SURFACE SPIN-WAVES IN A HEISENBERG FERRIMAGNET WITH A SINGLE-ION-ANISOTROPY (UNIAXIAL AND NONUNIAXIAL)

Surface spin waves on the (001) surface layer of a semi-infinite CsCl-type (bcc) Heisenberg ferrimagnet with a single-ion anisotropy, uniaxial and nonuniaxial, are investigated by use of Green's-function techniques. It is found that the spectrum of the surface spin waves is related to the strength of the anisotropy, and that only the optical branches can exist in the presence of anisotropy on the surface or in the bulk. In the uniaxial anisotropy case, there is a critical value of the surface-anisotropy parameter, below which the optical branches of the surface spin waves cannot be excited. For the nonuniaxial anisotropy case, it is found that surface spin waves cannot be excited in the vicinity of the Brillouin-zone boundary, if the surface anisotropy is weak enough. The effect of surface-exchange interactions on the surface spin-wave spectrum has also been discussed.