  The magnetic spectral response of black-phase mixed-valence Sm0.83Y0.17S has
been measured by inelastic neutron scattering on a single crystal. Two magnetic
peaks are observed in the energy range of the Sm2+ spin-orbit transition (25-40
meV). Both of them exhibit significant dispersion along the three main symmetry
directions, reminiscent of the spin-orbit exciton branch found in pure divalent
SmS. The results can be reproduced by a simple phenomenological model
accounting for the existence of sizeable Sm-Sm exchange interactions, and a
microscopic mechanism is proposed on the basis of the "local-bound-state"
theory developed previously for SmB6.
