Elastic properties of the ferromagnetic heavy fermion system SmOs4Sb12

Ultrasonic measurements were made on a single crystal of the filled skutterudite compound SmOs4Sb12. A remarkable elastic softening toward low temperature was observed in the elastic constants C11 (C11-C12)/2 and C44 as a function of temperature, followed by a sharp drop which is associated with the onset of ferromagnetic ordering, and then display a monotonic increase below Tc. The present results indicate that the crystalline electric field (CRF) effect would not support straightforward the observed elastic softening toward low temperature. We suggest that this fact is originated from valence instability of the Sm ion at low temperature in SmOs4Sb12. The 4f electronic ground state of the Sm ion and its elastic property are discussed