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Domain theory of ferromagnetism was developed by Pierre Weiss. It is understandable from the thermodynamic principle. There are four types of energies, namely:
The interaction of energy which makes adjacent dipoles adjust themselves is called exchange energy.
Magnetic anisotropy. In ferromagnetic crystals, there are two directions of magnetisation:
Note: For easy direction, weak magnetic field has to be applied and for hard direction, strong magnetic field has to be applied.
Domain wall energy
Domain wall is a transition layer which separates the adjacent domains magnetised in different directions. Domain wall energy is due to both exchange energy and anisotropy energy.
When domain walls are magnetised in different directions, they will either expand or shrink. This results in deformation. This effect is called magnetostriction and energy produced is called magnetostriction energy.