Anomalous Low-Lying Thermal Excitations Deep Inside the Ferromagnetic State in Filled Skutterudite NdFe4As12

Abstract

The low-temperature properties of NdFe4As12 single 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 NdFe4As12 shows a ferromagnetic transition at \(T_{\text{c}}=14.6\) K. Moreover, we found additional anomalies, a shoulder in the resistivity and a Schottky-like peak in magnetic specific heat below \(T_{\text{c}}\). It is rare that the corresponding peak of the specific heat is larger than the specific heat jump at \(T_{\text{c}}\). 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 µB/f.u.) is larger than the Nd3+ free-ion value (3.67 µB/ion), indica...