Entropy-driven phase transition in a non-collinear antiferromagnet due to higher-order exchange interactions

TL;DR

This paper investigates a temperature-driven phase transition in a non-collinear antiferromagnet caused by entropy effects, using Monte Carlo simulations and analytical modeling, revealing a transition between triple-Q and row-wise states.

Contribution

It demonstrates that entropy drives a phase transition in a non-coplanar spin state, supported by simulations and analytical expressions, expanding understanding of multi-Q magnetic states.

Findings

Entropy drives the phase transition between triple-Q and row-wise states.

The transition is predicted to occur across various magnetic parameters.

The study provides an analytical expression for the partition function.

Abstract

The triple-Q state arises due to the superposition of three symmetry equivalent spin spirals stabilized by higher-order exchange interactions. It has been predicted more than 20 years ago but was only recently discovered in a Mn monolayer on the Re(0001) surface. To date little is known about the thermodynamic properties of this intriguing non-coplanar spin state. Here, we reveal a low-temperature phase transition between the triple-Q and the row-wise antiferromagnetic state in this system via Monte Carlo simulations based on an atomistic spin model parametrized by density functional theory. By modeling the free energy landscape in terms of thermal excitations we derive an analytical expression of the partition function, which allows us to prove that the phase transition is driven by entropy. The predicted phase transition is not unique to Mn/Re(0001) but appears for a wide range of magnetic interaction parameters and is expected to occur also for other multi-Q states.