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

Abstract The 3Q 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 3Q 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.