Neutron scattering study of the magnetic excitation spectra in mixed-valence ¹⁵⁴Sm₃Te₄

The magnetic properties of the unconventional mixed-valence compound Sm₃Te₄ have been studied by inelastic neutron scattering. Time-of-flight experiments with different incident energies (E_i = 300 meV, 67 meV and 3.2 meV) and energy resolutions have been performed on isotopic (low-absorption) ¹⁵⁴Sm₃Te₄ and on the reference compound La₃Te₄. Spectral contributions from two coexisting valence states Sm²⁺ and Sm³⁺ were clearly identified. In contrast to previous results for SmB₆, the spectra can be analysed in terms of well defined 4f⁵ and 4f⁶ local moments subjected to the crystal field. However, the data reveal that di- and trivalent ions, which occupy identical crystallographic sites, feel quite different crystal-field potentials. Quasielastic scattering was found to exist down to the minimum temperature (1.5 K) of the measurements, with a half-width of about 0.45 meV. This implies that spin fluctuations, with a much higher fluctuation rate than the valence fluctuations reported in a recent NMR study, exist just above the spin-glass transition.