A property of certain substances which are enormously more magnetic than any other
known substance. Ferromagnetic substances, such as the chemical elements
iron, nickel, cobalt, some of the rare earths,
and ceratin alloys, achieve maximum → magnetization
at relatively low magnetic field strengths. Their large
→ magnetic permeabilityies (greater than unity)
vary with the strength of the applied field. When the temperature of a ferromagnet
is increased the property vanishes gradually due to randomizing effects of thermal
agitation. Beyond a definite
temperature for each substance ( → Curie temperature) it
ceases to behave as a ferromagnet and becomes a → paramagnet.
Ferromagnetism is due to the alignment of the → magnetic moments
of uncompensated electrons in the crystal lattice. Under the influence of
an external magnetizing field, all of the uncompensated electrons line up with their
→ spins in the direction of the field. In contrast with
paramagnetic substances, in which spins interact only with an external magnetic field,
in ferromagnets the spins interact with each others, each of them trying to align
the others in its own direction. This coupling gives rise to a spontaneous
alignment of the moments over macroscopic regions called domains. The domains
undergo further alignment when the substance is subjected to an applied field.
Ferromagnets retain their magnetisation even when the external magnetic field has
been removed. See also → antiferromagnetism ;
→ diamagnetism;
→ magnetism.
See also: → ferro-; → magnetism.
