Interface spin waves in a bilayer of two-sublattice ferrimagnets.

Abstract

Interface spin waves (ISW) in a bilayer of two-sublattice ferrimagnets are investigated on the Heisenberg model with nearest-neighbor exchange interactions. We first derive the Green's functions for the bilayer system in different coupling schemes and express them in terms of Green's functions for the two semi-infinite ferrimagnetic systems that are already known from previous work. Interface-spin-wave spectra are then calculated from these functions for various interface-coupling schemes. When the two semi-infinite systems are different, it is found that there are two ISW's localized in the interface, one on each side, for any finite interface exchange coupling ${\mathit{J}}_{12}$ and that they always appear outside the energy range of the corresponding surface spin waves (SSW) of the two semi-infinite ferrimagnets. Hence the SSW's serve as the lower bounds of the ISW's in the energy spectra. In the limit of ${\mathit{J}}_{12}$\ensuremath{\rightarrow}0, the ISW's approach their corresponding SSW's as expected.