Explanation of Ferromagnetism

Ferromagnetic substances are very strongly magnetic. The best-known examples of ferromagnets are the transition metals, Fe, Co, and Ni. A ferromagnet has a spontaneous magnetic moment – a magnetic moment even in zero applied fields. The atoms (or molecules) of ferromagnetic materials have a net intrinsic magnetic dipole moment which is primarily due to the spin of the electrons. The interaction between the neighboring atomic magnetic dipoles is very strong. It is called spin exchange interaction and is present even in the absence of an external magnetic field.  It turns out that the energy of two neighboring atomic magnets due to this interaction is the least when their magnetic moments are parallel. The neighboring magnetic moments are, therefore, strongly constrained to take parallel orientation. This effect of the exchange interaction to align the neighboring magnetic dipole moments parallel to one another spreads over a small finite volume of the bulk. This small volume of the bulk is called a domain. All magnetic moments with in a domain will point in the same direction, resulting in a large magnetic moment. Thus the bulk material consists of many domains. The domains are oriented in different directions. The total magnetic moment of a sample of the substance is the vector sum of the magnetic moments of the components domains.

In an unmagnetised piece of ferromagnetic material, the magnet moments of the domains themselves are not aligned. When an external field is applied, those domains that are aligned with the field increases in size at the expense of the others. In a very strong field, all the domains lined up in the direction of the field and provide the high observation magnetizations.

If a ferromagnetic material is heated to very high temperature, the thermal vibrations may become strong enough to offset the alignment within a domain. At such temperature, the material loses its ferromagnetic property and behaves like paramagnetic material. The critical temperature above which a ferromagnetic material becomes paramagnetic is called the Curie temperature.