Elastic properties of ferromagnetic heavy fermion system SmOs$_4$Sb$_{12}$

We report the elastic constants of the heavy fermion system SmOs$_4$Sb$_{12}$ by means of an ultrasonic measurement. A steep decrease associated with the ferromagnetic transition was observed at around 2 K in elastic constants C11, (C11-C12)/2 and C44. Furthermore a characteristic increase, possibly due to the ``rattling-motion'' was observed around 15 K in the elastic constants. The variation of the onset temperature and a degree of the increase as a function of the ultrasonic frequency is reasonably reproduced in terms of the Debye-type dispersion. The obtained parameters describing the rattling-motion such as a relaxation time, an activation energy and a mean square displacement will be discussed as compared with those of an isostructural compound PrOs$_4$Sb$_{12}$.