Sb NQR Study of Ferromagnetic Instability in Alkaline-Earth-Filled Skutterudites SrFe4Sb12and BaFe4Sb12

We report a 121,123 Sb nuclear quadrupole resonance (NQR) study of SrFe 4 Sb 12 and BaFe 4 Sb 12 . The absence of hyperfine splitting or marked broadening in NQR spectra even at the lowest temperature (1.5 K) suggests the absence of uniform ferromagnetic ordering in both compounds. The nuclear spin-lattice relaxation rate divided by temperature, 1/ T 1 T , does not follow the Korringa law, but shows a broad maximum at around 60 K. At temperatures higher than 80 K, 1/ T 1 T can be decomposed into two terms, (1/ T 1 T ) 0 =const. and T -dependent (1/ T 1 T ) spin . The component (1/ T 1 T ) spin is proportional to χ spin , which suggests that the present system is close to ferromagnetic instability, whereas below 60 K, (1/ T 1 T ) spin decreases to zero, being unexpected in typical nearly or weakly ferromagnetism.