Site susceptibility tensors and magnetic structure of U3Al2Si3:: a polarized neutron diffraction study

The nuclear and magnetic structures of U3Al2Si3 have been investigated by means of polarized and unpolarized neutron diffraction for single crystals. Measurements were made in the temperature range from 7 to 250 K and in external magnetic fields up to 6 T parallel to [100], [(1) over bar 10] and [001]. Above the magnetic ordering temperature (T-C = 36 K) the crystals have the fully ordered U3Ga2Ge3 structure (space group I4/m). Below 35 K both polarized and unpolarized neutron measurements show that the magnetic moments on the U3 atoms, which occupy general positions in the space group, order in the ab-plane in a non-collinear arrangement with a net ferromagnetic component. The moments of the U1 and U2 atoms are found to be small. A polarized neutron study of the magnetization induced by applied fields above T-C shows marked anisotropy and makes it likely that it is this strong local anisotropy of the U3 ions which leads to the non-collinearity of the magnetic structure.