanostructures;8.Spintronics;9.Magnetocaloricmaterials;10.Magnetostrictionmaterials.YoucanwriteyourreportinChinesethoughuseofEnglishisencouraged!NoteInformaljournalpaperstyle;Recentprogress;Noplagiarismallowed.8TermMagnetcomesfromtheancientGreekcityofMagnesia,atwhichmanynaturalmagnetswerefound.PlinytheElder(23-79ADRoman)wroteofahillneartheriverIndusthatwasmadeentirelyofastonethatattractediron.KnowninChinaandEurope-800BCLodestoneLet’sgetstart:Astory9LODESTONENow,werefertothesenaturalmagnetsaslodestones(alsospelledloadstone;lodemeanstoleadortoattract)whichcontainmagnetite,anaturalmagneticmaterialFe3O4.Whenlightningstrikestheearthitcouldcreateamagneticfieldlargeenoughtosaturatethemagnetizationoflodestone.Typicalcurrent~1,000,000Amp.Oncein1–10millionyears10HistoryChineseasearlyas121ADknewthatanironrodwhichhadbeenbroughtnearoneofthesenaturalmagnetswouldacquireandretainthemagneticproperty…andthatsucharodwhensuspendedfromastringwouldalignitselfinanorth-southdirection.Useofmagnetstoaidinnavigationcanbetracedbacktoatleastthe11thcentury.Basically,weknewthephenomenonexistedandwelearnedusefulapplicationsforit.Wedidnotunderstandit.司南11

ElectrifiedAmberattractssmallobjectsLodestoneattractsironAConnection?HansChristianØrsted(1777–1851)

Finally,theScience-

Oersted’sExperimentDanishscientistHansChristianOerstedobservedthatacompassneedleinthevicinityofawirecarryingelectricalcurrentwasdeflected!aconnectionbetweenelectricalandmagneticphenomenashown.Oersted’sexperiment(1820)12AquantitativerelationshipbetweenachangingmagneticfieldandtheelectricfieldcreatedbythechangeMichaelFaraday

(1791-1867)Faraday:

EffectofachangingmagneticfieldIn1831,Faradaydiscoveredthatamomentarycurrentexistedinacircuit,whenthecurrentinanearbycircuitwasstartedorstopped.Shortlythereafter,hediscoveredthatmotionofamagnettowardorawayfromacircuitcouldproducethesameeffect.13Henry’swork:alesson!JosephHenry(1797-1878)JosephHenryfailedtopublishwhathehaddiscovered6-12monthsbeforeFaradayHenrywasalwaysslowinpublishinghisresults,andhewasunawareofFaraday'swork.TodayFaradayisrecognizedasthediscovererofmutualinductance(thebasisoftransformers),whileHenryiscreditedwiththediscoveryofself-inductance.14TheconnectionismadeSUMMARY:Oerstedshowedthatmagneticeffectscouldbeproducedbymovingelectricalcharges;FaradayandHenryshowedthatelectriccurrentscouldbeproducedbymovingmagnetsSo....Allmagneticphenomenaresultfromforcesbetweenelectricchargesinmotion.15Ampere:MolecularCurrents

Amperefirstsuggestedin1820thatmagneticpropertiesofmatterwereduetotinyatomiccurrents: ExistenceofsmallmolecularcurrentsEachatom/moleculewouldbehaveasasmallpermanentmagnetWouldaligninthepresenceofamagneticfieldAndreMarieAmpere

