Pressure-induced magnetic structures in UNiGa

UNiGa orders magnetically below 40 K and exhibits several antiferromagnetic phases with decreasing temperature. These phases are characterized by various stackings (along the c axis) of basal-plane ferromagnetic layers of U magnetic moments aligned along c. The related magnetic phase transitions are accompanied by pronounced magnetoelastic anomalies that are well seen in the linear thermal expansion. Thermal expansion experiments on UNiGa under hydrostatic pressure revealed a pronounced pressure influence on the stability of individual antiferromagnetic phases (although the Neel temperature is only a little reduced) and indicated a new magnetic phase induced by pressures higher than 1.8 GPa. We report the results of neutron-diffraction studies of a UNiGa single crystal under hydrostatic pressures up to 2.3 GPa in conjunction with magnetoelastic data. The new magnetic phase is characterized by a simple +- stacking of ferromagnetic sheets and for 2.3 GPa it is the only magnetically ordered phase in UNiGa. The U magnetic moment in the new magnetic phase is only a little reduced with respect to the ground-state phase. Pressure-induced changes of exchange interactions in UNiGa are discussed considering the known values of linear compressibility.