Frequency dependence of spin relaxation in periodic systems

We show that in the presence of a periodic scattering potential the spin relaxation in ultrathin ferromagnets is not a monotonous function of the frequency as has been usually assumed taking intrinsic Gilbert and extrinsic two-magnon processes into account. The spin relaxation rate is found to substantially increase at characteristic frequencies related to the periodicity of the magnon scattering potential. We propose a theoretical model which is experimentally conrmed in Ni80Fe20 thin lms by articially introducing dierent