(1775-1836)16Themagneticfieldinspacearoundanelectriccurrentisproportionaltotheelectriccurrentwhichservesasitssource.Ampere'sLawForanyclosedlooppath,thesumofthelengthelementstimesthemagneticfieldinthedirectionofthelengthelementisequaltothepermeabilitytimestheelectriccurrentenclosedintheloop.17DonotforgetthemØrstedshowedthatmagneticeffectscouldbeproducedbymovingelectricalcharges;FaradayandHenryshowedthatelectriccurrentscouldbeproducedbymovingmagnets.Allmagneticphenomenaresultfromforcesbetweenelectricchargesinmotion.Amperefirstsuggestedin1820thatmagneticpropertiesofmatterwereduetotinyatomiccurrents.18Toptenlist:whatweshouldhaveknownaboutmagnetism？1.ThereareNorthPolesandSouthPoles.2.Likepolesrepel,unlikepolesattract.3.Magneticforcesattractonlymagneticmaterials.4.Magneticforcesactatadistance.5.Whilemagnetized,temporarymagnetsactlikepermanentmagnets.6.Acoilofwirewithanelectriccurrentflowingthroughitbecomesamagnet.7.Puttingironinsideacurrent-carryingcoilincreasesthestrengthoftheelectromagnet.8.Achangingmagneticfieldinducesanelectriccurrentinaconductor.9.Achargedparticleexperiencesnomagneticforcewhenmovingparalleltoamagneticfield,butwhenitismovingperpendiculartothefielditexperiencesaforceperpendiculartoboththefieldandthedirectionofmotion.10.Acurrent-carryingwireinaperpendicularmagneticfieldexperiencesaforceinadirectionperpendiculartoboththewireandthefield.19OriginofMagnetismofMatterAmpere:molecularcurrentsModernphysics:magneticmomentsinatoms〔“磁矩学说”或“磁偶极矩学说”〕1)unpairedelectronspinsmainly2)theorbitalmotionofelectronswithinthematerialtoalesserextent20物理学原理：任何带电体的运动都必然在周围的空间产生磁场。电动力学定律：一个环形电流具有一定的磁矩，它在磁场中行为像个磁性偶极子。设环形电流的强度为I〔A〕，它所包围的面积为A〔m2〕,那么该环流的磁矩为：m=I*A〔Am2〕玻尔〔Bohr〕原子模型：原子内的电子在固定的轨道上绕原子核作旋转运动，同时还绕自身的轴线作自旋运动。前一种运动产生“轨道磁矩”，后一种运动产生“自旋磁矩”。物质磁性来源的同一性：尽管宏观物质的磁性是多种多样的，但这些磁性都来源于电子的运动。OriginofMagnetismofMatter21原子磁矩Macroscopicpropertiesaretheresultofelectronmagneticmoments。Momentscomefrom2sources:Orbitalmotionaroundanucleus〔轨道磁矩〕与Spinningaroundanaxis〔自旋磁矩〕。原子核磁矩比电子磁矩小3个数量级，一般情况下忽略不计。因此，原子磁矩主要来源于原子核外电子的。原子中的电子成对地存在。这些成对电子的自旋磁矩和轨道磁矩方向相反而互相抵消，使原子中的电子总磁矩为零。——非磁原子。原子中的电子磁矩没有完全抵消使原子中电子的总磁矩〔有时叫净磁矩，剩余磁矩〕不为零。——磁性原子。22原子的总磁矩应是按照原子结构和量子力学规律将原子中各个电子的轨道磁矩和自旋磁矩相加起来的合磁矩。原子磁矩ThenetmagneticmomentforanatomisthesumofthemagneticmomentsofconstituentelectronsAtomswithcompletelyfilledelectronshellsareincapablepermanentmagnetizationAllmaterialsexhibitsomeformofmagnetization.Threetypesofresponse;ferro,diaandparamagnetic.23原子的磁矩电子和原子核均有磁矩，但原子核的磁矩仅有电子磁矩的1/1836.5。电子轨道磁矩：

l：轨道角量子数，0,1,2,3,4…n-1(s,p,d,f,…电子态)；n:主量子数；波尔磁子

B=9.273210-24A/m2轨道磁矩在外磁场方向的投影:

l,H=ml

B

ml:角动量方向量子数或磁量子数=0,1,2,…l电子自旋磁矩：

s：自旋量子数，s=1/2自旋磁矩在外磁场方向的投影:

s,H=2ms

B

ms:自旋角动量方向量子数=1/224原子的磁矩原子磁矩=电子轨道磁矩+自旋磁矩对于3d过渡族和4f稀土金属及合金，原子磁矩：

式中:

称为郎德因子。J：原子总角量子数；L：原子总轨道角量子数；S：原子总自旋量子数；波尔磁子

B=9.273210-24A/m2量子力学证明，原子磁矩在外磁场方向的投影也是量子化的

J,H=gJmJ

B

mJ:原子角动量方向量子数或原子磁量子数=0,1,2,…JIfJ,LandSareknown,JandJ,Hcanbecalculated.25在一个填满的电子壳层中，电子的轨道磁矩和自旋磁矩为零。对于次电子层〔等〕未填满电子的原子，在基态下，其总角量子数J、总轨道量子数L和总自旋量子数S存在如下关系：〔1〕在未填满电子的那些次电子层内，在Pauli原理允许的条件下S和L均取最大值；〔2〕次电子层未填满一半时，J=L-S；〔3〕次电子层填满一半或一半以上时，J=L+SHund规那么

