Lecture handout including QS - Department of Materials Science ...

BH42 Course B: Materials for Devices BH42
Aside on magnetic parameters:
If a magnetic field, H, is applied to a material, the resultant magnetic flux density, B, is
B = µH = µ 0 ( H + M ) with units of Tesla, T
µ = permeability of material; µ 0 = permeability of free space (4π×10 -7 H m -1 ); M = χH (BH36)
( ) = µ 0
H( 1+ χ) and
µ = µ 0 ( 1+ χ)
B = µ 0
H + χH
€
Although B (also called the inductance) is the parameter that we measure, it is useful to consider M,
the magnetisation, since it can be calculated from first principles:
€ M (A m -1 ) = magnetic moment per atom (A m€
2 ) × no. of atoms per unit volume (m -3 )
Hysteresis loops
Different shapes for different applications:
M
M
M
H
H
Large M r and H c for
permanent magnets.
High, stable magnetisation.
Well defined switching field,
(i.e. sharp change in magnetisation)
for memory devices.
High M sat and low H c
for easy switching in
transformer cores
Best soft magnetic materials: want very small loop area. Therefore get rid of imperfections
(crystalline defects, grain boundaries, impurities, vacancies, strains, etc.) that would pin domain walls.
Use long heat treatments (annealing) to form (ideally) ‘perfect’ crystal, through which domain walls
sweep without hindrance. Or, use very homogeneous, amorphous materials (e.g. H C
≈ 3 A m -1 ).
Best hard magnetic materials: want to pin domain walls. Therefore incorporate imperfections,
defects & stresses by quenching (cooling very rapidly from a high processing temperature, which
doesn’t allow re-organisation). Or, by compressing fine powders (sintering) to leave many
independent particles (e.g. H C
≈ 1.10 6 A m -1 )..