Effect of glassy topological disorder on low-energy magnetic excitations

A comparative study of specific-heat and low-frequency Raman scattering at low temperatures of lanthanide metaphosphate glasses and pentaphosphate sin gle crystals has revealed the presence of low-energy magnetic excitations. In an analogous manner to that found for vibrational modes, topological dis order softens the crystal field magnetic excitations, producing a broad ban d of low-energy states with a low-energy tail. A self-consistent data analy sis using a broad Gaussian distribution of singlet-singlet excitations allo ws a complete interpretation in Pr glass, which exhibits strong magnetic ef fects in the temperature range explored. This distribution is used to evalu ate the magnetic contribution to the specific heat; about one-fifth of the magnetic ions are characterised by singlet-singlet states with very low ene rgies and contribute to the magnetic entropy.