Frequency dependence of spin relaxation in periodic systems

As a common method to separate intrinsic and extrinsic spin relaxation processes in a ferromagnet, their frequency dependence is employed. We show, that this current approach may be insufficient for a large class of ferromagnetic systems, since the extrinsic spin relaxation can exhibit non-monotonous, periodic frequency dependence, as demonstrated by a good agreement of ferromagnetic resonance experiment and phenomenologic theory.