Magnetism in Matter Electric polarisation (P) - electric dipole moment per unit vol. Magnetic ‘polarisation’ (M) - magnetic dipole moment per unit vol. M magnetisation Am -1 c.f. P polarisation Cm -2 Element of magnetisation is magnetic dipole moment m When all moments have same magnitude & direction M=Nm N number density of magnetic moments Dielectric polarisation described in terms of surface (uniform) or bulk (non-uniform) bound charge densities Magnetisation described in terms of surface (uniform) or bulk (non-uniform) magnetisation current densities
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Magnetism in Matter Electric polarisation (P) - electric dipole moment per unit vol. Magnetic ‘polarisation’ (M) - magnetic dipole moment per unit vol.
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Magnetism in Matter
Electric polarisation (P) - electric dipole moment per unit vol.Magnetic ‘polarisation’ (M) - magnetic dipole moment per unit vol.M magnetisation Am-1 c.f. P polarisation Cm-2
Element of magnetisation is magnetic dipole moment mWhen all moments have same magnitude & direction M=NmN number density of magnetic moments
Dielectric polarisation described in terms of surface (uniform) or bulk (non-uniform) bound charge densities
Magnetisation described in terms of surface (uniform) or bulk (non-uniform) magnetisation current densities
Magnetism in MatterParamagnetism
Found in atoms, molecules with unpaired electron spins (magnetic moments)Examples O2, haemoglobin (Fe ion)
Paramagnetic substances become weakly magnetised in an applied fieldMagnetic moments align parallel to applied magnetic field to lower energy
Paramagnetic susceptibility is therefore positive
Moments fluctuate because system is at finite temperature
Energy of magnetic moment in B field Um = -m.BUm = -9.27.10-24 J for a moment of 1 mB aligned in a field of 1 TUthermal = kT = 4.14.10-21 J at 300K >> Um Um/kT=2.24.10-3
This implies little net magnetisation at room temperature
Magnetism in MatterDiamagnetism
Found in atoms, molecules, solids with paired electron spinsExamples H2O, N2
Induced electric currents shield interior of a body from applied magnetic fieldMagnetic field of induced current opposes the applied field (Lenz’s Law)
Diamagnetic susceptibilty is therefore negative
Generally small except for type I superconductor where interior is completely shielded from magnetic fields by surface currents in superconducting state
Strong, non-uniform magnetic fields can be used to levitate bodies via diamagnetism
Found in solids with magnetic ions (with unpaired electron spins)Examples Fe, Fe3O4 (magnetite), La2CuO4
When interactions H = -J mi.mj between magnetic ions are (J) >= kTThermal energy required to flip moment is Nm.B >> m.BN is number of ions in a cluster to be flipped and Um/kT > 1
Ferromagnet has J > 0 (moments align parallel)Anti-ferromagnet has J < 0 (moments align anti-parallel)Ferrimagnet has J < 0 but moments of different sizes giving net magnetisation
Magnetic susceptibilities non-linear because of domain formation
Magnetism in MatterElectric polarisation P(r) Magnetisation M(r)
p electric dipole moment of m magnetic dipole moment of
localised charge distribution localised current distribution
rrrp
rPrj
n.rjn.rP
)d(
t
)()(
dt)()(
allspace
pol
0
pol
ˆˆ
space all
)(x2
1
)(x )(
)(x2
1)(
dr rj rm
rMrj
rj r rM
M
Magnetisation
Electric polarisation Magnetisation
)(Amm
A.m
VCm
m
C.m
V1-
3
2i
i2-
3i
i
mM
pP )(
I
z
yx
xyΔx
yΔM
I
z
zI
Magnetisation is a current per unit length
For uniform magnetisation, all current localised on surface of magnetised body(c.f. induced charge in uniform polarisation)
Magnetisation
Uniform magnetisation and surface current density
Symbol: aM current density (vector )Units: A m-1
Consider a cylinder of radius r and uniform magnetisation Mwhere M is parallel to cylinder axis
Since M arises from individual m,(which in turn arise in current loops) draw these loops on the end face
Current loops cancel in interior,leaving only net (macroscopic) surface current
M
m
Magnetisation
magnitude aM = M but for a vector must also determine its direction
aM is perpendicular to both M and the surface normal n
Normally, current density is “current per unit area” in this case it is “current per unit length”, length along the cylinder - analogous to current in a solenoid.
nP nM . polM c.f.
M n
aM
MagnetisationNon-uniform magnetisation and bulk current density
Rectangular slab of material with M directed along y-axisM increases in magnitude along x-axis
Individual loop currents increase from left to right There is a net current along the z-direction Magnetisation current density
z
x
My
zMj
I1 I2 I3
I1-I2 I2-I3
Magnetisation
Consider 3 identical element boxes, centres separated by dx
If the circulating current on the central box is , on the left and rightboxes, respectively, it is
dyMy
dx dx
dy dxx
MManddydx
x
MM y
yy
y
Magnetisation
Magnetisation current is the difference in neighbouring circulating currents, where the half takes care of the fact thateach box is used twice! This simplifies to
dyMdxx
MMdx
x
MMM2
1y
yy
yyy
x
Mjdxdyjdxdy
x
Mdydx
x
M22
1 yMM
yy
zz
Magnetisation
Rectangular slab of material with M directed along x-axisM increases in magnitude along y-axis
z
x
My
I1 I2 I3
I1-I2 I2-I3z
y
-Mx
xx
Mj yMz
y
Mj xMz
y
M
x
Mj xyMz
Total magnetisation current || z
Similar analysis for x, y components yields MMj
Magnetic SusceptibilitySolenoid in vacuum
With magnetic core (red), Ampere’s Law integration contour encloses two types of current, “conduction current” in the coils and “magnetisation current” on the surface of the core
> 1: aM and I in same direction (paramagnetic) < 1: aM and I in opposite directions (diamagnetic)
is the relative permeability, c.f. e the relative permittivity
Substitute for aM
INB ovac
MNB o I
vacMo
Moenclo
BNB
LNLBLB.d
I
IIB
IL
Magnetic SusceptibilityMacroscopic electric field EMac= EApplied + EDep = E - P/o
Macroscopic magnetic field BMac= BApplied + BMagnetisation
BMagnetisation is the contribution to BMac from the magnetisation