Impurity relaxation mechanism for dynamic magnetization reversal in a single domain grain

The interaction of coherent magnetization rotation with a system of two-lev el impurities is studied. Two different, but not contradictory mechanisms, the "slow-relaxing ion'' and the ''fast-relaxing ion," are utilized to deri ve a system of integrodifferential equations for the magnetization. These e xpressions do not contain any phenomenological parameter and apply to any d ynamic magnetization process, including complete reversal. In the case that the impurity relaxation rate is much greater than the magnetization preces sion frequency, these equations can be written in a form similar to the Lan dau-Lifshitz-Gilbert equation with damping. The damping parameter can be di rectly calculated from these microscopic impurity relaxation processes. A r ealistic estimate of the damping parameter for a magnetic nanograin is obta ined.