  We report a resonant x-ray scattering (RXS) study of 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 behaviour
determined previously from neutron diffraction, i.e. the moments along [001] 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 magnetic
field of 9 Tesla along the [010] direction we observe a change in the
population of different [110]-type domains. We observe also a magnetic dipole
signal at the Ga K-edge, similarly to the reported UGa_3 case, that can be
interpreted within a semi-localized 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.
