Neutron diffraction study of magnetic structure and successive 5f electronic transition in the itinerant antiferromagnet NpRh Ga5

Neutron diffraction experiments have been carried out in a single crystal of NpRh Ga5 in order to reveal the magnetic structure and 5f electronic state. This material exhibits two successive antiferromagnetic transitions at TN1 =36 K and TN2 =33 K. An A-type antiferromagnetic structure with the propagation vector q = (0 0 1 2), appears below TN1. We found a remarkable change in the moment direction at TN2, namely, the Np moment is parallel to the c axis for TN2 <T< TN1 (AF1), but parallel to the 1 1 0 direction in the basal plane for T< TN2 (AF2). The magnitude of the magnetic moment also exhibits a remarkable and discontinuous change at TN2, indicating the moment reorientation is associated with a change of the 5f electronic state. This electronic transition might be the consequence of the competition between the magnetic and/or quadrupolar interaction in the low-moment (AF1) and high-moment (AF2) states. The observed magnetic form factor indicates a dominant contribution from the orbital moment, with a small reduction from the Np3+ free ion state.