Theoretical Problems in Magnetism.

The magnetic properties of materials with large anisotropy fields but small exchange fields are analyzed. These systems are either paramagnetic or ordered ones. The use and limitations of the matrix elements matching method when using multi-boson spin representations to describe spin-wave excitations in systems with weak field exchange fields are discussed, as well as the possible effects of an oscillatory field on systems with weak exchange coupling. A theory for systems with weak antiferromagnetic exchange fields is developed and systems with large, but transversely anisotrophic antiferromagnetic exchange fields are analyzed. The spontaneous magnetic moment in some finite range of concentrations of mixed valency crystals is examined.