Lecture1磁性物理和磁學基本概念

1、磁性材料：原理、工藝與應(yīng)用磁性材料：原理、工藝與應(yīng)用Magnetic Materials: Basic theory, Processing and ApplicationsLecture 1 磁學基本概念與磁性物理基礎(chǔ)磁學基本概念與磁性物理基礎(chǔ)磁性材料：原理、工藝與應(yīng)用2OutlinenIntroduction to this coursenOrigin of MagnetismnVarious types of magnetismnVarious magnetic materialsnSummary磁性材料：原理、工藝與應(yīng)用3Before start, some questionsnWha

2、t metals are magnetic?nSince Fe are magnetic metal, why does it not attract a small piece of iron?nPure iron is magnetic and some steel is not, Why?nFe, W, Mo, V, Nb are all b.c.c structured and have unpaired electrons, why is only Fe magnetic?nWhy do NdFeB magnets packed with Fe foil when posted?磁性

3、材料：原理、工藝與應(yīng)用4A SurveyHave you ever studied -nFerromagnetism鐵磁學?nMagnetism磁學?nElectromagnetism電磁學?nSolid Physics固體物理?nMaterials Physics材料物理?nGeneral Physics普通物理（大學物理）?磁性材料：原理、工藝與應(yīng)用5About This CourseIt is not -nElectromagnetism電磁學!nMagnetism磁學!nMagnetic Physics磁性物理！It is nMagnetic Materials！We emphasiz

4、e theory, processing and application!磁性材料：原理、工藝與應(yīng)用6Course Structure共32學時，含：n論文報告4學時n學術(shù)報告4學時n討論2學時n講授24學時內(nèi)容： 磁性基礎(chǔ)、軟磁材料、硬磁材料、磁記錄、磁致伸縮材料、磁性-物性相互作用、巨磁阻材料、吸波材料、磁性薄膜、磁性納米結(jié)構(gòu)、磁性材料最新進展磁性材料：原理、工藝與應(yīng)用7What is your opinion about this course?nWhat do you want to learn?nHow to teach?Let me know by email!磁性材料：原理、工藝

5、與應(yīng)用8All About Examn論文報告，70%；平時，30%。Topics for your report: Recent progress in advanced - 1. Nanocomposite Rare-earth permanent magnetic materials; 2. Nanocomposite soft magnetic materials; 3. perpendicular magnetic recording; 4. Magnetoelectric materials; 5. Magnetic thin films for microwave absorbe

6、r; 6. GMR materials; 7. One-dimension magnetic nanostructures; 8. Spintronics; 9. Magnetocaloric materials; 10. Magnetostriction materials.You can write your report in Chinese though use of English is encouraged !Note1)In formal journal paper style;2)Recent progress;3)No plagiarism allowed.磁性材料：原理、工

7、藝與應(yīng)用9nTerm Magnet comes from the ancient Greek city of Magnesia, at which many natural magnets were found. nPliny the Elder (23-79 AD Roman) wrote of a hill near the river Indus that was made entirely of a stone that attracted iron.nKnown in China and Europe -800 BCLodestoneLets get start: A story 磁

8、性材料：原理、工藝與應(yīng)用10LODESTONEnNow, we refer to these natural magnets as lodestones (also spelled loadstone; lode means to lead or to attract) which contain magnetite, a natural magnetic material Fe3O4.nWhen lightning strikes the earth it could create a magnetic field large enough to saturate the magnetiza

9、tion of lodestone .Typical current 1,000,000 Amp.Once in 1 10 million years磁性材料：原理、工藝與應(yīng)用11Historyn Chinese as early as 121 AD knew that an iron rod which had been brought near one of these natural magnets would acquire and retain the magnetic propertyand that such a rod when suspended from a string

10、would align itself in a north-south direction.n Use of magnets to aid in navigation can be traced back to at least the 11th century.Basically, we knew the phenomenon existed andwe learned useful applications for it. We did not understand it.司南司南磁性材料：原理、工藝與應(yīng)用12Electrified Amber attracts small objects

