Interface spin waves in a bilayer of two-sublattice ferrimagnets

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 Greens functions for the bilayer system in different coupling schemes and express them in terms of Greens 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 ISWs localized in the interface, one on each side, for any finite interface exchange coupling J12 and that they always appear outside the energy range of the corresponding surface spin waves (SSW) of the two semi-infinite ferrimagnets. Hence the SSWs serve as the lower bounds of the ISWs in the energy spectra. In the limit of J120, the ISWs approach their corresponding SSWs as expected.