  In the ordered phase of 2D XY ferromagnet the dipolar interaction between
spins induces a strong, relevant interaction between spin-waves. We study
quasi-excitations of the interacting spin-wave 'liquid' in the long wavelength
limit. We employ the Janssen-De-Dominicis method for classical Langevin
equation to find the transformation of the spin-wave excitation into a new
soft-mode excitation in the intermediate range of wavelengths; and into an
anomalous anisotropic diffusion mode excitation at long wavelengths. The
dissipation of a spin-wave at short wavelengths is found to be highly
anisotropic.
