All magnetic materials contain magnetic moments, which behave in a way similar to microscopic bar magnetis. In order to define a ferromagnetism as a class of magnetism, it is easiest to compare the various properties of different possible types of magnetic material. These are principally: paramagnets, ferromagnets, antiferromagnets and ferrimagnets.
Paramagnetism
In a paramagnet, the magnetic moments tend to be randomly orientated due to
thermal fluctuations when there is no magnetic field. In an applied magnetic
field these moments start to align parallel to the field such that the magnetisation
of the material is proportional to the applied field.
Ferromagnetism
The magnetic moments in a ferromagnet have the tendency to become aligned parallel
to each other under the influence of a magnetic field. However, unlike the moments
in a paramagnet, these moments will then remain parallel when a magnetic field
is not applied (this will be discussed later).
Antiferromagnetism
Adjacent magnetic moments from the magnetic ions tend to align anti-parallel
to each other without an applied field. In the simplest case, adjacent magnetic
moments are equal in magnitude and opposite therefore there is no overall magnetisation.
Ferrimagnetism
The aligned magnetic moments are not of the same size; that is to say there
is more than one type of magnetic ion. An overall magnetisation is produced
but not all the magnetic moments may give a positive contribution to the overall
magnetisation.
Below is a periodic table showing the elements and the types of magnetism at room temperature:
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