Spin waves of two-sublattice Heisenberg ferrimagnets

The method of the retarded-Green's-function equation of motion is applied to investigate the spin-wave spectrum and other properties of two-sublattice Heisenberg ferrimagnets. The algebraic procedure is greatly simplified by introducing the matrix equation for the ferrimagnet for both NaCl and CsCl structures. We find that the spin-wave spectrum breaks into acoustical and optical branches as expected. The mean spin values of individual sublattices are calculated numerically for different single-particle spin values. It is seen that in the special case of an antiferromagnet, our results agree completely with existing work.