11、Lodestone attracts iron A Connection ?Hans Christian rsted ( 1777 1851) Oersteds Experimentn Danish scientist Hans Christian Oersted observed that a compass needle in the vicinity of a wire carrying electrical current was deflected!n a connection between electrical and magnetic phenomena shown.Oerst

12、eds experiment (1820)磁性材料：原理、工藝與應(yīng)用13A quantitative relationship between a changing magnetic field and the electric field created by the changeMichael Faraday(1791-1867) Faraday: Effect of a changing magnetic fieldnIn 1831, Faraday discovered that a momentary current existed in a circuit, when the cu

13、rrent in a nearby circuit was started or stopped. nShortly thereafter, he discovered that motion of a magnet toward or away from a circuit could produce the same effect.磁性材料：原理、工藝與應(yīng)用14Henrys work: a lesson!Joseph Henry (1797-1878)nJoseph Henry failed to publish what he had discovered 6-12 months bef

14、ore FaradaynHenry was always slow in publishing his results, and he was unaware of Faradays work. nToday Faraday is recognized as the discoverer of mutual inductance (the basis of transformers), while Henry is credited with the discovery of self-inductance.磁性材料：原理、工藝與應(yīng)用15The connection is madeSUMMAR

15、Y: Oersted showed that magnetic effects could be produced by moving electrical charges; Faraday and Henry showed that electric currents could be produced by moving magnetsAll magnetic phenomena result from forces between electric charges in motion.磁性材料：原理、工藝與應(yīng)用16Ampere: Molecular CurrentsAmpere firs

16、t suggested in 1820 that magnetic properties of matter were due to tiny atomic currents:nExistence of small molecular currents nEach atom/molecule would behave as a small permanent magnetnWould align in the presence of a magnetic fieldAndre Marie Ampere(1775-1836) 磁性材料：原理、工藝與應(yīng)用17The magnetic field i

17、n space around an electric current is proportional to the electric current which serves as its source. Amperes LawFor any closed loop path, the sum of the length elements times the magnetic field in the direction of the length element is equal to the permeability times the electric current enclosed

18、in the loop.磁性材料：原理、工藝與應(yīng)用18Do not forget themnrsted showed that magnetic effects could be produced by moving electrical charges; nFaraday and Henry showed that electric currents could be produced by moving magnets. All magnetic phenomena result from forces between electric charges in motion.nAmpere

19、first suggested in 1820 that magnetic properties of matter were due to tiny atomic currents.磁性材料：原理、工藝與應(yīng)用19Top ten list: what we should have known about magnetism？1. There are North Poles and South Poles. 2. Like poles repel, unlike poles attract. 3. Magnetic forces attract only magnetic materials.

20、4. Magnetic forces act at a distance. 5. While magnetized, temporary magnets act like permanent magnets. 6. A coil of wire with an electric current flowing through it becomes a magnet. 7. Putting iron inside a current-carrying coil increases the strength of the electromagnet. 8. A changing magnetic

21、field induces an electric current in a conductor.9. A charged particle experiences no magnetic force when moving parallel to a magnetic field, but when it is moving perpendicular to the field it experiences a force perpendicular to both the field and the direction of motion. 10. A current-carrying w

22、ire in a perpendicular magnetic field experiences a force in a direction perpendicular to both the wire and the field.磁性材料：原理、工藝與應(yīng)用20Origin of Magnetism of MatternAmpere: molecular currentsnModern physics: magnetic moments in atoms （“磁矩學說磁矩學說”或或“磁偶極矩學說磁偶極矩學說”） 1) unpaired electron spins mainly 2) th

