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

The interaction of coherent magnetization rotation with a system of two-level impurities is studied. Two different, but not contradictory mechanisms, the ``slow-relaxing ion'' and the ``fast-relaxing ion,'' are utilized to derive a system of integrodifferential equations for the magnetization. These expressions do not contain any phenomenological parameter and apply to any dynamic magnetization process, including complete reversal. In the case that the impurity relaxation rate is much greater than the magnetization precession frequency, these equations can be written in a form similar to the Landau-Lifshitz-Gilbert equation with damping. The damping parameter can be directly calculated from these microscopic impurity relaxation processes. A realistic estimate of the damping parameter for a magnetic nanograin is obtained.