Chapter 19 - Ferromagnetism

Abstract

This chapter starts with the Weiss molecular field theory, a phenomenological model that predicts the Weiss-Curie behavior of magnetic susceptibility in ferromagnetic solids. The classical and quantum theories of ferromagnetism have been briefly discussed and the results have been compared with experiment. The magnetic susceptibility shows the Curie-Weiss behavior at high temperatures. The relationship of the transition temperature with the Curie constant and the molecular field constant has been established. The Heisenberg theory of ferromagnetism has been described in reasonable detail and the concept of exchange interactions has been introduced. The general theory of spin waves and the quantization of the theory have been developed, following which the theory is then applied to one-, two- and three-dimensional lattices to evaluate the magnon dispersion relations. The idea of magnetic domains and hysteresis loss was presented at the end of this chapter.