23、e orbital motion of electrons within the material to a lesser extent磁性材料：原理、工藝與應(yīng)用21n物理學原理：任何帶電體的運動都必然在周圍的空間產(chǎn)生磁場。n電動力學定律：一個環(huán)形電流具有一定的磁矩，它在磁場中行為像個磁性偶極子。設(shè)環(huán)形電流的強度為I（A），它所包圍的面積為A（m2）,則該環(huán)流的磁矩為：m=I*A （A m2）n玻爾（Bohr）原子模型：原子內(nèi)的電子在固定的軌道上繞原子核作旋轉(zhuǎn)運動，同時還繞自身的軸線作自旋運動。前一種運動產(chǎn)生“軌道磁矩”，后一種運動產(chǎn)生“自旋磁矩”。n物質(zhì)磁性來源的同一性：盡管宏觀物質(zhì)的磁性是

24、多種多樣的，但這些磁性都來源于電子的運動。A(m2)I(A)mOrigin of Magnetism of Matter磁性材料：原理、工藝與應(yīng)用22原子磁矩Macroscopic properties are the result of electron magnetic moments。Moments come from 2 sources: Orbital motion around a nucleus（軌道磁矩）與Spinning around an axis（自旋磁矩）。原子核磁矩比電子磁矩小3個數(shù)量級，一般情況下忽略不計。因此，原子磁矩主要來源于原子核外電子的。n 原子中的電子成對

25、地存在。這些成對電子的自旋磁矩和軌道磁矩方向相反而互相抵消，使原子中的電子總磁矩為零。 非磁原子。n 原子中的電子磁矩沒有完全抵消使原子中電子的總磁矩（有時叫凈磁矩，剩余磁矩）不為零。磁性原子。磁性材料：原理、工藝與應(yīng)用23原子的總磁矩應(yīng)是按照原子結(jié)構(gòu)和量子原子的總磁矩應(yīng)是按照原子結(jié)構(gòu)和量子力學規(guī)律將原子中各個電子的軌道磁矩力學規(guī)律將原子中各個電子的軌道磁矩和自旋磁矩相加起來的合磁矩。和自旋磁矩相加起來的合磁矩。原子磁矩nThe net magnetic moment for an atom is the sum of the magnetic moments of constituent e

26、lectronsn Atoms with completely filled electron shells are incapable permanent magnetizationn All materials exhibit some form of magnetization.n Three types of response; ferro, dia and paramagnetic.磁性材料：原理、工藝與應(yīng)用24原子的磁矩n電子和原子核均有磁矩，但原子核的磁矩僅有電子磁矩的1/1836.5。n電子軌道磁矩： l：軌道角量子數(shù)，0, 1, 2, 3, 4n-1 (s, p, d, f,

27、電子態(tài))；n: 主量子數(shù)；波爾磁子B=9.273210-24 A/m2 軌道磁矩在外磁場方向的投影: l,H=mlB ml: 角動量方向量子數(shù)或磁量子數(shù)=0,1, 2, ln電子自旋磁矩： s：自旋量子數(shù)， s=1/2 自旋磁矩在外磁場方向的投影: s,H=2msB ms: 自旋角動量方向量子數(shù)= 1/2Blll) 1( Bsss) 1(2磁性材料：原理、工藝與應(yīng)用25原子的磁矩n原子磁矩=電子軌道磁矩+自旋磁矩n對于3d過渡族和4f稀土金屬及合金，原子磁矩： 式中: 稱為郎德因子。J：原子總角量子數(shù)；L：原子總軌道角量子數(shù)；S：原子總自旋量子數(shù)；波爾磁子B=9.273210-24 A/m2n

28、量子力學證明，原子磁矩在外磁場方向的投影也是量子化的 J,H=gJmJB mJ: 原子角動量方向量子數(shù)或原子磁量子數(shù)=0,1, 2, JnIf J, L and S are known, J and J,H can be calculated.BJJJJg) 1() 1(2) 1() 1() 1(1JJLLSSJJgJ磁性材料：原理、工藝與應(yīng)用26n 在一個填滿的電子殼層中，電子的軌道磁矩和自旋磁矩為零。n 對于次電子層（等）未填滿電子的原子，在基態(tài)下，其總角量子數(shù)J、總軌道量子數(shù)L和總自旋量子數(shù)S存在如下關(guān)系： （1）在未填滿電子的那些次電子層內(nèi)，在Pauli原理允許的條件下S和L均取最大值

