Critical behavior of a triangular lattice Ising AF/FM bilayer

• We study critical behavior of a triangular lattice Ising AF/FM bilayer.
• Critical properties are studied by Monte Carlo and parallel tempering methods.
• Critical exponents are determined from finite-size scaling analysis.
• At higher temperature Ising phase transitions in both FM and AF layers are found.
• At lower temperature a three-state Potts phase transition in AF layer is found.

We study a bilayer Ising spin system consisting of antiferromagnetic (AF) and ferromagnetic (FM) triangular planes, coupled by ferromagnetic exchange interaction, by standard Monte Carlo and parallel tempering methods. The AF/FM bilayer is found to display the critical behavior completely different from both the single FM and AF constituents as well as the FM/FM and AF/AF bilayers. Namely, by finite-size scaling (FSS) analysis we identify at the same temperature a standard Ising transition from the paramagnetic to FM state in the FM plane that induces a ferrimagnetic state with a finite net magnetic moment in the AF plane. At lower temperatures there is another phase transition, that takes place only in the AF plane, to different ferrimagnetic state with spins on two sublattices pointing parallel and on one sublattice antiparallel to the spins on the FM plane. FSS indicates that the corresponding critical exponents are close to the two-dimensional three-state ferromagnetic Potts model values.