5006 Magnetism and Magnetic Materials. Chapter 1: Introduction

Transcription

1 5006 Magnetism and Magnetic Materials Chapter 1: Introduction 1. A Brief History of Magnetism 2. Magnetism and Hysteresis 3. Magnet Applications 4. Magnetism, Physics and Technology Comments and corrections please: jcoey@tcd.ie Dublin January

2 Some introductory texts: David Jiles Introduction to Magnetism and Magnetic Materials, Chapman and Hall 1991; 1997 A detailed introduction, written in a question and answer format. Stephen Blundell Magnetism in Condensed Matter, Oxford 2001 A new book providing a good treatment of the basics History: A. Kloss Geschichte des Magnetismus, VDE, Berlin 1994 Light reading: J. D. Livingstone. Driving Force, Harvard University Press Alberto Guimaraes, From Lodestone to Supermagnets, Wiley 2005 Dublin January

3 1. A Brief History of Magnetism Ancient Early scientific Electromagnetic Understanding High-frequency Applications Spin electronics Dublin January

4 Age Date Names Driver Achievements Materials Applications Ancient to 1500 Shen Kua, Petrus Peregrinus State Force field, induced magntism, TRM Iron, lodestone South pointer, Compass Early scientific 1500 to 1820 Gilbert, Descartes D.Bernouilli Navy Earth s field Iron, lodestone Dip circle, Horseshoe magnet Electromagnetic 1820 to 1900 Oersted, Ampere, Faraday, Maxwell Industry/infrastructure E-M induction, Maxwells =ns Electrical steel Motors generators, telegraph, wireless, magnetic recording Understanding 1900 to 1935 Weiss, Bohr, Dirac, Heisenberg, Pauli, Landau Academy Spin, Exchange interactions [Alnico] High-frequency 1935 to 1960 Bloch,, Pound, Purcell Military Microwaves, epr, fmr, nmr Ferrites Radar, television Applications 1960 to 1995 Gorter, Sagawa, Croat Consumers New materials, miniaturization Sm-Co, Nd-Fe-B Consumer electronics Spin electronics 1995 to?? Fert, Parkin. Consumers Thin film devices Multilayers High-density recording, MRAM? Dublin January

5 The Ancient Age to 1500 Key names Shen Kua Petrus Peregrinus Applications South-Pointer Compass Scientific achievements Force field Induced magnetism Thermoremanence Driver The State Dublin January

6 1820 Dublin January

7 The Electromagnetic Age Key names Oersted, Ampere Faraday, Maxwell Hertz Applications Motors, Generators Telegraph, Wireless Magnetic recording Scientific Achievements Electromagnetic Induction Maxwells Equations Driver Industry (Infrastructure) Dublin January

8 Maxwell s equations. B = 0 0. E = (1/µ 0 ) B = j + 0 E/ t E = - B/ t From a long view of the history of mankind, there can be little doubt that the most significant event of the 19th century will be judged as Maxwell s discovery of the laws of electrodynamics. Richard Feynmann Written in terms of two fields B (kg C -1 s -1 ) and E (V m -1 ), they are valid in free space. They relate these fields to the charge density (C m -3 ) and the current density j (A m -2 ) at a point. c = ( 0 µ 0 ) 1/2 c = m s -1 c = Also, the force on a moving charge q, velocity v F = q(e + v B) Dublin January

9 The Age of Understanding Key Players Weiss, Bohr Heisenberg Dirac, Pauli Landau H = -2JS i S j Scientific Achievements Mean Field Theory, Spin, Exchange Interactions Applications Driver Academy Dublin January

10 The 1930 Solvay conference consecrated our physical understand-ing of magnetism in terms of quantum mechanics (exchange) and relativity (spin) The m-j paradigm: m represents the magnetic moment, mainly localized on the atoms J represents the exchange coupling of electron spins Solvay Conference Dirac Heisenberg At this point it seems that the whole of chemistry and much of physics is understood in principle. The problem is that the equations are much to difficult to solve.. P. A. M. Dirac Dublin January

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12 Key Players Gorter, Sagawa, Croat The Age of Applications Applications Consumer Electronics Scientific Achievements New materials Miniaturisation of Magnetic Circuits Driver Industry (Consumer) Dublin January

13 How many magnets do you own? Dublin January

14 The Age of Spin Electronics ? Albert Fert Peter Gruneberg Stuart Parkin Applications High-density recording MRAM? Scientific Achievements Thin film devices Driver Industry (Consumer) Dublin January

