Heavy Fermion Behavior, Crystalline Electric Field Effects, and Weak Ferromagnetism in SmOs$_4$Sb$_{12}$

The filled skutterudite compound SmOs$_4$Sb$_{12}$ was prepared in single crystal form and characterized. The SmOs$_4$Sb$_{12}$ crystals have the LaFe$_4$P$_{12}$-type structure with lattice parameter $a = 9.3085$ \AA. Specific heat measurements indicate a large electronic specific heat coefficient of $\approx 880$ mJ/mol K$^{2}$, from which an enhanced effective mass $m^{*} \approx 170m_{e}$ is estimated. The specific heat data also suggest crystalline electric field (CEF) splitting of the Sm$^{3+} J = 5/2$ multiplet into a $\Gamma_{7}$ doublet ground state and a $\Gamma_{8}$ quartet excited state separated by $\sim 37$ K. Electrical resistivity $\rho$(T) measurements reveal a decrease in $\rho$(T) below $\sim 50$ K that is consistent with CEF splitting of $\sim 33$ K between a $\Gamma_{7}$ doublet ground state and $\Gamma_{8}$ quartet excited state. Specific heat and magnetic susceptibility measurements display a possible weak ferromagnetic transition at $\sim 2.6$ K, which could be an intrinsic property of SmOs$_4$Sb$_{12}$ or possibly due to an unknown impurity phase. This research was supported by the U.S. DOE (DE-FG02-04ER46105), the NSF (DMR-0335173), and the NEDO International Joint Research Program. Work at the NHMFL-LANL was supported by the NSF, the State of FL and the US DOE.