Ordering phenomena in a heterostructure of frustrated and unfrustrated triangular-lattice Ising layers

TL;DR

This study investigates the critical and magnetic behaviors of a bilayer triangular-lattice Ising system with frustrated and unfrustrated layers, revealing complex ordering phenomena including a ferrimagnetic phase and a BKT-like crossover.

Contribution

It provides the first detailed Monte Carlo analysis of a bilayer Ising system with mixed frustration, uncovering the interplay between frustration, interlayer coupling, and critical phenomena.

Findings

Unfrustrated FM layer orders at high temperature.

Ferrimagnetic order induced in frustrated AF layer at low temperature.

Presence of a BKT-like crossover regime with varying critical exponents.

Abstract

We study critical and magnetic properties of a bilayer Ising system consisting of two triangular planes A and B, with the antiferromagnetic (AF) coupling $J_{\rm A}$ and the ferromagnetic (FM) one $J_{\rm B}$ for the respective layers, which are coupled by the interlayer interaction $J_{\rm AB}$ by using Monte Carlo simulations. When $J_{\rm A}$ and $J_{\rm B}$ are of the same order, the unfrustrated FM plane orders first at a high temperature $T_{c1} \sim J_{\rm B}$. The spontaneous FM order then exerts influence on the other frustrated AF plane as an effective magnetic field, which subsequently induces a ferrimagnetic order in this plane at low temperatures below $T_{c2}$. When short-range order is developed in the AF plane while the influence of the FM plane is still small, there appears a preemptive Berezinskii-Kosterlitz-Thouless-like pseudocritical crossover regime just above the ferrimagnetic phase transition point, where the short-distance behavior up to a rather large length scale exponentially diverging in $\propto \JA / T$ is controlled by a line of Gaussian fixed points at $T = 0$. In the crossover region, a continuous variation in the effective critical exponent $4/9 \lesssim \eta^{\rm eff} \lesssim 1/2$ is observed. The phase diagram by changing the ratio $J_{\rm A}/J_{\rm B}$ is also investigated.