Examination of the magnetic properties of NpCo2

The authors report a series of measurements on both polycrystalline and single-crystal samples of NpCo2 in an effort to understand the low-temperature magnetic properties. Magnetization and Mossbauer experiments strongly suggest a transition to an ordered state. A variety of neutron experiments, including small-angle scattering and searches along the principal crystallographic directions with a single crystal, have failed to reveal any direct evidence for long-range ordering of the Np moments. These measurements do not exclude short-range order or a spin-glass state, although the latter appears to be excluded by the magnetization experiments. An account is given of the polarized-neutron experiments in an applied field that give direct evidence of hybridization between Np 5f and Co 3d electrons.