J,H=gJmJ

BmJ=0,1,2,…J26原子磁矩尽管上述计算方法有其深奥的量子力学来源，但与实验值之间的符合并不十分好。对铁磁和反铁磁材料，有时也使用更简化的方程：μ=gs

或者干脆将g作为可调参数以与实验结果吻合。27众所周知，电子轨道运动是量子化的，因而只有分立的轨道存在，换言之、角动量是量子化的，并由下式给出普郎克(Planck)常数：玻尔磁子(Bohrmagneton)电子的轨道磁矩电子的角动量是：电子的轨道磁矩：°●PMLeiv电子的轨道磁矩28与自旋相联系的角动量的大小是ħ/2，因而自旋角动量可写为：S是自旋角动量量子数自旋磁矩通常磁矩

和P之间的关系由下式给出：这里g因子(g-factor)对自旋运动是2，而对轨道运动是1。不管是自旋磁矩，还是轨道磁矩，都是玻尔磁子B的整数倍。P

se电子的自旋磁矩29TheUniversality:MagnetismAllmatteraremagnetic物质磁性无处不在〔1〕物质的各种形态，无论是固态、液态、气态、等离子态、超高密度态和反物质态都具有磁性；〔2〕物质的各个层次，无论是原子、原子核、根本粒子和根底粒子等都会具有磁性。〔3〕无限广袤的宇宙，无论是天体，还是星际空间都存在着或强或弱的磁场。地球磁场强度：~240A/m，太阳的磁场强度~80A/m，中子星磁场强度高达~1013-1014A/m。物质的磁性与其他属性之间存在着广泛的联系，并构成多种多样的耦合效应和双重〔多重〕效应〔如磁电效应、磁光效应、和磁热效应等〕。这些效应是了解物质结构和性能关系的重要途径，又是开展各种应用技术和功能器件〔如磁光存储技术、磁记录技术和霍尔器件等〕的根底。30MagneticPoles(磁极)theexternalmagneticfieldisstrongestatthepolesThetwotypesofmagneticpolescannotexistseparately–alwayscoupledtogetherasadipole.Isolatedmagneticmonopoleshavenotyetbeendetected.表示磁极强弱的物理量称为“磁极强度”。两个强弱相同的磁极，在真空中相距1厘米时，如果它们之间相互作用力为1达因，那么每个磁极的强度就规定为一个电磁系单位制的磁极强度单位。磁极强度〔Wboremu〕为m1、m2的磁极间相互作用力：F=km1m2/r2k=1/4μ0,0=410-7H/m31Amagneticdipole〔磁偶极子〕AloopofelectriccurrentgeneratesamagneticdipolefieldFieldlinesrunfromtheNorthpoletotheSouthpoleFieldlinesindicatethedirectionofforcethatwouldbeexperiencedbyaNorthmagneticmonopole32MagneticMoment〔磁矩〕电流在其四周产生环绕的磁场。如果把通电导线圈成一个半径为r的圆环，其周围的铁屑那么展示了其产生的磁场的形态。这个磁场等效于一个磁矩为M的磁铁产生的磁场。由电流i产生的磁场，其强度和圆环的面积相关〔圆环越大，磁矩就越大〕，即M=iπr2。由n个圆环产生的总磁矩是由这些单一圆环产生的磁矩的迭加，即：M=niπr2因此，磁矩M的单位为Am2。环电流磁矩：M=IA棒状磁铁磁矩：M=mllmAI33MagneticField〔磁场〕,HAmagneticfieldHisgeneratedwheneverthereiselectricchargeinmotion(electriccurrents).Thiscanbeduetomacroscopiccurrentsinaconductor,ormicroscopiccurrentsassociatedwithelectronsinatomicorbits,orbeproducedbyapermanentmagnet.HismeasuredinA/morOe(SIsystemorcgssystem).1A/m=0.01257OeForasolenoid:H=NI/L34MagneticField〔磁场〕,H电流能够产生磁场，因此可以借助于电场来定义由其产生的磁场。当导线通以电流时，根据右手法那么，右手的大拇指指向电流方向〔即正方向，与电子流动方向相反〕，其它成环状的四指那么指示了相应的磁场方向。磁场H同时垂直于电流方向和径向单位矢量r，其强度与电流强度成正比。磁场强度H可以由安培定律给出：因此，磁场强度H的单位为A/m。35MagneticField,HAforcefieldsimilartothegravitationalandelectricalfield,detectedbyaprobe.Amagneticfieldexertsatorquewhichorientsdipoleswiththefield.Directionofmagneticfieldatanypointisdefinedasthedirectionofmotionofachargedparticleonwhichthemagneticfieldwouldnotexertaforce.Magneticfieldlinesdescribethestructureofmagneticfieldsinthreedimensions.Foramagnet:H=F/m1=k

