Magnetization and specific heat measurements on a UIrSi 3 single crystal reveal Ising-like antiferromagnetism below T N = 41.7 K with easy magnetization direction along the c-axis of tetragonal structure. The antiferromagentic ordering is suppressed by magnetic fields > H c (µ 0 H 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 µ 0 H 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,  p a ~ -51 K and  p c ~ +38 K, respectively. This result and the small value of µ 0 H 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 ferromagentic 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.

We report the first observation of the de Haas-van Alphen (dHvA) effect in the novel spin-triplet superconductor UTe 2 using high quality single crystals with the high residual resistivity ratio (RRR) over 200. The dHvA frequencies, named α and β, are detected for the field directions between c and a-axes. The frequency of branch β increases rapidly with the field angle tilted from c to a-axis, while branch α splits, owing to the maximal and minimal cross-sectional areas from the same Fermi surface. Both dHvA branches, α and β reveal two kinds of cylindrical Fermi surfaces with a strong corrugation at least for branch α. The angular dependence of the dHvA frequencies is in very good agreement with that calculated by the generalized gradient approximation (GGA) method taking into account the on-site Coulomb repulsion of U = 2 eV, indicating the main Fermi surfaces are experimentally detected. The detected cyclotron effective masses are large in the range from 32 to 57 m 0 . They are approximately 10-20 times lager than the corresponding band masses, consistent with the mass enhancement obtained from the Sommerfeld coefficient, γ and the calculated density of states at the Fermi level. The local density approximation (LDA) calculations of ThTe 2 assuming U 4+ with the 5 f 2 localized model are in less agreement with our experimental results, in spite of the prediction for two cylindrical Fermi surfaces, suggesting a mixed valence states of U 4+ and U 3+ in UTe 2 .

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Evolution of magnetism in single crystals of the van der Waals compound VI3 in external pressure up to 7.3 GPa studied by measuring magnetization and ac magnetic susceptibility is reported. Four magnetic phase transitions, at T1 = 54.5 K, T2 = 53 K, TC = 49.5 K, and TFM = 26 K, respectively have been observed at ambient pressure. The first two have been attributed to the onset of ferromagnetism in specific crystal-surface layers. The bulk ferromagnetism is characterized by the magnetic ordering transition at Curie temperature TC and the transition between two different ferromagnetic phases TFM, accompanied by a structure transition from monoclinic to triclinic symmetry upon cooling. The pressure effects on magnetic parameters were studied with three independent techniques. TC was found to be almost unaffected by pressures up to 0.6 GPa whereas TFM increases rapidly with increasing pressure and reaches TC at a triple point at  0.85 GPa. At higher pressures, only one magnetic phase transition is observed moving to higher temperatures with increasing pressure to reach 99 K at 7.3 GPa. In contrast, the low-temperature bulk magnetization is dramatically reduced by applying pressure (by more than 50% at 2.5 GPa) suggesting a possible pressure-induced reduction of vanadium magnetic moment. We discussed these results in light of recent theoretical studies to analyze exchange interactions and provide how to increase the Curie temperature of VI3.