³¹P-NMR study of the neptunium-based filled-skutterudite NpFe₄P₁₂

³¹P-NMR experiments have been performed on a single crystal of NpFe₄P₁₂ with applied fields parallel to the 〈111〉 direction for the cubic crystal axis. From K-_χ plot above 20 K, the isotropic and anisotropic components of the hyperfine coupling constants between Np-5f electrons and P nuclei have been estimated. A relatively large value of the isotropic component suggests the importance of a transferred hyperfine mechanism through the 3s orbitals of P atoms. In the ferromagnetic ordered state, an additional NMR line splitting associated with the reorientation of the Np moments has been observed below T* = 19 K. Analysis of the NMR line splitting reveals that the Np-5f moments are ferromagnetically aligned along the applied field above T*, while they are reoriented along the ferromagnetic easy-axis of the 〈100〉 direction below T*.