m1/r2F=km1m2/r236Fluxdensity〔磁通密度〕,B磁通量〔Magneticflux，〕 磁场是一个矢量场，在任何一点它都由方向和强度共同定义。其方向由磁力线箭头确定，而其强度那么由磁力线的密度确定。磁力线即为磁通量，其密度可用来衡量磁场的强度〔即磁感应强度B〕。Densityofflux(orfield)linesdeterminesforcesonmagneticpolesDirectionoffluxindicatesdirectionofforceonaNorthpoleHigherfluxdensityexertsmoreforceonmagneticpoles37FluxdensityBBdependsonGeometryandcurrentinsolenoidMagneticpropertiesofthematerialGeometryofmaterial38MagneticInduction〔磁感应强度〕,BThemagneticinductionB,alsoknownasthefluxdensity,measuredinTesla(SI)orGauss(cgs),istheresponseofamediumtothepresenceofamagneticfield.1T=10000GsHfieldcreatesmagneticinduction Bisthemagneticinduction;themagnitudeoftheinternalfieldwithinasubstance39MagneticPermeability〔磁导率〕,B=Histhepermeability〔磁导率〕ofthemedium(Henriespermeter)B0=0H0isthepermeabilityofavacuumr=/0ristherelativepermeability40Magnetization(磁化强度),MWedefinemagnetizationasthetotalmagneticdipolemoment(magneticmoment)perunitvolumewithinthematerialItismeasuredinA/m(SI)oremu/cm3(cgs).41Magnetizationdependson……..NumberdensityofmagneticdipolemomentswithinmaterialMagnitudeofthemagneticdipolemomentswithinthematerialThearrangementofthemagneticdipoleswithinthematerial42Polarization〔磁极化强度〕,JThemagneticpolarisationJ,measuredinTesla,isgivenbyJ=

oM,where

o(=1.23664

10-6H/m)isthepermeabilityoffreespace.Mincreasesasmoreelectronicmagneticmomentsarealigned.Whenallmagneticmomentsarealignedinthesamedirection,thesaturationmagnetisation(polarisation)Ms

(Js)isachieved.43HowdoesMrespondtoH?ThereisavarietyofwaysthatMrespondstoHResponsedependsontypeofmaterialResponsedependsontemperatureResponsecansometimesdependontheprevioushistoryofmagneticfieldstrengthsanddirectionsappliedtothematerial44Non-linearresponsesGenerally,theresponseofMtoHisnon-linearOnlyatsmallvaluesofHorhightemperaturesisresponsesometimeslinearMtendstosaturateathighfieldsandlowtemperatures45B,H,M,JRelationshipsB(inT)consistsoftwocontributions:onefrommagneticfieldH(A/m),theotherfrommagnetisationM(A/m).Thisleadstooneofthemostimportantrelationsinmagnetism:IfthereisnomagnetizationM…..46MagneticSusceptibility〔磁化率〕,B=

0(H

+M)ReplaceB=

H→

H=

0(H

+M) →r

0H=

0(H

+M) →0M=0(r-1)H →M=

(r

-1)H

MagneticSusceptibilityM=

H

=

r−1

,Susceptibility,measuresthematerialresponserelativetoavacuum(Dimensionless)47VariousMagnetismBasedon抗磁性〔Diamagnetism〕顺磁性〔Paramagnetism〕铁磁性〔Ferromagnetism〕亚铁磁性〔Ferrimagnetism〕反铁磁性〔Antiferromagnetism〕>0,typically10-3-10-5>>1,typically

50-104<0,typically

-10-7Magneticorderingmaterials48VariousMagnetismDiamagnetsarematerialswhichhavenonetmagneticmomentontheiratoms,becausetheelectronsareallpairedwithantiparallelspins.WhenamagneticfieldHisapplied,theorbitsoftheelectronschangeinaccordancewithLenz’slaw,andtheysetupanorbitalmagneticmomentwhichopposesthefield,andthereforegivesverysmallnegativesusceptibility.Paramagnetsarematerialswhichhaveanetmagneticmomentperatomduetounpairedelectronspins.Inzerofieldthesemagneticmomentsarerandomlyorientedbut,undertheactionofanexternalfieldH,theycanbealignedinthefielddirection.Asaresultofthisalignment,themagnetisationMisparalleltothefieldand,hence,thesusceptibilityispositive.Ingeneral,verylargefieldsareneededtoalignallthemomentsandthereforethesusceptibility,althoughpositive,isverysmall.Orderedmagneticmaterials(

