27.3 Theory of Ferromagnetism & Curie temperature of Ferromagnets
The essential origin of Ferromagnetism is the same as that of Diamagnetism or Paramagnetism.
However, as greater stability arises out of minimum potential energy requirement of lattice structures in
substances (such as Iron, Cobalt, Nickel etc.), it is found that not only does each atom have net
# 0 but also that in small regions (called domains) that are within the substance all the
s of a large number of atoms are all aligned parallel to each other. Ordinarily the domains are randomly oriented and hence net = 0 in
the substance. When external field is applied then its effect are
(i) Alignment of domains initially not parallel to the field in the direction of the field.
(ii) growth in size of domains which by chance had their s
parallel to applied .
The substance as a result acquires large induced
that is parallel to
and is therefore strongly attracted.
The acquired induced by a ferromagnet does not vanish when is reduced to zero, i.e. the
randomness in domains does not revert as the thermal agitations of atoms are not sufficiently strong to break
the domain alignments. Therefore ferromagnets are found to be permanent magnets.
When heated, the domain alignments gradually begin to break and the ferromagnet gradually loses
its magnetism. At a certain critical temperature, the domain structure also breaks down leaving
randomly oriented atomic dipoles i.e. the substance becomes a paramagnet. This critical temperature is called