Chapter 27 Magnetism in Matter. Topics Magnetization Saturation magnetization Types of materials.
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Transcript of Chapter 27 Magnetism in Matter. Topics Magnetization Saturation magnetization Types of materials.
Chapter 27Magnetism in Matter
Topics
Magnetization
Saturation magnetization
Types of materials
Magnetization
Atoms have magnetic dipole moments due to
orbital motion of the electrons
magnetic moment of the electron
Material placed in magnetic field magnetic dipole moments aligned material “magnetized”
Magnetization
magnetization = net magnetic dipole moment per unit volume
magnetization contributes an additional magnetic, Bm, given by
dM
dV
m oB M
Magnetization
Magnetic susceptibility
Magnetization M depends on applied field Bapp and the susceptibility m of the material
a
o
ppm
BM
m m appBB
Magnetic susceptibility
For paramagnets: m > 0
For diamagnets: m < 0 Total field:
Km = relative permeability of the material
(1 )app app apo m m pB B BB M K
1m mK
Magnetic moment and angular momentum
Angular momentum For circular motion L = rmv Magnetic moment of current
loop = IA = Iπr2
For single charge q on circular orbit I = q/T = qv/(2πr) =Iπr2 = qvπr2/(2πr) = qvr/2 = qL/(2m)
L r p
2
qL
m
Saturation magnetization
Magnetization grows with applied field until all magnetic moments are aligned --“saturation”
At saturation, the magnetization is Ms = n•μ, where n is number of atoms per unit volume and μ is the magnetic moment of each atom
Types of Materials
Materials exhibit three types of magnetism:paramagnetic
diamagnetic
ferromagnetic
Paramagnetism
Paramagnetic materials have permanent magnetic
moments
moments randomly oriented at normal temperatures
adds a small additional field to applied magnetic field
Paramagnetism
Small effect (changes B by only 0.01%)
Example materialsOxygen (STP), aluminum,
tungsten, platinum
Diamagnetism
Diamagnetic materialsno permanent magnetic
moments
magnetic moments induced by applied magnetic field B
applied field creates magnetic moments opposed to the field
Diamagnetism
Common to all materials.
Applied B field induces a magnetic field opposite the applied field, thereby weakening the overall magnetic field
But the effect is very small:Bm ≈ -10-4 Bapp
Diamagnetism
Example materials High temperature
superconductorscoppersilver
Ferromagnetism
Ferromagnetic materialshave permanent magnetic
moments
align at normal temperatures when an external field is applied and strongly enhances applied magnetic field
Ferromagnetism
Ferromagnetic materials (e.g. Fe, Ni, Co, alloys) have domains of randomly aligned magnetization (due to strong interaction of magnetic moments of neighboring atoms)
Ferromagnetism
Applying a magnetic field causes domains aligned with the applied fieldto grow at the expense of othersthat shrink
Saturation magnetization is reachedwhen the aligned domains have replaced all others
Ferromagnetism
In ferromagnets, some magnetization will remain after the applied field is reduced to zero, yielding permanent magnets
Such materials exhibithysteresis
Material properties
Summary
The magnetism of materials is due to the magnetic dipole moments of atoms, which arise from:the orbital motion of electrons
and the intrinsic magnetic moment of each electron
Summary
Three classes of materialsDiamagnetic M = - const • Bext,
small effect (10-4)Paramagnetic M = + const •
Bext small effect (10-2)
Ferromagnetic M ≠ const • Bext
large effect (1000)