Collective magnetic excitations in mixed-valence ${\mathrm{Sm}}_{0.83}{\mathrm{Y}}_{0.17}\mathrm{S}$

The magnetic spectral response of black-phase mixed-valence ${\mathrm{Sm}}_{0.83}{\mathrm{Y}}_{0.17}\mathrm{S}$ has been measured by inelastic neutron scattering on a single crystal. Two magnetic peaks are observed in the energy range of the ${\mathrm{Sm}}^{2+}$ 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 taking into account the existence of sizable Sm-Sm exchange interactions, and a microscopic mechanism is proposed on the basis of the “local-bound-state” theory developed previously for ${\mathrm{SmB}}_6.$