29、；（2）次電子層未填滿一半時， J=L-S；（3）次電子層填滿一半或一半以上時， J=L+SHund規(guī)則J,H=gJmJBmJ=0,1, 2, J磁性材料：原理、工藝與應(yīng)用27原子磁矩盡管上述計算方法有其深奧的量子力學來源，但與實驗值之間的符合并不十分好。對鐵磁和反鐵磁材料，有時也使用更簡化的方程：= g s 或者干脆將g作為可調(diào)參數(shù)以與實驗結(jié)果吻合。磁性材料：原理、工藝與應(yīng)用28眾所周知，電子軌道運動是量子化的，因而只有分立的軌道存在，換言之、角動量是量子化的，并由下式給出Pl 普郎克(Planck)常數(shù)：玻爾磁子 (Bohr magneton)(10055. 1234JSxh電子的軌道磁矩

30、220022e reMr 2Pm r電子的角動量是：電子的軌道磁矩：PMLeiv電子的軌道磁矩Pmel20WbmmeB29010165. 12llmeBl20磁性材料：原理、工藝與應(yīng)用29與自旋相聯(lián)系的角動量的大小是/2，因而自旋角動量可寫為：sP S是自旋角動量量子數(shù)21自旋磁矩自旋磁矩PmeS0通常磁矩和P之間的關(guān)系由下式給出：PmegS20這里g因子( g-factor)對自旋運動是自旋運動是2，而對軌道運動是軌道運動是1。ssmexBs2220lMlmexMBL210不論是自旋磁矩，還是軌道磁矩，都是玻爾磁子 B的整數(shù)倍。P se電子的自旋磁矩磁性材料：原理、工藝與應(yīng)用30The Un

31、iversality: MagnetismnAll matter are magnetic 物質(zhì)磁性無處不在 （1）物質(zhì)的各種形態(tài)，無論是固態(tài)、液態(tài)、氣態(tài)、等離子態(tài)、超高密度態(tài)和反物質(zhì)態(tài)都具有磁性； （2）物質(zhì)的各個層次，無論是原子、原子核、基本粒子和基礎(chǔ)粒子等都會具有磁性。 （3）無限廣袤的宇宙，無論是天體，還是星際空間都存在著或強或弱的磁場。地球磁場強度：240A/m，太陽的磁場強度80A/m，中子星磁場強度高達1013-1014A/m。n物質(zhì)的磁性與其他屬性之間存在著廣泛的聯(lián)系，并構(gòu)成多種多樣的耦合效應(yīng)和雙重（多重）效應(yīng)（如磁電效應(yīng)、磁光效應(yīng)、和磁熱效應(yīng)等）。這些效應(yīng)是了解物質(zhì)結(jié)構(gòu)和性能

32、關(guān)系的重要途徑，又是發(fā)展各種應(yīng)用技術(shù)和功能器件（如磁光存儲技術(shù)、磁記錄技術(shù)和霍爾器件等）的基礎(chǔ)。磁性材料：原理、工藝與應(yīng)用31Magnetic Poles (磁極磁極)nthe external magnetic field is strongest at the polesnThe two types of magnetic poles cannot exist separately always coupled together as a dipole. Isolated magnetic monopoles have not yet been detected.n表示磁極強弱的物理量稱為

33、“磁極強度” 。兩個強弱相同的磁極，在真空中相距1厘米時，如果它們之間相互作用力為1達因，則每個磁極的強度就規(guī)定為一個電磁系單位制的磁極強度單位。n磁極強度（Wb or emu）為m1、m2的磁極間相互作用力：F=km1m2/r2k=1/40, 0=410-7 H/m磁性材料：原理、工藝與應(yīng)用32A magnetic dipole （磁偶極子）l A loop of electric current generates a magnetic dipole fieldl Field lines run from the North pole to the South polel Field li

