P 31 -NMR study of hyperfine interactions and magnetic fluctuations in the neptunium-based filled skutterudite NpFe4 P12

P 31 -NMR measurements have been performed on a single crystal of the neptunium-based filled skutterudite NpFe4 P12. The compound undergoes a ferromagnetic phase transition at TC =23 K. From the field-orientation dependence of the P 31 -NMR line splitting, the angular dependence of the hyperfine interactions between Np5f spin and P 31 nuclear moments has been investigated. We have observed anisotropic transferred hyperfine interactions at the P sites, which lead to an estimate of the local spin density in the P3p orbitals. It is shown that a fraction of the Np5f spin moments is transferred mostly into the P3p orbitals extending toward the inside of a P cage. The weak hybridization between Np5f and P3p orbitals suggests a localized character for Np5f electrons in NpFe4 P12. We have also measured the field and temperature dependences of the nuclear spin-lattice relaxation rate (1/ T1) in several magnetic fields between 18.5 and 78.0 kOe. The 1/ T1 data reveal that the low-energy spin fluctuations of Np5f spin moments are strongly suppressed by applied fields over a relatively wide temperature range up to 4 TC. In this compound, a large and negative magnetoresistance has been known to occur in the same temperature range. The present NMR results demonstrate that the negative magnetoresistance comes from a reduction in the magnetic scattering from Np5f spin moments by an applied field.