Brillouin-zone mapping of the existence conditions for interface bilayer spin waves.

Abstract

We consider the model of a ferromagnetic bilayer film, described by a standard Heisenberg Hamiltonian involving interactions between nearest neighbors. Our theory holds for arbitrary (with respect to the film normal) configuration of the film magnetization, arbitrary (ferromagnetic or antiferromagnetic) interface exchange coupling, and arbitrary (easy-axis/easy-plane) uniaxial interface anisotropy. Exact conditions for the existence of interface spin waves (ISW) are established in the limit of a very thick bilayer film. The existence of ISW is found to be related to the propagation direction of the spin wave in the plane of the film. We propose a Brillouin-zone mapping (BZM) of the regions of existence of ISW's on the two-dimensional Brillouin zone for the three interface orientations sc (100), fcc (100), and bcc (100). These regions are studied as to their size versus the respective interface parameters as well as the film magnetization configuration. We show that the emergence of ISW occurs much more easily on the edges of the BZ than at its center and moreover that antiferromagnetic interface coupling considerably broadens the regions of interface spin waves existence (towards the BZ center).