Dissipation function of magnetic media

A general method of constructing a dissipation function is developed for disordered magnetic media and for magnetically ordered systems. As an example it is shown for a ferromagnet that not only the invariance with respect to uniform rotations of the body but also the law of conservation of magnetization must be taken into account in order to construct a dissipation function. It is found that in ferromagnets the dissipation term in the equations of motion for the magnetization is a sum of Bloch and Landau-Lifshitz-Gilbert relaxation terms. The region of applicability of the relaxation term in the Landau-Lifshitz form is determined. The damping of spin waves in a ferromagnet with tetragonal symmetry is calculated. A procedure is formulated for transitioning from a ferromagnet with lower symmetry to a ferromagnet with a continuous degeneracy parameter. In this case the relaxation process can be systematically described by means of the dissipation function described in this article. It is shown how the relaxation term of a general form for ferromagnets becomes the Bloch relaxation term for paramagnets. It is shown that the magnetization vector relaxes in two stages. First the magnetic moment relaxes in magnitude quite rapidly as a result of exchange enhancement and then the magnetization relaxes slowly to its equilibrium direction. The second stage qualitatively corresponds to the relaxation picture described by the Landau-Lifshitz model. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3421029]