ABSTRACT
This chapter presents the spin wave theory as applied to ferromagnets, antiferromagnets and, to a lesser extent, ferrimagnets. At low temperatures, spin waves yield the main thermal behavior of a system of ordered spins such as in a ferromagnet. Such collective excitations or elementary excitations are common phenomena observed in systems of interacting particles or quasi-particles in condensed matter. One can mention phonons (waves of the atomic motions around the atoms’ equilibrium positions in a system of the interacting atoms) and the plasmons (waves of the charge density). Since the macroscopic physical properties are calculated by averaging over the spin wave excitations.
