A model for the mediated artificial square ice phenomenology

In the present letter we discuss the origin of the vertex population inversion observed experimentally in the mediated artificial square ice (Östman E. et al., Nat. Phys., 14 (2018) 375). An interaction modifier is a disc-shaped magnetic nanoisland (a dot) which is placed at the center of a vertex to mediate the interaction between the nearby islands. We show that the inversion is of entropic origin, and can be explained via the renormalization of the vertex configuration energies due to local interaction between the nanoislands and the dot. We show that in a model with mixed Heisenberg and Ising spins (a spin-dot interaction) that as a function of the island size, entropic effects become important. Because of the renormalization of the energies, we observe a level crossing between Type I and Type II vertices which is qualitatively similar to the experimental results. We also discuss possible implications of spin-dot interactions in the eight- and sixteen-vertex-models phase diagrams, using both the exact order parameter in the former case and the numerically inferred one in the latter.