Effects of Filling Fraction on Magnetic Properties of Filled Skutterudite<i>R</i>Fe₄Sb₁₂(<i>R</i>= La, Ce) Synthesized under High Pressure

Abstract

We report the results of the 121,123 Sb-nuclear quadrupole resonance (NQR) measurements on the filled skutterudite R Fe 4 Sb 12 ( R = La, Ce) synthesized under high pressure (HP), and compare them with those for the sample synthesized under ambient pressure (AP) in order to understand the effects of the R -site filling fraction on magnetic properties. The NQR spectra for the HP samples are much sharper than those for the AP samples, suggesting that the R -site filling fraction is high close to unity and the electronic states around Sb nuclei become uniform. In LaFe 4 Sb 12 -HP, the nuclear spin–lattice relaxation rate divided by temperature, 1/ T 1 T , exhibits the Curie–Weiss temperature dependence 1/ T 1 T = C /( T +θ) with the Weiss temperature θ=12 K above 30 K, indicating the development of ferromagnetic fluctuations. The magnitude of θ for the HP sample is smaller than that for the AP sample, which suggests that the system becomes close to the vicinity of ferromagnetic instability as the La-site filling fraction increases. In CeFe 4 Sb 12 -HP, 1/ T 1 follows the exponential decrease 1/ T 1 ∝exp (-Δ/ k B T ) above 100 K with the gap Δ/ k B of 270 K, which is larger than that for the AP sample. This result suggests that c – f hybridization is enhanced by increasing the Ce-site filling fraction.