Ferromagnetism and possible heavy fermion behavior in single crystals of NdOs$_4$Sb$_{12}$

Single crystals of the filled-skutterudite compound NdOs$_4$Sb$_{12}$ have been investigated by means of electrical resistivity, magnetization, and specific heat measurements. The NdOs$_4$Sb$_{12}$ crystals have the LaFe$_4$P$_{12}$-type cubic structure with a lattice parameter of 9.3 AA. Possible heavy-fermion behavior is inferred from specific heat measurements, which reveal a large electronic specific heat coefficient $gamma approx 520$ mJ/mol-K$^2$, corresponding to an effective mass $m^* sim$ 98 $m_e$. Features related to a ferromagnetic transition at {$sim$ 0.9 K} can be observed in electrical resistivity, magnetization and specific heat. Conventional Arrott-plot analysis indicates that NdOs$_4$Sb$_{12}$ conforms to mean-field ferromagnetism.