Magnetic phases and magnetoelastic phenomena in UNiGa under pressure

UNiGa exhibits several antiferromagnetic (AF) phases below T_N=39 K. The related magnetic phase transitions cause pronounced thermoexpansion anomalies of opposite sign for the c- and a-axes, respectively, leaving volume effects nearly negligible. Magnetic fields applied along the c-axis induce transitions from the AF phases to an uncompensated AF and/or to a ferromagnetic (F) phase accompanied by magnetostriction effects. Our results on pressure influence on the thermoexpansion and magnetostriction anomalies allow us to propose a tentative p-T magnetic phase diagram. A new AF phase induced by pressures above 1.8 GPa has been found. Neutron-diffraction studies under hydrostatic pressure up to 0.9 GPa and in magnetic fields up to 2 T confirmed that the critical temperatures and the critical fields of transitions of UNiGa are strongly pressure dependent (some phases are even suppressed) although relevant magnetic structures themselves remain essentially unchanged.