Critical behavior of a triangular lattice Ising AF/FM bilayer

TL;DR

This study investigates a bilayer Ising model with antiferromagnetic and ferromagnetic triangular planes, revealing unique critical behavior and phase transitions distinct from single-layer systems, using Monte Carlo simulations.

Contribution

It uncovers novel critical phenomena in AF/FM bilayers, showing phase transitions with exponents similar to the 2D three-state Potts model, which differs from known single-layer behaviors.

Findings

Identified a standard Ising transition in the FM plane inducing ferrimagnetism in the AF plane.

Discovered a second phase transition in the AF plane with Potts-like critical exponents.

Revealed that the bilayer's critical behavior differs fundamentally from single-layer and other bilayer configurations.

Abstract

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.