CsFeCl3

Cesium iron chloride is a remarkable material. The iron ions with S=1 are dominated by the easy plane anisotropy and despite the presence of antiferromagnetic couplings between them the magnet remains gapped and quantum disordered down to T=0. Interestingly, the couplings are strong enough to create a magnetic state at finite temperature for an isostructural material with Cs replaced by Rb. This opens up a possibility to study a continuous magnetic phase transition, induced by the variation of sample's chemical composition.  Plus, this story is taking place on a frustrated triangular lattice!