Neutron diffraction study of magnetic structure and successive 5 f electronic transition in the itinerant antiferromagnet Np Rh Ga 5

Neutron diffraction experiments have been carried out in a single crystal of $\mathrm{Np}\mathrm{Rh}{\mathrm{Ga}}_{5}$ in order to reveal the magnetic structure and $5f$ electronic state. This material exhibits two successive antiferromagnetic transitions at ${T}_{\mathrm{N}1}=36\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}2}=33\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. An A-type antiferromagnetic structure with the propagation vector $\stackrel{P\vec}{q}=(0\phantom{\rule{0.3em}{0ex}}0\phantom{\rule{0.3em}{0ex}}1∕2)$, appears below ${T}_{\mathrm{N}1}$. We found a remarkable change in the moment direction at ${T}_{\mathrm{N}2}$, namely, the Np moment is parallel to the $c$ axis for ${T}_{\mathrm{N}2}lTl{T}_{\mathrm{N}1}$ (AF1), but parallel to the ⟨1 1 0⟩ direction in the basal plane for $Tl{T}_{\mathrm{N}2}$ (AF2). The magnitude of the magnetic moment also exhibits a remarkable and discontinuous change at ${T}_{\mathrm{N}2}$, 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 ${\mathrm{Np}}^{3+}$ free ion state.