Anomalous Damping of Spin Waves in Magnetic Metals

An analysis of long-wavelength magnon damping in magnetic metals is presented in the context of an itinerant-electron picture of ferromagnetism. The decay is into single-particle excitations, and is a result of spin-orbit forces. The predicted dependence of the damping constant on the electron mean free path is in accord with recent experiments on ferromagnetic resonance. In particular, for pure metals it increases as the temperature is lowered, and then saturates. This anomalous rise is closely related to the anomalous electrical conductivity.