Antiferromagnetism and phase transitions in noncentrosymmetric UIrSi3

Magnetization and specific-heat measurements on a UIrSi3 single crystal reveal Ising-like antiferromagnetism below T-N = 41.7 K with the easy magnetization direction along the c axis of the tetragonal structure. The antiferromagnetic ordering is suppressed by magnetic fields > H-c (mu H-0(c) = 7.3 T at 2 K) applied along the c axis. The first-order metamagnetic transition at H-c exhibits asymmetric hysteresis reflecting a slow reentry of the complex ground-state antiferromagnetic structure with decreasing field. The hysteresis narrows with increasing temperature and vanishes at 28 K. A second-order metamagnetic transition is observed at higher temperatures. The point of change of the order of transition in the established H-T magnetic phase diagram is considered as the tricritical point (at T-tc = 28 K and mu H-0(tc) = 5.8 T). The modified-Curie-Weiss law fits of temperature dependence of the a- and c-axis susceptibility provide opposite signs of Weiss temperatures, Theta(a)(p) similar to -51 K and Theta(c)(p) similar to + 38 K, respectively. This result and the small value of mu H-0(c) contrasting to the high T-N indicate competing ferromagnetic and antiferromagnetic interactions responsible for the complex antiferromagnetic ground state. The simultaneous electronic-structure calculations focused on the total energy of ferromagnetic and various antiferromagnetic states, the U magnetic moment, and magnetocrystalline anisotropy provide results consistent with experimental findings and the suggested physical picture of the system.