Two-magnon scattering and viscous Gilbert damping in ultrathin ferromagnets

Ferromagnetic resonance experiments of magnetic nanostructures over a large frequency range from 1 to 225 GHz are presented. We find unambiguous evidence for a nonlinear frequency dependence of the linewidth. The viscous Gilbert damping and two-magnon scattering are clearly separated. Both angular and frequency dependent measurements give a transverse scattering rate within the magnetic subsystem of the order of 10(9) s(-1), whereas the longitudinal Gilbert relaxation into the thermal bath is one to two orders of magnitude smaller.