34、nes indicate the direction of force that would be experienced by a North magnetic monopole磁性材料：原理、工藝與應(yīng)用33Magnetic Moment （磁矩）n電流在其四周產(chǎn)生環(huán)繞的磁場。如果把通電導(dǎo)線圈成一個半徑為r的圓環(huán)，其周圍的鐵屑則展示了其產(chǎn)生的磁場的形態(tài)。這個磁場等效于一個磁矩為M的磁鐵產(chǎn)生的磁場。n由電流i產(chǎn)生的磁場，其強度和圓環(huán)的面積相關(guān)（圓環(huán)越大，磁矩就越大），即M = ir2。由n個圓環(huán)產(chǎn)生的總磁矩是由這些單一圓環(huán)產(chǎn)生的磁矩的迭加，即：M=nir2 因此，磁矩M的單位為Am2。n環(huán)電流

35、磁矩：M=IA 棒狀磁鐵磁矩：M=mllmAI磁性材料：原理、工藝與應(yīng)用34Magnetic Field（磁場）, HnA magnetic field H is generated whenever there is electric charge in motion (electric currents). This can be due to macroscopic currents in a conductor, or microscopic currents associated with electrons in atomic orbits, or be produced by a

36、 permanent magnet. nH is measured in A/m or Oe (SI system or cgs system). 1 A/m = 0.01257 Oe For a solenoid:H=NI/L磁性材料：原理、工藝與應(yīng)用35Magnetic Field（磁場）, Hn電流能夠產(chǎn)生磁場，因此可以借助于電場來定義由其產(chǎn)生的磁場。當導(dǎo)線通以電流時，根據(jù)右手法則，右手的大拇指指向電流方向（即正方向，與電子流動方向相反），其它成環(huán)狀的四指則指示了相應(yīng)的磁場方向。n磁場H同時垂直于電流方向和徑向單位矢量r，其強度與電流強度成正比。磁場強度H可以由安培定律給出： 因此，磁場

37、強度H的單位為A/m。磁性材料：原理、工藝與應(yīng)用36Magnetic Field, HnA force field similar to the gravitational and electrical field, detected by a probe. nA magnetic field exerts a torque which orients dipoles with the field.nDirection of magnetic field at any point is defined as the direction of motion of a charged partic

38、le on which the magnetic field would not exert a force. nMagnetic field lines describe the structure of magnetic fields in three dimensions.For a magnet: H=F/m1=k m1/r2F=km1m2/r2磁性材料：原理、工藝與應(yīng)用37Flux density （磁通密度）（磁通密度）, Bn磁通量（磁通量（Magnetic flux， ）磁場是一個矢量場，在任何一點它都由方向和強度共同定義。其方向由磁力線箭頭確定，而其強度則由磁力線的密度確定。

39、磁力線即為磁通量，其密度可用來衡量磁場的強度（即磁感應(yīng)強度B）。nDensity of flux (or field) lines determines forces on magnetic polesnDirection of flux indicates direction of force on a North polenHigher flux density exerts more force on magnetic polesB A磁性材料：原理、工藝與應(yīng)用38Flux density BB depends onnGeometry and current in solenoidnMa

40、gnetic properties of the materialnGeometry of material磁性材料：原理、工藝與應(yīng)用39Magnetic Induction（磁感應(yīng)強度）（磁感應(yīng)強度）, BnThe magnetic induction B, also known as the flux density, measured in Tesla (SI) or Gauss (cgs), is the response of a medium to the presence of a magnetic field. 1T=10000GsnH field creates magnet

41、ic inductionnB is the magnetic induction; the magnitude of the internal field within a substance磁性材料：原理、工藝與應(yīng)用40Magnetic Permeability（磁導(dǎo)率）, B= Hn is the permeability （磁導(dǎo)率）（磁導(dǎo)率） of the medium (Henries per meter)B0 = 0Hn 0 is the permeability of a vacuum r= / 0n r is the relative permeability磁性材料：原理、工藝

