Order from disorder phenomena in BaCoS₂

At T-N similar or equal to 300K the layered insulator BaCoS2 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 magnetic order, but also the inter-layer coherence responsible for the finite N & eacute;el transition temperature. This uncommon ordering mechanism, actively contributed by orbital degrees of freedom, hints at an abundance of low energy excitations above and across the N & eacute;el transition, in agreement with experimental evidence.