Mechanisms for Induced Magnetization
The nature of material magnetization is in general complex, governed by atomic
properties, and well beyond the scope of this series of notes. Suffice it to
say, there are three types of magnetic materials: paramagnetic, diamagnetic,
and ferromagnetic.
- Diamagnetism - Discovered by Michael Faraday in 1846. This form of
magnetism is a fundamental property of all materials and is caused by the
alignment of magnetic moments associated with orbital electrons* in the presence
of an external magnetic field. For those elements with no unpaired electrons
in their outer electron shells, this is the only form of magnetism observed.
The susceptibilities of diamagnetic materials are relatively small and negative.
Quartz and salt are two common diamagnetic earth materials.
- Paramagnetism - This is a form of magnetism associated with elements
that have an odd number of electrons in their outer electron shells. Paramagnetism
is associated with the alignment of electron spin directions in the presence
of an external magnetic field. It can only be observed at relatively low temperatures.
The temperature above which paramagnetism is no longer observed is called
the Curie Temperature. The susceptibilities of paramagnetic substances
are small and positive.
- Ferromagnetism - This is a special case of paramagnetism in which
there is an almost perfect alignment of electron spin directions within large
portions of the material referred to as domains. Like paramagnetism,
ferromagnetism is observed only at temperatures below the Curie temperature.
There are three varieties of ferromagnetism.
- Pure Ferromagnetism - The directions of electron spin alignment
within each domain are almost all parallel to the direction of the external
inducing field. Pure ferromagnetic substances have large (approaching
1) positive susceptibilities. Ferromagnetic minerals do not exist, but
iron, cobalt, and nickel are examples of common ferromagnetic elements.
- Antiferromagnetism - The directions of electron alignment within
adjacent domains are opposite and the relative abundance of domains with
each spin direction is approximately equal. The observed magnetic intensity
for the material is almost zero. Thus, the susceptibilities of antiferromagnetic
materials are almost zero. Hematite is an antiferromagnetic material.
- Ferromagnetism - Like antiferromagnetic materials, adjacent domains
produce magnetic intensities in opposite directions. The intensities associated
with domains polarized in a direction opposite that of the external field,
however, are weaker. The observed magnetic intensity for the entire material
is in the direction of the inducing field but is much weaker than that
observed for pure ferromagnetic materials. Thus, the susceptibilities
for ferromagnetic materials are small and positive. The most important
magnetic minerals are ferromagnetic and include magnetite, titanomagnetite,
ilmenite, and pyrrhotite.
* We have not mentioned previously that the source of all magnetic
fields is most probably circulating electric currents - the field due to a current
loop is identical with the field due to a dipole pair. Orbiting electrons in an
atom are therefore conceptually responsible for a magnetic field which can react
with the ambient field, or which can be acted on by the ambient field.