42、與應(yīng)用41Magnetization (磁化強度磁化強度), MnWe define magnetization as the total magnetic dipole moment (magnetic moment) per unit volume within the materialnIt is measured in A/m (SI) or emu/cm3 (cgs).VolumeMNii1磁性材料：原理、工藝與應(yīng)用42Magnetization depends on.nNumber density of magnetic dipole moments within material

43、nMagnitude of the magnetic dipole moments within the materialnThe arrangement of the magnetic dipoles within the material磁性材料：原理、工藝與應(yīng)用43Polarization（磁極化強度）, JnThe magnetic polarisation J, measured in Tesla, is given by J= oM, where o (=1.23664 10-6 H/m) is the permeability of free space. nM increase

44、s as more electronic magnetic moments are aligned. nWhen all magnetic moments are aligned in the same direction, the saturation magnetisation (polarisation) Ms (Js) is achieved.磁性材料：原理、工藝與應(yīng)用44How does M respond to H?nThere is a variety of ways that M responds to HnResponse depends on type of materia

45、lnResponse depends on temperaturenResponse can sometimes depend on the previous history of magnetic field strengths and directions applied to the material磁性材料：原理、工藝與應(yīng)用45Non-linear responsesnGenerally, the response of M to H is non-linearnOnly at small values of H or high temperatures is response som

46、etimes linearnM tends to saturate at high fields and low temperatures磁性材料：原理、工藝與應(yīng)用46B, H, M, J RelationshipsnB (in T) consists of two contributions: one from magnetic field H (A/m), the other from magnetisation M (A/m). This leads to one of the most important relations in magnetism:JHMHB00)(n If the

47、re is no magnetization M .B0H磁性材料：原理、工藝與應(yīng)用47Magnetic Susceptibility（磁化率）（磁化率）, nB = 0 ( H + M ) nReplace B= H H = 0 ( H + M ) r 0 H = 0 ( H + M ) 0 M = 0 ( r -1) H M = ( r -1) H Magnetic SusceptibilityM=H =r1n, Susceptibility, measures the material response relative to a vacuum (Dimensionless)磁性材料：原

48、理、工藝與應(yīng)用48Various MagnetismBased on n抗磁性（Diamagnetism）n順磁性（Paramagnetism）n鐵磁性（Ferromagnetism）n亞鐵磁性（Ferrimagnetism）n反鐵磁性（Antiferromagnetism） 0, typically 10-3-10-5 1, typically 50-104 1, typically 50-104) show large, intrinsic magnetic moments, and can behave as if they were spontaneously magnetised.

49、Various types of magnetic moment ordering have been observed: (1) Ferromagnetic; (2) Ferrimagnetic; (3) Antiferromagnetic.磁性材料：原理、工藝與應(yīng)用50Various Magnetism磁性材料：原理、工藝與應(yīng)用51Diamagnetism （抗磁性）（抗磁性）nDiamagnets have no net magnetic moment on their atoms, because the electrons are all paired with antiparall

50、el spins.n拉莫爾進動 When a magnetic field H is applied, the orbits of the electrons change in accordance with Lenzs law, and they set up an orbital magnetic moment which opposes the field, and therefore gives very small negative susceptibility (磁化率0，數(shù)值很小，約為10-3-10-6。順磁性也可以分為三類： 1、郎之萬（Langevin）順磁性 包括O2和N

51、2氣體、三價Pt和Pd、稀土元素，許多金屬鹽以及居里溫度以上的鐵磁性和亞鐵磁性物質(zhì)。原子磁矩可自由地進行熱振動，值與溫度有關(guān)，服從居里（Curie）定律： =C/T 或居里-外斯（Curie-Weiss）定律： =C/（T+） 式中：C居里常數(shù)（K）， T絕對溫度（K）， 外斯常數(shù)（K）1/T(K)斜率C居里（Curie）定律居里-外斯（Curie-Weiss）Paramagnetism (順磁性順磁性)磁性材料：原理、工藝與應(yīng)用56 2、 泡利（Pauli）順磁性 典型代表物為堿金屬，它們的磁化率相對較前一種為低，并且其值幾乎不隨溫度變化。 3、 超順磁性 在常態(tài)下為鐵磁性的物質(zhì)，當呈現(xiàn)為極

