Phase transitions in few-monolayer spin ice films

Vertex models are an important class of statistical mechanical system that admit exact solutions and exotic physics. Applications include water ice, ferro- and antiferro-electrics, spin ice and artificial spin ice. Here we show that it is possible to engineer spin ice films with atomic-layer precision down to the monolayer limit. Specific heat measurements show that these films, which have a fundamentally different symmetry to bulk spin ice, realise systems close to the two-dimensional F-model, with exotic phase transitions on topologically-constrained configurational manifolds. Our results show how spin ice thin films can release the celebrated Pauling entropy of spin ice without an anomaly in the specific heat. They also significantly expand the class of vertex models available to experiment. Magnetic spin ice compounds are described by vertex models, which have been intensively studied for their exotic properties. Bovo et al. show thin films of Dy2Ti2O7 have structures distinct from bulk crystals and come close to realising the two-dimensional F-model, which has an unusual ordering transition in the Berezinskii–Kosterlitz–Thouless class.