NONLINEAR DAMPING IN SPIN SYSTEMS - FERROMAGNETIC INFINITE RANGE EXCHANGE INTERACTIONS

To establish an appropriate form of the nonlinear, dynamical, effective field theory for strongly interacting spins a spin model with isotropic infinite-ranged exchange interaction driven by an external field and coupled to a bath is investigated. From a microscopic starting point the nonlinear equation of motion for the homogeneous magnetization is calculated. New nonlinear damping terms result from the spin-spin interaction in addition to the usual Landau-Lifshitz terms. An interpretation of the general results is presented in terms of an effective rotating field and some simplifications for special cases are worked out. © 1990.