U4+ FORM FACTOR IN UGa2

UGa2 crystallizes in the hexagonal AIB2 type structure and a large orthorhombic distortion is observed in the ferromagnetic state (Tc = 125 K) due to a giant magnetostriction. A polarized neutron study has been performed at 4.2 K in a magnetic field of 46.2 kOe> applied parallel to the easy magnetization axis a. Integrated intensities have been measured for the refinement of the nuclear structure factors in the distorted structure. The projection of the magnetic density on the (100) plane evidences a large and almost spherical density on U sites, no density on the Ga sites, and a slight positive polarization between the atoms. The deduced U form factor has been analyzed in order to determine the magnetic ground state. We think that the uranium atoms are in the 4+ state. The observed form factor corresponds to a larger derealization of the 5f electrons than that calculated in the RussellSaunders coupling approximation. A first calculation using the dipole approximation in the intermediate coupling framework, associated with the contribution of a positive polarization can account for the observed variation of the U form factor.