Surface spin waves of semi-infinite two-sublattice ferrimagnets

We investigate the surface spin waves on the (001) free surface of semi-infinite two-lattice ferrimagnets on the Heisenberg model with nearest-neighbor exchange interactions. Energy spectra are calculated for both NaCl and CsCl structures by means of the method of the retarded-Greens-function equation of motion. It is found that the surface-spin-wave spectrum for NaCl structure has both optical and acoustic branches, while it can only have one of these branches in the case of CsCl structure. The nature of the energy spectrum branch depends upon which sublattice the surface layer belongs to. It is also found in CsCl structure that the surface optical branch crosses into the bulk acoustic continuum and produces the resonant state. Possible extension of the theory to interface and superlattice problems is discussed.