Resonant X-ray scattering study of NpRhGa and NpCoGa

Detlefs, B.*; Wilkins, S. B.*; Caciuffo, R.*; Paixo, J. A.*; Kaneko, Koji   ; Honda, Fuminori*; Metoki, Naoto   ; Bernhoeft, N.*; Rebizant, J.*; Lander, G. H.*

We report a resonant X-ray scattering study of the antiferromagnetic neptunium compounds NpCoGa and NpRhGa. Large resonant signals of magnetic dipole character are observed below the Nel temperatures. The signals at the Np edges confirm the behavior determined previously from neutron diffraction. On application of a magnetic field of 9 T along the [010] direction in NpRhGa, we observe a change in the population of different [110]-type domains. We also observe a magnetic dipole signal at the Ga edge, similarly to the reported UGa case, that can be interpreted within a semilocalized model as an orbital polarization of the Ga 4 states induced via strong hybridization with the Np 5 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. The ratios of the signals on the two inequivalent Ga sites are not the same for the different compounds.