# Order from disorder phenomena in BaCoS$_2$

## TL;DR

This paper reveals that the magnetic transition in BaCoS$_2$ is driven by an order-from-disorder mechanism involving orbital degrees of freedom, explaining its finite Néel temperature despite magnetic frustration.

## Contribution

It demonstrates that the magnetic transition in BaCoS$_2$ is an order-from-disorder phenomenon involving orbital effects, a novel insight into frustrated layered magnets.

## Key findings

- Magnetic transition driven by order-from-disorder mechanism.
- Orbital degrees of freedom contribute actively to the transition.
- Presence of abundant low-energy excitations around $T_N$.

## Abstract

At $T_N\simeq 305~\text{K}$ the layered insulator BaCoS$_2$ transitions to a columnar antiferromagnet that signals non-negligible magnetic frustration despite the relatively high $T_N$, all the more surprising given its quasi two-dimensional structure. Here, we show by combining ab initio and model calculations that the magnetic transition is an order-from-disorder phenomenon, which not only drives the columnar $C_4\to C_2$ symmetry breaking, but also, and more importantly, the inter-layer coherence responsible for the finite N\'eel transition temperature. This uncommon ordering mechanism, actively contributed by orbital degrees of freedom, hints at an abundance of low energy excitations below and, especially, above $T_N$, not in disagreement with experimental evidences, and might as well emerge in other layered correlated compounds showing frustrated magnetism at low temperature.