>>1,typically

50-104)showlarge,intrinsicmagneticmoments,andcanbehaveasiftheywerespontaneouslymagnetised.Varioustypesofmagneticmomentorderinghavebeenobserved:(1)Ferromagnetic;(2)Ferrimagnetic;(3)Antiferromagnetic.49VariousMagnetism50Diamagnetism〔抗磁性〕Diamagnets

havenonetmagneticmomentontheiratoms,becausetheelectronsareallpairedwithantiparallelspins.拉莫尔进动

WhenamagneticfieldHisapplied,theorbitsoftheelectronschangeinaccordancewithLenz’slaw,andtheysetupanorbitalmagneticmomentwhichopposesthefield,andthereforegivesverysmallnegativesusceptibility(磁化率χ<0).M与H的方向相反，所以由此而产生的物质磁性称作抗磁性。抗磁性存在于一切物质中，但只有在抗磁性物质中才能从宏观上表现出来，在另外的物质中，这种磁性被其他磁性所掩盖。51Thesusceptibility,isnegativeItdoesnotchangemuchwithtemperatureWhenadiamagneticmaterialisplacednearamagnet,itwillberepelledfromtheregionofgreatermagneticfield,justoppositetoaferromagneticmaterial.Examples:Peopleandfrogsarediamagnetic.

Metalssuchasbismuth,copper,gold,silverandlead,aswellasmanynonmetalssuchaswaterandmostorganiccompoundsarediamagnetic.water,inertgases

TDiamagnetism抗磁性52根据抗磁性物质χ值的大小及其与温度的关系可将抗磁性物质分为三种类型：1、弱抗磁性例如惰性气体、金属铜、锌、银、金、汞等和大量的有机化合物，磁化率极低，约为-10-6，并根本与温度无关；2、反常抗磁性例如金属铋、镓、碲、石墨以及γ-铜锌合金，其磁化率较前者约大10-100倍，Bi的磁化率χ比较反常，是场强H的周期函数，并与温度强烈相关；3、超导体抗磁性许多金属在其临界温度和临界磁场以下时呈现超导性，具有超导体完全抗磁性，其χ=-1.Diamagnetism抗磁性53VeryhighfieldswouldsaturatemagnetizationHeatingthegaswouldtendtodisorderthemomentsandhencedecreasemagnetizationWhenaparamagneticmaterialisplacednearamagnet,itwillbeattractedtotheregionofgreatermagneticfield,likeaferromagneticmaterial.Thedifferenceisthattheattractionisweak.

Itisexhibitedbymaterialscontainingtransitionelements,rareearthelementsandactinideelements.

Liquidoxygenandaluminumareexamplesofparamagneticmaterials.

Paramagnetism(顺磁性)54原子、分子或离子具有不等于零的磁矩，并在外磁场作用下沿轴向排列时便产生顺磁性。顺磁性物质的磁化率χ>0，数值很小，约为10-3-10-6。顺磁性也可以分为三类：1、郎之万〔Langevin〕顺磁性包括O2和N2气体、三价Pt和Pd、稀土元素，许多金属盐以及居里温度以上的铁磁性和亚铁磁性物质。原子磁矩可自由地进行热振动，χ值与温度有关，服从居里〔Curie〕定律：χ=C/T或居里-外斯〔Curie-Weiss〕定律：χ=C/〔T+θ〕式中：C—居里常数〔K〕，T—绝对温度〔K〕，θ—外斯常数〔K〕1/T(K)θΧ斜率C居里（Curie）定律居里-外斯（Curie-Weiss）Paramagnetism(顺磁性)552、泡利〔Pauli〕顺磁性典型代表物为碱金属，它们的磁化率相对较前一种为低，并且其值几乎不随温度变化。3、超顺磁性在常态下为铁磁性的物质，当呈现为极微细的粒子时那么表现为超顺磁性。此时粒子的自发极化本身作热运动，产生郎之万磁性行为，初始磁化率随温度降低而升高。Paramagnetism(顺磁性)56