52、微細的粒子時則表現(xiàn)為超順磁性。此時粒子的自發(fā)極化本身作熱運動，產(chǎn)生郎之萬磁性行為，初始磁化率隨溫度降低而升高。Paramagnetism (順磁性順磁性)磁性材料：原理、工藝與應(yīng)用57n M is proportional to the applied field Hn = Lim H 0 M / Hn = C / TCURIES LAWPIERRE CURIEnNormal paramagnetic substances obey the Curie Law nExamples : Aluminum, platinum, manganese, chromium =C/T1/ =T/C 1/

53、T in KParamagnetism (順磁性順磁性): Curies Law磁性材料：原理、工藝與應(yīng)用58強磁性強磁性(Magnetic ordering materials)n在強磁性物質(zhì)中，原子間的交換作用使得原子磁矩保持有秩序地排列，即產(chǎn)生所謂自發(fā)磁化。nMagnetic domain: 原子磁矩方向排列規(guī)律一致的自發(fā)磁化區(qū)域叫做磁疇。n存在飽和磁化強度Ms。n強磁性物質(zhì)的磁化率值是很大的正值，并且易于在外磁場作用下達到飽和磁化。強磁性可以分為如下三種類型：鐵磁性、亞鐵磁性、弱鐵磁性。磁性材料：原理、工藝與應(yīng)用59where q is the angle between spins

54、and Jex is the exchange integral. For Jex0, ferromagnetic order results in an energy minimum; for Jex 0，交換作用使得相鄰原子磁矩平行排列，產(chǎn)生鐵磁性（Ferromagnetism）。 ii）Jex 0, r 1DIAmagneticn 0 , r 0, r 1磁性材料：原理、工藝與應(yīng)用74Magnetic domainsApplying a field changes domain structure; Domains with magnetization in direction of f

55、ield grow; Other domains shrinkApplying very strong fields can saturate magnetization by creating single domainFerromagnetic materials tend to form magnetic domains; Each domain is magnetized in a different direction; Domain structure minimizes energy due to stray fields磁性材料：原理、工藝與應(yīng)用75Magnetic domai

56、nsnRemoving the field does not necessarily return domain structure to original statenHence results in magnetic hysteresis磁性材料：原理、工藝與應(yīng)用76Magnetic hysteresisnM depends on previous state of magnetizationnRemanent magnetization Mr remains when applied field is removednNeed to apply a field (coercive fie

57、ld) in opposite direction to reduce M to zero.磁性材料：原理、工藝與應(yīng)用77nHeating a magnetized material generally decreases its magnetization.nRemnant magnetization is reduced to zero above Curie temperature TcnHeating a sample above its Curie temperature is a way of demagnetizing itnThermal demagnetizationEffe

58、ct of temperature on remanent magnetization磁性材料：原理、工藝與應(yīng)用78Generating a uniform magnetic field in the laboratorynAn electric current run through a conducting coil (solenoid) generates a uniform flux density within the coil 磁性材料：原理、工藝與應(yīng)用79Inserting a specimen into the coilnGenerally, the orbital and s

59、pin magnetic moments within atoms respond to an applied magnetic fieldnFlux lines are perturbed by specimen磁性材料：原理、工藝與應(yīng)用80Specimen in magnetic fieldnIf specimen has no magnetic response, flux lines are not perturbed磁性材料：原理、工藝與應(yīng)用81“Magnetic” materialsn“magnetic” materials tend to concentrate flux lin

60、esnExamples: materials containing high concentrations of magnetic atoms such as iron, cobalt磁性材料：原理、工藝與應(yīng)用82Diamagnetic materialsnDiamagnetic materials tend to repel flux lines weaklynExamples: water, protein, fat磁性材料：原理、工藝與應(yīng)用83Magnetic MaterialsROOM TEMPERATURE磁性材料：原理、工藝與應(yīng)用84n 對于磁學單位，考慮強度為p1, p2的磁極，