New perspective on the Green’s function dipole-exchange spin wave theory for thin films (abstract)

Abstract

In contrast with the scalar potential theory for dipole-exchange spin waves in thin films, the Green’s function (GF) approach in combination with perturbation methods (PM) gives simple closed form dispersion equations, analytical expressions for the dipole-exchange frequency gaps, and good intuitive physics. However, in the case of in-plane magnetized films and perpendicular propagation, the GF/PM results present two major problems. (1) The multibranch dipole-exchange solutions bear little resemblance to the single dispersion curve for the well-established Damon–Eshbach magnetostatic surface modes and the nonreciprocal mode profiles predicted by the simple theory and observed experimentally. (2) The lowest order exchange branch has a curvature that is much greater than expected from either simple spin wave considerations or scalar potential theory. To resolve these problems, a simple method has been developed to solve the coupled mode equations obtained from the GF formalism without recourse to perturbation methods. The coupled mode dispersion relations obtained with this method are valid for an arbitrary field configuration and propagation direction, and are not subject to the restrictions imposed by perturbation methods. The solutions agree well with the surface mode theory for in-plane magnetized films. Good agreement is also found with the Brillouin light scattering data for iron thin films.