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

BH40 Course B: Materials for Devices BH40
Small particles:
A domain structure won’t form in a small particle, or grain, if domain wall energy > stray field
energy. The domain structure of a polycrystalline material depends upon grain size: grains may be
too small to support a domain boundary ⇒ form uniformly magnetised, single domain grains.
Ferromagnetic Hysteresis
In the presence of an applied field, domains with magnetisation antiparallel to the field DO NOT
spontaneously flip. It is energetically cheaper for the magnetisation to switch by moving domain
walls. Plotting Magnetisation, M as a function of Applied Field, H:
Extrapolation of the curve from M sat back
to the axis defines the Spontaneous
M
a
b
c
d
dipole rotation
growth of favourably
Magnetisation (the net magnetisation oriented domains
within a uniformly magnetised microscopic
volume, in zero field) irreversible wall motion
reversible wall motion
H
H H H H
a b c d
*Domains in unmagnetised
sample cancel
out ⇒ zero net
magnetisation, M
*Moments are aligned
along easy axes
*Wall motion and M
are reversible
*Favourably oriented
domains grow by wall
motion
*M increases sharply
*Wall motion is irreversible
(due to pinning
by imperfections)
*Whole sample is
aligned as a single
magnetic domain
(along easy axis, not
along external field
direction)
*Moment is pulled
away from easy axis,
into line with external
field
*Saturation magnetisation
M sat produced
by rotation of moments
to lie along H