Anomalous low-lying thermal excitations deep inside the ferromagnetic state in filled skutterudite NdFe₄As₁₂

The low-temperature properties of NdFe₄As₁₂single crystals grown by the As self-flux method under high pressure have been studied by magnetization, electrical resistivity, and specific heat measurements. We found that NdFe₄As₁₂shows a ferromagnetic transition at T_c-14.6 K. Moreover, we found additional anomalies, a shoulder in the resistivity and a Schottky-like peak in magnetic specific heat below T _c. It is rare that the corresponding peak of the specific heat is larger than the specific heat jump at T_c. From the crystal electric field analysis of Nd 4f electrons with a simple mean field approximation, we found that a model with a quasi-degenerate sextet (doublet and quartet) reproduces these anomalous features qualitatively, but quantitative differences, probably attributable to the contribution from Fe d electrons, still remain. We also found that the effective magnetic moment (4.43 μ_B/f.u.) is larger than the Nd³⁺ freeion value (3.67μ_B/ion), indicating the contribution of Fe itinerant magnetism.