Effects of spin-wave energy degeneracy induced by in-plane wave propagation in ferromagnetic thin films

This work presents an investigation of the collapse effect predicted to occur in the spin-wave band structure of magnetic cubic thin films. The system under consideration is described by the Heisenberg model with exchange interactions between the nearest (NN) and next-nearest neighbors (NNN) taken into account. In the collapse effect, which occurs for some specific directions of in-plane propagation of spin waves, the effective coupling between spins in adjacent layers vanishes dynamically and each mode becomes confined to a single atomic plane; this means that all the bulk modes and, independently, all the surface modes become energetically degenerate. This effect of ‘directional collapse’ can only occur when the exchange interactions between NN or NNN include bonds oblique with respect to the direction of in-plane propagation.