Heavy-fermion behavior, crystalline electric field effects, and weak ferromagnetism in Sm Os 4 Sb 12

The filled skutterudite compound $\mathrm{Sm}{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}$ was prepared in single-crystal form and characterized using x-ray diffraction, specific-heat, electrical resistivity, and magnetization measurements. The $\mathrm{Sm}{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}$ crystals have the $\mathrm{La}{\mathrm{Fe}}_{4}{\mathrm{P}}_{12}$-type structure with lattice parameter $a=9.3085\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. Specific-heat measurements indicate a large electronic specific-heat coefficient of $\ensuremath{\approx}880\phantom{\rule{0.3em}{0ex}}\mathrm{mJ}∕\mathrm{mol}\phantom{\rule{0.2em}{0ex}}{\mathrm{K}}^{2}$, from which an enhanced effective mass ${m}^{*}\ensuremath{\approx}170{m}_{e}$ is estimated. The specific-heat data also suggest crystalline electric field (CEF) splitting of the ${\mathrm{Sm}}^{3+}J=5∕2$ multiplet into a ${\ensuremath{\Gamma}}_{7}$ doublet ground state and a ${\ensuremath{\Gamma}}_{8}$ quartet excited state separated by $\ensuremath{\sim}37\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Electrical resistivity $\ensuremath{\rho}(T)$ measurements reveal a decrease in $\ensuremath{\rho}(T)$ below $\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ that is consistent with CEF splitting of $\ensuremath{\sim}33\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ between a ${\ensuremath{\Gamma}}_{7}$ doublet ground state and ${\ensuremath{\Gamma}}_{8}$ quartet excited state. Specific-heat and magnetic susceptibility measurements display a possible weak ferromagnetic transition at $\ensuremath{\sim}2.6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which could be an intrinsic property of $\mathrm{Sm}{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}$ or possibly due to an unknown impurity phase.