Magnetization and specific-heat measurements on a UIrSi3 single crystal reveal Ising-like antiferromagnetism below TN=41.7K with the easy magnetization direction along the c axis of the tetragonal structure. The antiferromagnetic ordering is suppressed by magnetic fields >Hc (μ0Hc=7.3T at 2 K) applied along the c axis. The first-order metamagnetic transition at Hc 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 Ttc=28K and μ0Htc=5.8T). The modified-Curie-Weiss law fits of temperature dependence of the a- and c-axis susceptibility provide opposite signs of Weiss temperatures, Θpa∼-51K and Θpc∼+38K, respectively. This result and the small value of μ0Hc contrasting to the high TN 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.
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