The phenomenological theory of magnetization relaxation (Review Article)

The review is devoted to systematic description of results on relaxation of magnetization in magnetically ordered crystals previously obtained by the authors. The ideas of the phenomenological theory of magnetism formulated by Landau and Lifshitz are analyzed. A general method of constructing the dissipation function for both magnetically ordered systems and paramagnets is described. In the case of magnetically ordered systems the dissipation of exchange and relativistic nature is considered. It is found that for constructing the dissipation function it is necessary to take into account not only the symmetry of a crystal, but also the laws of conservation of magnetization. It is shown that in the case of a ferromagnet, the ground state is characterized by a continuous degeneracy parameter; the Landau-Lifshitz relaxation term gives qualitatively incorrect results (abnormally large attenuation of spin waves). According to the proposed method the spectra of spin waves and their attenuation were calculated and analyzed for ferromagnets with uniaxial, tetragonal and cubic symmetry as well as for two-sublattice uniaxial ferrites. It was found that the relaxation of the magnetization vector has a two-step character in ferromagnets and a multistep character in ferrites. In ferrites, the fastest process is the relaxation of length of the antiferromagnetic vector. It is shown that this relaxation is caused by the exchange interaction between sublattices of a ferrite and is enhanced by the exchange interactions within the sublattices. The relaxation of the total magnetization of a ferrite is much slower and, as in the case of a simple ferromagnet, is described by non-uniform exchange interactions and relativistic interactions. The results obtained are in a good agreement with recent experimental data.