Unusual field-driven anisotropy of the low-lying ferromagnetic state in NdOs4As12

We present the low-temperature specific heat C(T) of the filled skutterudite compound NdOs4As12, which undergoes a ferromagnetic transition at Tc 1.10K. In the absence of an external field B, the C(T) results show a Schottky-like anomaly at 0.93 K whose origin is attributed to a lowering of the Th cubic point symmetry of the Nd3+ ions rather than to an impact of the molecular field. More intriguingly, however, for B 0.25 T applied parallel to the [111] direction, a sharp peak in C(T) occurs at T* ≈ 0.7K. This is in striking contrast to B || [100] where the Curie temperature is essentially unchanged. While more experimental input is needed to resolve the nature of a T* phase, our observations point at anomalous field- driven ground-state properties of ferromagnetic NdOs4As12.