15 2. Magnetism and Hysteresis 2.1 The hysteresis loop spontaneous magnetization remanence coercivity virgin curve initial susceptibility major loop The hysteresis loop shows the irreversible, nonlinear response of a ferromagnet to a magnetic field. It reflects the arrangement of the magnetization in ferromagnetic domains. The magnet cannot be in thermodynamic equilibrium anywhere around the open part of the curve! M and H have the same units (A m -1 ). Dublin January

16 Soft and hard magnets. The area of the hysteresis loop represents the energy loss per cycle. For efficient soft magnetic materials, this needs to be as small as possible. 1 M (MA m -1 ) 1 M (MA m -1 ) H (A m -1 ) H (MA m -1 ) -1-1 For a useful hard magnet. H c > M r /2 Dublin January

17 2.2 Curie temperature Ferromagnetic materials possess a spontaneous magnetization M, which falls to zero at the Curie point T C - a phase transition. M(T)/M(0) Fe Co Ni Gd T C (K) M(0) MA m A specific heat anomaly appears at T C S mag = (C/T)dT R ln 2 Dublin January

18 2.3 Coercivity The progress in magnetism in the 20th century which has spawned so many magnet applications has been due to mastery of coercivity. Dublin January

19 The shape barrier. N < 0.1 Shen Kwa 1060 S N Daniel Bernouilli 1743 Gowind Knight 1760 Dublin January

20 2.4 Anisotropy The direction of magnetization M(r) in a macoscopic ferromagnetic domain lies along one or other easy axes. E a = K 1 sin 2 Easy axis M 1 kjm -3 < K 1 < 10 MJm mk < K 1 < 10 K Dublin January

21 2.5 Susceptibility Above T C the ferromagnetic material becomes paramagnetic. The susceptibility is defined in small fields as = M/H. Note that has no units. It is known as the relative or dimensionless susceptibility. It is a number which is characteristic of a particular material. At temperatures above T C, the susceptibility often follows a Curie-Weiss Law = C/(T-T c ). The Curie constant is of order 1 K. Solids that do not order magnetically are either paramagnetic or diamagnetic. Their susceptibility is small and positive or negative, repectively. (magnitude ). Dublin January

22 2.6 Other Types of Magnetic Order Ordered T < T C M 0 M = 0 Disordered T > T C Dublin January

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24 2.7 Magnetic elements Eight elements (blue) and many compounds are ferromagnetic. They possess a spontaneous magnetization - eleven elements (purple) are antiferromagnetic Dublin January

25 3. Magnet Applications 3.1 The world market Magnet applications; A 30 B market Amorphous Ni-Fe/Fe-Co Soft ferrite Others Hard ferrite Hard Magnets Fe-Si (oriented) Nd-Fe-B Sm-Co Alnico Soft Magnets Others Co- Fe 2 O 3 (tapes, floppy discs) CrO2 (tapes) Iron (tapes) Fe-Si Co-Cr (hard discs) Iron Others Ni-Fe/Fe-Co (heads) Magnetic Recording Dublin January

26 Global domestic product 2000 Continent GDP Population GDP/person (T$) (millions)($) Asia (incl. Australia) Europe (incl. Russia) North America South & Cent America Africa Average production per person (approximate): 30 g hard ferrite, 2 g rare earth magnet, 1 m 2 flexible medium, 1/10 hard disc, 1/10 read/write head, 0.25 m 2 electrical sheet steel, 30 g soft ferrite, 0.1 g metallic glass. Dublin January

27 3.2 Economics Abundances of magnetic ions in the Earth s crust Fe O Si Al 3+ Al Fe Iron (Fe 2+ /Fe 3+ ) is most abundant magnetic element. It is 40 times as abundant as all other magnetic elements together. Si 4+ O 2- Mg Ca K Na H Others Composition in atomic % of the Earth s crust. Iron (Fe 2+ /Fe 3+ ) is the fourth most abundant element. Cr Mn Price scales roughly inversely with abundance. Dublin January

28 A useful magnetic material needs to be able to operate from -50 C to 120 C. The Curie temperature needs to be > 500 K Dublin January

29 Core losses in electrical machinery Global energy production kw hr Efficiency of transformers > 99% yet losses cost > 10 B$ per year. Dublin January

30 Energy Product of Permanent Magnets A permanent magnet is useful because of the stray field it produces. A useful figure of merit is the maximum energy product (BH) max. This is twice the maximum energy in the stray field produced by unit volume of magnet. B (T) Working point (BH) maz H (A m -1 ) Dublin January

31 New icon for permanent magnets! Dublin January

32 Magnetic recording density 160 Gb 40 Mb Dublin January

33 3. Magnetism, Physics and Technology 30,000 people worldwide Dublin January

34 Typical values of B Human brain 1 ft Earth 50 µt Helmholtz coils 0.01 Am - Magnetar T Electromagnet 1 T Superconducting magnet 10 T Dublin January