Low-energy magnetic excitations in the Pr metaphosphate glass

Large magnetic contributions to the low-temperature specific heats of rare-earth metaphosphate $\rm (R_2O_3)_{0.25}(P_2O_5)_{0.75}$ glasses have been observed. These effects result from electronic transitions between 4f levels multiply split due to local disorder, which also give rise to a strong electronic contribution to the low-frequency Raman scattering at low temperatures in the Pr metaphosphate glass. By contrast Sm and Gd metaphosphate glasses show the usual spectral features for a glass: the quasielastic scattering and the boson peak. Both the thermal and optical characteristics in the Pr glass are associated with a very large density of low-lying magnetic excitations, induced by the topological disorder. Using the electronic low-frequency Raman scattering at 1.5 K, which reflects this high density of low-energy states, it has proved possible to fit the magnetic contribution to the low-temperature specific heat in the Pr glass