Mixed spin-1/2 and spin-1 Ising ferromagnets on a triangular lattice

By Monte Carlo simulations we study critical properties of the mixed spin-1/2 and spin-1 Ising model on a triangular lattice, considering two different ways of the spin-value distributions on the three sublattices: $(1\mathrm{/}2,1\mathrm{/}2,1)$ and $(1\mathrm{/}2,1,1)$. In the former case, we find standard Ising universality class phase transitions between paramagnetic and magnetic phases but no phase transitions between two distinct magnetic phases $(\pm 1\mathrm{/}2,\pm 1\mathrm{/}2,\pm 1)$ and $(\pm 1\mathrm{/}2,\pm 1\mathrm{/}2,0)$ at any finite temperature, except for some interesting non-critical anomalies displayed by response functions. On the other hand, the latter case turns out to be a rare (or perhaps the only) example of a two-dimensional mixed spin-1/2 and spin-1 Ising model on a standard lattice that displays tricritical behavior.