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. (C) 1998 Elsevier Science S.A.