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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: [email protected]
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Some introductory texts:
• David Jiles Introduction to Magnetism and Magnetic Materials, Chapman and Hall 1991; 1997A detailed introduction, written in a question and answer format.
• Stephen Blundell Magnetism in Condensed Matter, Oxford 2001A 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 1996.
• Alberto Guimaraes, From Lodestone to Supermagnets, Wiley 2005
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1. A Brief History of Magnetism
-1000 0 1500 1820 1900 1935 1960 1995
Ancien
t
Early
scien
tific
Electro
mag
netic
Under
stand
ing
High
-freq
uenc
y
Applic
ations
Spin
electro
nics
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High-densityrecording, MRAM?
MultilayersThin film devicesConsumersFert, Parkin …. 1995 to ??Spin electronics
Consumerelectronics
Sm-Co, Nd-Fe-BNew materials,miniaturization
ConsumersGorter, Sagawa,Croat
1960 to 1995Applications
Radar, televisionFerritesMicrowaves, epr,fmr, nmr
MilitaryBloch,, Pound,Purcell
1935 to 1960High-frequency
[Alnico]Spin, Exchangeinteractions
AcademyWeiss, Bohr,Dirac,Heisenberg,Pauli, Landau
1900 to 1935Understanding
Motorsgenerators,telegraph,wireless, magneticrecording
Electrical steelE-M induction,Maxwells =ns
Industry/infra-structure
Oersted,Ampere, Faraday,Maxwell
1820 to 1900Electromagnetic
Dip circle, Horse-shoe magnet
Iron, lodestoneEarth’s fieldNavyGilbert,DescartesD.Bernouilli
1500 to 1820Early scientific
South pointer,Compass
Iron, lodestoneForce field,induced magntism,TRM
StateShen Kua, PetrusPeregrinus
-1000 to 1500Ancient
ApplicationsMaterialsAchievementsDriverNamesDateAge
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The Ancient Age
-1000 to 1500
Applications
South-Pointer
Compass
Driver
The State
Scientific achievements
Force field
Induced magnetism
Thermoremanence
Key names
Shen Kua
Petrus Peregrinus
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The Electromagnetic Age
1820 - 1900
Applications
Motors, Generators
Telegraph, Wireless
Magnetic recording
Driver
Industry
(Infrastructure)
Scientific Achievements
Electromagnetic Induction
Maxwells Equations
Key names
Oersted, Ampere
Faraday, Maxwell
Hertz
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Maxwell’s equations
! . B = 0"0! . E = #
(1/µ0 )! $ B = j + "0%E/%t ! $ E = -%B/%t
Written in terms of two fields B (kg C-1 s-1 ) and E (V m-1), they are validin 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 µ0)1/2 c = 2.998 108 m s-1 c = &'
Also, the force on a moving charge q, velocity v
F = q(E + v $ B)
From a long view of the history ofmankind, there can be little doubt thatthe most significant event of the 19thcentury will be judged as Maxwell’sdiscovery of the laws of electrodynamics.Richard Feynmann
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The Age of Understanding
1900 - 1935
Applications
Driver
Academy
Scientific Achievements
Mean Field Theory, Spin,
Exchange Interactions
Key Players
Weiss, Bohr
Heisenberg
Dirac, Pauli
Landau
H = -2JSiSj
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The m-J paradigm:
m represents the
magnetic moment,mainly localized onthe atoms
J represents theexchange couplingof electron spins.
At this point it seems that the whole of chemistry and much of physics is understood in principle. The problem isthat the equations are much to difficult to solve….. P. A. M. Dirac
1930 Solvay Conference
Dirac Heisenberg
The 1930 Solvay conference consecrated our physical understand-ing ofmagnetism in terms of quantum mechanics (exchange) and relativity (spin)
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The Age of Applications1960 - 1995
Key Players
Gorter,
Sagawa,
Croat
Driver
Industry
(Consumer)
Scientific Achievements
New materials
Miniaturisation of Magnetic Circuits
Applications
Consumer
Electronics
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The Age of Spin Electronics1995 - ?
Albert Fert
Peter Gruneberg
Stuart Parkin
Driver
Industry
(Consumer)
Scientific Achievements
Thin film devices
Applications
High-density
recording
MRAM ?
