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

BH36 Course B: Materials for Devices BH36
Magnetic Materials
Magnetic moments are produced by spinning electrons orbiting the atomic nucleus. Some atoms /
ions behave as magnetic dipoles.
Electron spin +
orbital angular
momentum
e
-
+
The magnetisation of a material is a function of how strong the separate moments are, and how well
aligned they are. The relative orientation may be influenced by neighbouring moments and by an
external field. They may be randomly oriented, or perfectly aligned, or somewhere in between.
For atoms with complete shells, all electron states are full (paired up, with opposite spins), so both
orbital and spin angular momentum average out to zero. Hence magnetism depends upon electrons in
incomplete shells (valence electrons).
Magnetisation, M = magnetic moment per unit volume: i.e. A m 2 per m 3
Magnetic Moment:
current × area
which has the same units as an applied Magnetic Field, H i.e. Am -1
M = χH
Susceptibility, χ (dimensionless)
current,
A
area,
m 2
Diamagnetism
€
Orbital shells are filled, no unpaired electrons - with no external field there can be no net moment. In
an applied magnetic field electron orbits react to oppose the external flux ⇒ moments develop to
oppose the field ⇒ χ is negative, e.g. ~ −10 -5 (e.g. graphite, noble gases)
Paramagnetism
Some unpaired electrons in partially filled shells, so
dipoles exist, but they’re isolated and non-interacting
⇒ they are randomly oriented.
With no external field the overall moment is zero.
In an applied field there is partial alignment of
moments in the direction of the field ⇒ χ is positive,
but small, e.g. ~10 -5 (e.g. many metals, Na, O 2
)