Surface spin-wave spectra in trifluorides MF3 (M=Fe, Cr, V)

Theoretical investigations of surface spin-waves localized on a (100) surface of an antiferromagnetic semi-infinite trifluoride MF3 (M=Fe, Cr, V) lattice have been carried out. Using a Green function formalism in addition to a matching technique in the mean field approximation, the excitation spectrum of the surface spin-waves has been obtained and compared with that for bulk spin-waves. Dispersion curves of surface magnons are obtained within a single framework by matching the evanescent and travelling solutions, respectively, obtained from the secular equation, satisfying the boundary conditions brought about by the surface. Bulk magnons of the same energy travelling to and away from the surface are related to one another by reflection coefficients, for which sum rules are derived. The numerical results for the evolution of acoustic and optical modes are presented as a function of the bulk-surface exchange integrals.