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The hysteresis loop shows the irreversible, nonlinear response of a ferromagnet to amagnetic field . It reflects the arrangement of the magnetization in ferromagnetic domains.The magnet cannot be in thermodynamic equilibrium anywhere around the open part ofthe curve! M and H have the same units (A m-1).
coercivity
spontaneous magnetization
remanence
major loop
virgin curveinitial susceptibility
2. Magnetism and Hysteresis 2.1 The hysteresis loop
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Soft and hard magnets.
The area of the hysteresis loop represents the energy loss per cycle. For efficient softmagnetic materials, this needs to be as small as possible.
M (MA m-1)
-1 0 1 H (MA m-1)
1
-1
M (MA m-1)
-50 0 50 H (A m-1)
1
-1
For a useful hard magnet.Hc > Mr/2
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2.2 Curie temperature
M(T)/M(0)
Ferromagnetic materials possess a spontaneous magnetization M, whichfalls to zero at the Curie point TC - a phase transition.
293
631
1390
1043
TC (K)
Gd
Ni
Co
Fe
2.0
0.5
1.3
1.8
M(0) MA m-1
A specific heat anomaly appears at TC
(Smag = )(C/T)dT ! R ln 2
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2.3 Coercivity
The progress in magnetism in the20th century which has spawned somany magnet applications has beendue to mastery of coercivity.
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Daniel Bernouilli1743
S N
Gowind Knight 1760
Shen Kwa 1060
N < 0.1
The shape barrier.
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2.4 Anisotropy
The direction of magnetization M(r) in a macoscopic ferromagneticdomain lies along one or other easy axes.
Ea = K1sin2*
M
*
1 kJm-3 < K1 < 10 MJm-3
10 mK < K1 < 10 K
Easy axis
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2.5 Susceptibility
Above TC the ferromagnetic material becomes paramagnetic. The susceptibility + is defined insmall fields as
,+ = M/H.
Note that + has no units. It is known as the relative or dimensionless susceptibility. It is anumber which is characteristic of a particular material.
At temperatures above TC, the susceptibility often follows a Curie-Weiss Law
,+ = C/(T-Tc).
The Curie constant is of order 1 K.
Solids that do not order magnetically are either paramagnetic or diamagnetic. Theirsusceptibility is small and positive or negative, repectively. (magnitude 10-3 - 10-7).
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2.7 Magnetic elements
Eight elements (blue) and many compounds are ferromagnetic. They possess aspontaneous magnetization - eleven elements (purple) are antiferromagnetic
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Ni-Fe/Fe-Co (heads)
Fe-Si
Fe-Si (oriented)
Ni-Fe/Fe-Co
Amorphous
Others
Others
Alnico
Sm-CoNd-Fe-B
Hard ferrite
Co- ! Fe 2 O 3
(tapes, floppy discs)
CrO2 (tapes)
Iron (tapes)
Co-Cr (hard discs)
Soft ferrite
Others
Iron
Soft Magnets
HardMagnets
MagneticRecording
Magnet applications; A 30 B# market
3. Magnet Applications
3.1 The world market
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Global domestic product 2000
Continent GDP Population GDP/person
(T$) (millions)($)
Asia (incl. Australia) 14.6 3716 3930
Europe (incl. Russia) 10.9 728 14970
North America 10.5 316 33200
South & Cent America 3.4 525 6100
Africa 0.2 819 1730
39.6 6104 6488
Average production per person (approximate):
30 g hard ferrite, 2 g rare earth magnet, 1 m2 flexible medium, 1/10 hard disc, 1/10 read/write head, 0.25 m2
electrical sheet steel, 30 g soft ferrite, 0.1 g metallic glass.
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Abundances of magnetic ions in theEarth’s crust
Price scales roughly inversely with abundance.
O
Si
Al
Fe
Mg
Ca
K
Na
H
Others
O2-Si4+
Al3+
Fe
Iron (Fe2+/Fe3+) is most abundantmagnetic element. It is 40 times asabundant as all other magneticelements together.
Composition in atomic % of theEarth’s crust. Iron (Fe2+/Fe3+) is thefourth most abundant element.
Cr Mn
3.2 Economics
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A useful magnetic material needs tobe able to operate from -50 C to 120C.
The Curie temperature needs to be >500 K
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Core losses in electrical machinery
Global energy production 18 1012 kW hr
Efficiency of transformers > 99%
yet losses cost > 10 B$ per year.
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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 themaximum energy in the stray field produced by unit volume of magnet.
H (A m-1)
B (
T)
Working point
(BH)maz
Energy Product of Permanent Magnets