Quantum Theory of Magnetism

Magnetism is one of the oldest and most fundamental problems of Solid State Physics although not being fully understood up to now. On the other hand it is one of the hottest topic of current research. Practically all branches of modern technological developments are based on ferromagnetism, especially what concerns information technology.

Magnetic properties are exhibited to a greater or lesser ex- tent by all substances. Among solids, we can distinguish a group of magnetic substances whose magnetic properties are particular- ly pronounced: ferromagnets, ferrimagnets, and antiferromag- nets. The present treatment deals with some of the problems aris- ing in the theory of the magnetic phenomena in such substances. We shall call the theory of phenomena in strongly magnetic sub- stances the theory of strong magnetism or, simply, the theory of magnetism, because the weakly magnetic substances (paramagnets and diamagnets) will not be considered. The emphasis is on the methods giving general solutions and on the features character- istic of magnetic problems. The microscopic approach is used throughout.