Low-temperature specific heat and magnetic properties of the filled skutterudite ferromagnet NdRu4As12

We present the low-temperature specific heat and magnetic properties of the filled skutterudite compound NdRu4As12 that exhibits a ferromagnetic transition at TC≃2.3K. Magnetic entropy considerations point at a quartet ground state of the Nd3+Nd3+ ions. Deep in the ferromagnetic state, the heat capacity shows a Schottky anomaly that we ascribe to the Zeeman splitting in the presence of a molecular field. Comparison of the specific heats of NdRu4As12 and its Os-based homologue near their Curie temperatures supports our earlier observation suggesting an unusual lowering of the Th cubic point symmetry in the latter filled skutterudite.