ABSTRACT
This chapter discusses theories of magnetic behavior of materials, particularly the alignment of magnetic moments within the material. These theories can provide useful phenomenological models of the magnetic properties, including the order-disorder transition that occurs at the Curie temperature. The Ising model provides a useful mathematical description of critical phenomena and is particularly relevant to magnetic materials. There are a number of different types of magnetic order in solids including ferromagnetism, antiferromagnetism, ferrimagnetism, and helimagnetism. In paramagnetism, the atomic magnetic moments are randomly oriented but can be aligned by a magnetic field. It was found that the susceptibilities of a number of paramagnetic metals obey a modified or generalized law known as the Curie-Weiss law. Diamagnetism leads to a very weak magnetization, which opposes the applied field. The mean-field approximation gives an explanation for the spontaneous magnetization within domains but ultimately is not very realistic because each moment does not interact equally with all other moments.
