Spin-wave propagation on imperfect ultrathin ferromagnetic films

The effects of localized imperfections on spin-wave propagation in very thin ferromagnetic films are examined. The imperfections are assumed to be localized to a few lattice sites and cause local changes in anisotropy and exchange fields. These imperfections may be due to thickness variations or other geometrical imperfections. We find that in very thin films the lifetime of long-wavelength spin waves is relatively insensitive to scattering from even large numbers of imperfections, and therefore cannot explain large observed linewidths observed in Brillouin light-scattering experiments. On the other hand, we find that a band of long-wavelength spin-wave modes can exist in an inhomogeneous film with a distribution of effective anisotropy fields. It is possible to have large bands with bandwidths on the order of 10 GHz in rough ultrathin films due to the sensitive dependence of effective anisotropy fields on thickness.