Frustrated mixed spin-1/2 and spin-1 Ising ferrimagnets on a triangular lattice

Mixed spin-1/2 and spin-1 Ising ferrimagnets on a triangular lattice with sublattices A, B, and C are studied for two spin-value distributions (S-A, S-B, S-C) = (1/2, 1/2, 1) and (1/2, 1, 1) by Monte Carlo simulations. The nonbipartite character of the lattice induces geometrical frustration in both systems, which leads to the critical behavior rather different from their ferromagnetic counterparts. We confirm second-order phase transitions belonging to the standard Ising universality class occurring at higher temperatures, however, in both models these change at tricritical points (TCP) to first-order transitions at lower temperatures. In the model (1/2, 1/2, 1), TCP occurs on the boundary between paramagnetic and ferrimagnetic (+/- 1/2, +/- 1/2, -/+ 1) phases. The boundary between two ferrimagnetic phases (+/- 1/2, +/- 1/2, -/+ 1) and (+/- 1/2, -/+ 1/2, 0) at lower temperatures is always first order and it is joined by a line of second-order phase transitions between the paramagnetic and the ferrimagnetic (+/- 1/2, -/+ 1/2, 0) phases at a critical endpoint. The tricritical behavior is also confirmed in the model (1/2, 1, 1) on the boundary between the paramagnetic and ferrimagnetic (0, +/- 1, -/+ 1) phases.