Microscopic theory of dipole-exchange spin waves in ferromagnetic films: Linear and nonlinear processes

The linear and nonlinear processes in ferromagnetic films at low temperatur es (T<<T-c) are studied in a microscopic theory. Both the long-range magnet ic dipole-dipole and the Heisenberg exchange interactions to nearest and ne xt-nearest neighbors are included. The results obtained for the linearized spin-wave spectrum are compared with previous macroscopic theories. For ult rathin films (or for large wave vectors) the microscopic theory provides im portant corrections. The nonlinear dynamics of the spin waves are studied t hrough a finite-temperature perturbation theory based on Feynman diagrams. We obtain explicit results for the energy shift and damping (or reciprocal lifetime) of the dipole-exchange spin waves due to all possible three-magno n and four-magnon processes involving combinations of the surface and quant ized bulk spin waves at low temperatures. To investigate different dipole i nteraction strengths (relative to the exchange) numerical results are prese nted using parameters for Fe, EuO, and GdCl3.