ABSTRACT
This chapter shows how the mean-field theory, or molecular-field approximation is applied to ferromagnets, antiferromagnets and ferrimagnets. In reality, a ferromagnetic crystal can have several ferromagnetic domains with spins pointing in different directions. This is due to the presence of defects, dislocations and imperfections during the formation of the crystal. The mean-field theory assumes that all spins have the same value, meaning that it neglects instantaneous fluctuations of each spin. Ferrimagnetic materials have complicated crystalline structures. There are often many sublattices of non equivalent spins interacting with each other via antiferromagnetic couplings. Ferrimagnets may also have an angular-momentum compensation point where the net angular momentum vanishes. This compensation point is a crucial point for achieving high speed magnetization reversal in magnetic memory device. The method with smallest clusters is sometimes called “Bethe–Peierls–Weiss” approximation. To obtain more precise properties of interacting spin systems, the readers can use other methods such as the spin-wave theory.