MisproportionaltotheappliedfieldH=Lim

H→0M/H=C/TCURIE’SLAWPIERRECURIENormalparamagneticsubstancesobeytheCurieLawExamples:Aluminum,platinum,manganese,chromium

=C/T1/=T/C

1/T

inKParamagnetism(顺磁性):Curie’sLaw57强磁性(Magneticorderingmaterials)在强磁性物质中，原子间的交换作用使得原子磁矩保持有秩序地排列，即产生所谓自发磁化。Magneticdomain:原子磁矩方向排列规律一致的自发磁化区域叫做磁畴。存在饱和磁化强度Ms。强磁性物质的磁化率χ值是很大的正值，并且易于在外磁场作用下到达饱和磁化。强磁性可以分为如下三种类型：铁磁性、亚铁磁性、弱铁磁性。58whereqistheanglebetweenspinsandJexistheexchangeintegral.ForJex>0,ferromagneticorderresultsinanenergyminimum;forJex<0,anantiferromagneticalignmentisfavoured.Whenconsideringasolid,itisthennecessarytosumtheexchangeoveralltheelectronswhichcancontributetothisenergy,sothat:ExchangeinteractionExchangeinteractionisresponsibleforthephenomenonofmagneticmomentordering.Itsphysicaloriginisquantum-mechanical.Theexchangeinteractionreducestheenergyassociatedwithparallelalignmentofspins,evenintheabsenceofanexternalfield.Thisresultsinanetmagneticmoment.In1928,Heisenberg[2]showedthatexchangeenergy(Eex)canberepresentedby:59Exchangeinteraction对于磁性物质，由于近邻原子共用电子〔交换电子〕所引起的磁矩之间的交换作用所产生能量，称作交换能〔Jex〕，因其以积分形式出现，也称交换积分。它取决于近邻原子未填满的电子壳层相互靠近的程度，并决定了原子磁矩的排列方式和物质的根本磁性。i〕Jex>0，交换作用使得相邻原子磁矩平行排列，产生铁磁性〔Ferromagnetism〕。ii〕Jex<0，交换作用使得相邻原子磁矩反平行排列，产生反铁磁性〔Antiferromagnetism〕。iii〕原子间距离足够大时，Jex值很小时，交换作用缺乏于克服热运动的干扰，原子磁矩随机取向排列，于是产生顺磁性〔Paramagnetism〕60铁氧体材料具有亚铁磁性〔Ferrimagnetism〕,其中金属离子具有几种不同的亚点阵晶格，因相邻的亚点阵晶格相距太远，因此在其格点的金属离子之间不能直接发生交换作用，但可以通过位于它们之间的氧原子间接发生交换作用，或称超交换作用〔Superexchange〕。反铁磁性〔Antiferromagnetism〕材料中也存在超交换作用。Superexchangeinteraction超交换作用反铁磁性NiO中的超交换作用61原子磁矩方向一致整齐排列，MaterialsthatretainamagnetizationinzerofieldQuantummechanicalexchangeinteractionsfavourparallelalignmentofmomentsExamples:iron,cobaltFerromagnetism〔铁磁性〕62ThermalenergycanbeusedtoovercomeexchangeinteractionsCurietempisameasureofexchangeinteractionstrength原子磁矩的排列为方向一致的整齐排列，随着温度的升高，这种排列受热扰动的影响而愈加紊乱，同时物质的自发磁化强度也愈来愈小。当温度上升到某一定值TC(居里温度)时，自发磁化消失，物质由铁磁型转变为顺磁性。大局部强磁性金属和合金属于这种磁性。Ferromagnetism〔铁磁性〕63MdecreasesrapidlywithHBeyondtheCurietemperatureitbehaveslikeaparamagneticsubstanceExamples:iron,cobalt,nickelBehaveslikeaparamagnetCurieTemperatureTCMdecreasesrapidlywithHFerromagnetism〔铁磁性〕64Theinternalexchangeinteractiontriestokeepthemagneticmomentsaligned,butthisorientationcanbedestroyedbyincreasingtemperature.Whenasufficientlyhightemperatureisreachedthethermalenergyovercomestheexchangeenergyandthematerialundergoesanorderedferromagneticphasetoadisorderedparamagneticphase.ThetemperatureisknownastheCurietemperatureTc.TheCurielawstatesthatthesusceptibilityofaparamagnetisproportionaltothereciprocalofthetemperatureT,i.e.whereCisconstant.Toincludethosematerialswhichundergoanorder-disordertransitiontoferromagnetismorferrimagnetismatTc,theaboverelationshipbecomescalledCurie-Weisslaw,whichisageneralisationoftheCurielaw.Ferromagnetism:Curietemperature〔居里温度〕，TC65Antiferromagnetism〔反铁磁性〕Insomematerials,exchangeinteractionsfavourantiparallelalignmentofatomicmagneticmomentsMaterialsaremagneticallyorderedbuthavezeroremnantmagnetizationandverylowManymetaloxidesareantiferromagnetic反铁磁性物质的原子磁矩具有完全相互抵消的有序排列，因而自发磁化强度为零。在外磁场作用下仍具有相当于强顺磁性物质的磁化率〔χ为10-3-10-6〕，所以这类磁性为弱磁性。Example:CobaltOxides66likeparamagnetsaboveacriticaltemperatureTNcalledNeéltemperature(奈耳温度).BelowTNissmall&T-dependenceisdifferentfromparamagnets.Thermalenergycanbeusedtoovercomeexchangeinteractions,MagneticorderisbrokendownattheNéeltemperature(c.f.Curietemp)随着温度升高，磁矩完全抵消的有序排列受到越来越大的破环，磁化率χ值也随之上升。当温度上升到TN时，χ值到达最大；超过TN，有序排列完全破环，而转化为顺磁性。HeatAntiferromagnetism〔反铁磁性〕67根据原子磁矩排列方式的不同，分为：1〕正常反铁磁性原子磁矩排列为互相平行而大小和数量相等的两组。MnO、NiO及FeS等化合物2〕螺旋磁性在晶体的一个平面内，原子磁矩的排列方向一致，而在相邻的另一平面内，原子磁矩较前一个平面内的原子磁矩一致性地旋转了一定的角度，形成螺旋式的旋转。重稀土金属Tb、Dy、Ho、Er、Tm等具有这种磁性。3〕自旋密度波原子磁矩密度〔自旋密度〕本身具有正旋波调制结构。在Cr及其合金中存在这种结构。Antiferromagnetism〔反铁磁性〕68Ferrimagnetism〔亚铁磁性〕AntiferromagneticexchangeinteractionsDifferentsizedmomentsoneachsublatticeresultinnetmagnetization原子占据两种或两种以上的亚点阵。同一种亚点阵上的原子磁矩平行排列，但不同亚点阵间原子磁矩的反平行排列。原子磁矩相加的结果表现为不等于零的自发磁化强度MS。由于每种亚点阵的磁化强度随温度变化的规律不同，因而总的磁化强度随温度的变化曲线具有不同于铁磁性的各种特殊形状，可以分为Q型、P型、R型和N型。Example:magnetite,maghemiteTTcR型P型N型TCOM69Likeferromagnets,buttheeffecttendstobesmaller.The1/curveisveryclosetozerobelowacriticaltemperature,alsocalledNeéltemperature.Examples:magnetite(Fe3O4)andspinelferritesFerrimagnetism〔亚铁磁性〕70BPARAmagneticDIAmagneticFERROmagneticFERRImagneticANTIFERROmagneticBComparison〔磁场作用〕71MagnetizationCurves(磁化曲线)抗磁顺磁铁磁亚铁磁反铁磁DIAmagneticFERROmagneticFERRImagneticPARAmagneticANTIFERROmagnetic72磁化率与磁行为类型磁性种类典型的χ值χ随温度的变化χ随场强的变化抗磁性-1×10-6无变化无关顺磁性10-4~10-5减小无关铁磁性102~106减小无关反铁磁性0~10-2增加有关Comparison磁化率与磁行为PARAmagnetic>0,r

