Neutron scattering study of single crystals of NpAs and NpSb

Neutron scattering experiments have been performed on single crystals of NpAs and NpSb. We report the critical exponents and a detailed investigation of the complex magnetic phase diagram of NpAs as a function of temperature. The anisotropy ratios in the critical scattering are 2.9(5) and 4.5(10) in NpAs and NpSb, respectively. The incommensurate-to-commensurate transition in NpAs is studied and found to be consistent with the McMillan approach, which is based on a Landau expansion. The 4+, 4- magnetic structure that exists over a narrow temperature range in NpAs is almost a complete square-wave modulation. These results, taken together with those for the isostructural uranium monopnictides, indicate the importance of invoking anisotropic interactions. Our experiments provide strong motivation for extending the theories to Np, the element following uranium, and in the trivalent state having the configuration 5f4. (author)