Amplitudes for magnon scattering by vortices in two-dimensional weakly easy-plane ferromagnets

We study magnon modes in the presence of a vortex in a circular easy-plane ferromagnet. The problem of vortex-magnon scattering is investigated for partial modes with different values of the azimuthal quantum number m over a wide range of wave numbers. The analysis was done by combining analytical and numerical calculations in the continuum limit with numerical diagonalization of adequately large discrete systems. The general laws governing vortex-magnon interactions are established. We give simple physical explanations of the scattering results: the splitting of doublets for the modes with opposite signs of m, which takes place for the long-wavelength limit, is an analog of the Zeeman splitting in the effective magnetic field of the vortex. A singular behavior for the scattering amplitude σ_m∝k takes place as k diverges; it corresponds to the generalized Levinson theorem and can be explained by the singular behavior of the effective magnetic field at the origin.