>1DIAmagnetic<0,r

<1

r

=0(superconductors)(r

-1)=→

r

=

+1FERROmagnetic>0,r

>>173MagneticdomainsApplyingafieldchangesdomainstructure;Domainswithmagnetizationindirectionoffieldgrow;OtherdomainsshrinkApplyingverystrongfieldscansaturatemagnetizationbycreatingsingledomainFerromagneticmaterialstendtoformmagneticdomains;Eachdomainismagnetizedinadifferentdirection;Domainstructureminimizesenergyduetostrayfields74MagneticdomainsRemovingthefielddoesnotnecessarilyreturndomainstructuretooriginalstateHenceresultsinmagnetichysteresis75MagnetichysteresisMdependsonpreviousstateofmagnetizationRemanentmagnetizationMrremainswhenappliedfieldisremovedNeedtoapplyafield(coercivefield)inoppositedirectiontoreduceMtozero.76Heatingamagnetizedmaterialgenerallydecreasesitsmagnetization.RemnantmagnetizationisreducedtozeroaboveCurietemperatureTcHeatingasampleaboveitsCurietemperatureisawayofdemagnetizingitThermaldemagnetizationEffectoftemperatureonremanentmagnetization77GeneratingauniformmagneticfieldinthelaboratoryAnelectriccurrentrunthroughaconductingcoil(solenoid)generatesauniformfluxdensitywithinthecoil78InsertingaspecimenintothecoilGenerally,theorbitalandspinmagneticmomentswithinatomsrespondtoanappliedmagneticfieldFluxlinesareperturbedbyspecimen79SpecimeninmagneticfieldIfspecimenhasnomagneticresponse,fluxlinesarenotperturbed80“Magnetic”materials“magnetic”materialstendtoconcentratefluxlinesExamples:materialscontaininghighconcentrationsofmagneticatomssuchasiron,cobalt81DiamagneticmaterialsDiamagneticmaterialstendtorepelfluxlinesweaklyExamples:water,protein,fat82MagneticMaterialsROOMTEMPERATURE83对于磁学单位，考虑强度为p1,p2的磁极，其单位为静电单位〔electrostaticunits，esu〕，那么上式变为:在cgs单位系统中，力的单位为达因〔dyn)〕，所以一个单位的磁极强度为1gm1/2cm3/2s-1。实际上，自然界没有独立的单磁极子。但是磁极强度的概念仍然是cgs磁学单位的核心。cgsUnits在cgs〔厘米-克-秒〕系统中推导磁学单位与在SI系统中完全不同。根据库仑定律，两个电荷〔q1，q2〕之间的力为：其中r为两个电荷之间的距离。在cgs单位系统中，比例常数k为1。而在SI单位系统中，其值为1/(4π0)，其中0=107/4c2，c是真空中的光速。因此0=8.859*10-12AsV-1m-1。[可见为什么大多数学者偏爱cgs单位！]84cgsUnits一个磁极子或者一个独立的电荷会在其周围空间产生一个磁感应强度0H。一个单位的磁场强度定义为〔1oersted或者Oe〕相当于在每单位磁极强度上施加一达因的力。因此三者之间的关系为：所以，具有一单位磁极强度的磁极放在1Oe的磁场中会受到一达因的力。这个力也等效于在距离具有一个磁极强度的磁极1cm的地方所受到的力。因此，在距离一个单极子1cm的地方的磁场为1Oe，并且按着1/r2的规律递减。现在我们可以定义1Oe是每平方cm上1line的力。假设一个半径为r的球包围一个磁单极子，球的外表积为4r2，这个球就为单位圆球〔aunitsphere〕〔r＝1〕，在球面上的磁场就为1Oe。那么一定有4lines的力穿过这个球。85cgsUnits至于磁矩，从cgs系统的观点看，我们假设一个长为l的磁铁，其两端磁极的强度为p。把这个磁铁放在

0H的磁场中，那么这个磁铁所受的扭力矩为：其中pl是磁矩m，的单位是能量〔在cgs系统中，其单位是ergs〕，所以，磁矩的定位是ergs/Oe。我们因此定义一电磁单位〔emu〕为1erg/Oe。注意，以上推导中的系数0，在使用cgs单位的Cullity〔1972〕以及很多书籍和文章中并不存在。原因是，在应用cgs系统时，这个系数值为1，所以oersteds(H)和gauss(B)经常被互换使用。然而在SI系统中，二者并不相同，因为这个系数的值为4x10-7。86UnitsandConversions87UnitsandConversionsSymbolDefinitionCGSSIConversionCGStoSI

FluxMaxwellWeber(Wb)10-8MMagnetisation,Numberofpolesinagivencross-sectionOersted(Oe)A/m103/4

BFluxDensityorInductionB=m0H+JMaxwell/cm2orGauss(G)Wb/m2orTesla(T)10-4HMagneticFieldStrengthOersted(Oe)A/m103/4

m0PermeabilityoffreespaceUnit4px10-7Henry/metre-JIntensityofmagnetisationormagneticvolumeperunitvolumeGauss(G)Tesla(T)10-4HcCoercivity,magneticfieldrequiredtoreduceBtozeroaftersaturationO