TABLE OF CONTENTS Magnetic ordering, such as ferromagnetism, antiferromagnetism and ferrimagnetism, arises from the broken symmetry of time-reversal or spin-rotation. There are two physical models to describe the magnetism of condensed matter: One is the localized; the other, the itinerant. The former has been used successfully in magnetic insulators, and the latter for magnetic metals. These two models are opposed but complementary to each other and illustrate the intrinsic properties of magnetism of materials, however, they are still in the stage of development with increasing sophistication. Certainly, in many cases, these two models cannot be separated from each other. There has been a trend to combine both to develop a unified theory to understand magnetism. In this chapter we are mainly concerned with the formation of magnetically-ordered phases and intrinsic magnetic properties. The problems of technical magnetization and micromagnetics will be discussed in Part VII.
Magnetic properties of materials are closely related to the spin of microscopic particles. Spin, as a new degree of freedom, is purely quantum mechanical, although sometimes it can be considered as a classical vector, such as in Langevin's treatment of paramagnetism, and also in many statistical models, like the classical Ising, XY, and Heisenberg models.
Magnetic behavior in solids is, in general, involved in the orientations of magnetic dipoles. Each of these dipoles, or magnetic moments, is composed of the electronic spins, electronic orbitals, and nuclear magnetic moments. Because the nuclear magnetic moment is so much smaller than...
