Neutron scattering study of magnetic structure and metamagnetic transition between low- and high-moment states of NpNiGa5

Systematic neutron scattering experiments have been carried out in order to reveal the magnetic properties of NpNiGa5. We have observed a ferromagnetic transition with the magnetic moment parallel to the tetragonal c axis at TC=30 K. We have also observed antiferromagnetic reflections with the propagation vector q=(1/2 1/2 1/2), coexisting with the ferromagnetic component below TN=18 K. The direction of the antiferromagnetic component is perpendicular to the c axis, most likely μAF(parallel sign)<1 1 0>. Spin-polarized neutron scattering experiments have revealed that only Np carries a magnetic moment. The polarization of Ni atom is less than 0.03 μB/Ni, judging from our experimental accuracy. Thus, we conclude that the magnetic moments of Np atoms exhibit the simple ferromagnetic ordering, TN<T<TC, while a canted antiferromagnetic structure appears where T<TN. A cant angle θ of ∼90 deg. between two adjacent Np moments and a magnetic moment μ of ∼0.8 μB/Np at 3 K have been estimated. The temperature and the magnetic field dependences of these two components have also been quantitatively analyzed. (i) It is quite unusual that the Np total moment increases with the transition from the ferromagnetic to canted antiferromagnetic structure. (ii) The metamagnetic transition cannot be explained in terms of a simple spin-flip mechanism with competing interactions and anisotropy. (iii) Furthermore, an unusual reduction of the Np magnetic moment is accompanied with the metamagnetic transition. From these results, we concluded that a remarkable change in the 5f electronic state between the ferromagnetic low-moment state and the canted antiferromagnetic high-moment state takes place at TN and metamagnetic transition