AbstractThe influence of the surface roughness on the low energy spin wave properties is treated within a quantized phenomenological spin wave theory. Up to second order in the roughness amplitude, the surface roughness can be replaced for thin films by a non‐local scattering Hamiltonian acting in two dimensions. For a surface with a random “mosaic structure”, results are obtained for the self‐energy of the magnons, for the density of states g(E), and for the magnetization as function of the temperature T. For rough films, due to the properties of g(E), the magnetization decreases faster with T than for a smooth film of equal thickness.