Magnetic phases in UNi₂Si₂

The tetragonal compound UNi₂Si₂ exhibits in zero magnetic field three different antiferromagnetic phases below T_N = 124 K. They are formed by ferromagnetic basal planes, which are antiferromagnetically coupled along the c-axis with the propagation vector q = (0, 0, q_z). Two additional order-order magnetic phase transitions are observed below T_N, namely at T₁ = 108 K and T₂ = 40 K in zero magnetic field. All three phases exhibit strong uniaxial anisotropy confining the U moments to a direction parallel to the c-axis. UNi₂Si₂ single crystals were studied in detail by measuring bulk thermodynamic properties, such as thermal expansion, resistivity, susceptibility, and specific heat. A microscopic study using neutron dffraction was performed in magnetic fields up to 14.5 T parallel to the c-axis, and a complex magnetic phase diagram has been determined. Here, we present the analysis of specific-heat data measured in magnetic fields up to 14 T compared with the results of the neutron-diffraction study and with other thermodynamic properties of UNi₂Si₂.