Resonant x-ray scattering study of NpRhGa(5) and NpCoGa(5)

We report a resonant x-ray scattering study of the antiferromagnetic neptunium compounds NpCoGa(5) and NpRhGa(5) at the Np M(4) and Ga K edges. Large resonant signals of magnetic dipole character are observed below the Neel temperatures at both edges. The signals at the Np edges confirm the behavior determined previously from neutron diffraction, i.e., moments along the c-axis in NpCoGa(5) and in NpRhGa(5) a reorientation of the moments from the c-axis direction to the ab plane. In the latter material, on application of a magnetic field of 9 T along the [010] direction, we observe a change in the population of different [110]-type domains. We also observe a magnetic dipole signal at the Ga K edge, similarly to the reported UGa(3) case, that can be interpreted within a semilocalized model as an orbital polarization of the Ga 4p states induced via strong hybridization with the Np 5f valence band. Quantitative analysis of the signal shows that the Ga dipole on the two different Ga sites follows closely the Np magnetic moment reorientation in NpRhGa(5). The ratios of the signals on the two inequivalent Ga sites are not the same for the different compounds.