Neutron scattering study of magnetic structure and metamagnetic transition between low- and high-moment states of NpNiGa

Honda, Fuminori; Metoki, Naoto   ; Kaneko, Koji   ; Jonen, Shintaro; Yamamoto, Etsuji  ; Aoki, Dai*; Homma, Yoshiya*; Haga, Yoshinori   ; Shiokawa, Yoshinobu*; Onuki, Yoshichika*

Systematic neutron scattering experiments have been carried out in order to reveal the magnetic properties of NpNiGa. We have observed successive ferromagnetic transitions with the magnetic moment parallel to the tetragonal -axis at = 30 K and 18 K. We have also observed antiferromagnetic reflections with the propergation vector = [1/2 1/2 1/2], coexisting with the ferromagnetic component below = 18 K. Spin polarized neutron scattering experiments have revealed that only Np carries a magnetic moment. We conclude that the magnetic moments of Np atoms exhibit simple ferromagnetic ordering for , while a canted antiferromagnetic structure appears for . The size of the Np magnetic moment 0.78 /Np at 3 K have been estimated. The temperature and the magnetic field dependences of these two components have also been quantitatively analyzed. (1) It is quite unusual that the Np total moment increases with the transition from the ferromagnetic to canted antiferromagnetic structure. (2) The metamagnetic transition cannot be explained in terms of a simple spin-flip mechanism with competing interactions and anisotropy. (3) Furthermore, unusual reduction of the Np magnetic moment has been accompanied by the metamagnetic transition. From these three results, we concluded that a remarkable change in the 5 electronic state was associated with the transition at and the metamagnetic transition between the ferromagnetic low-moment state and the canted antiferromagnetic high-moment state. An important role in the 5 itinerant system.