Spin-wave damping at spin-orientation phase transitions

The spectra or spin waves and their damping in the vicinity of spin-orientation phase transitions are investigated. It is shown that if the ground state is degenerate and the degeneracy is characterized by a continuous degeneracy parameter, then the spin-wave damping is not described by a Landau-Lifshitz-Gilbert relaxation term. The dissipative function is constructed for ferromagnetic crystals of different symmetry. The cases of finite and zero longitudinal magnetic susceptibility are considered. A method of finding the relaxation term in the Landau-Lifshitz equation for crystals of different symmetry is given. The spin-wave spectra and damping are calculated for ferromagnets with uniaxial and tetragonal symmetry. It is shown that the relaxation process has a two-step character. In the first step an equilibrium value of the magnetization is established on account of the exchange interaction. In the second step the spin-wave amplitude is damped as the magnetization variable precesses around its equilibrium value. The details of the Bogolyubov method of quasi-averages in application to the case of ferromagnets with a spontaneously degenerate vacuum are discussed. (c) 2006 American Institute of Physics.