Surface excitations in magnetic systems with a singlet ground state

We have investigated surface excitations in a system wherein the ionic ground state is a magnetic singlet. The pseudospin formalism is employed to account for the crystal-field and exchange interactions between the ions in a Heisenberg ferromagnet with the singlet-triplet crystal-field-only level scheme. The Hamiltonian was studied in the molecular field approximation to find the possible ground states. Surface excitations for the simple cubic structure were investigated for the (001) surface in the random phase approximation. Analytic expressions have been obtained for the thermodynamic Green functions. For a fully magnetized molecular field ground state, there are in general two bulk bands, the spin-wave and the excitonic. Surface modes were found to exist below the spin-wave band, above the excitonic band and between the bands. The dispersion curves can exist only over one or two limited regions of the two-dimensional wave-vector parallel to the crystal surface.