Non-Gilbert-type damping of the magnetic relaxation in ultrathin ferromagnets: Importance of magnon-magnon scattering

Ferromagnetic resonance (FMR) measured over a large range of frequencies from 1-70 GHz offers a unique possibility to study the dynamic response of ultrathin ferromagnetic films in the range from nanoseconds to picoseconds. The linewidth of the FMR signal is commonly believed to follow a linear omega dependence, the so-called Gilbert damping. Here we give experimental unambiguous evidence that other processes of spin dynamics such as two-magnon scattering are equally important at interfaces of ferromagnetic to nonmagnetic nanostructures. The relevance to spin transport and spin injection as well as the agreement with recent theoretical proposals are discussed.