Reentrant magnetic ordering and percolation in a spin-crossover system

Spin-crossover compounds, which are characterized by magnetic ions showing low-spin and high-spin states at thermally accessible energies, are ubiquitous in nature. We here focus on the effect of an exchange interaction on the collective properties for the case of nonmagnetic low-spin ions, which applies to Fe2+ compounds such as FeO under high pressure. Monte Carlo simulations are used to study a three-dimensional spin-crossover model for the full parameter range from essentially pure high spin to essentially pure low spin. We find that as the low-spin state becomes more favorable, the Curie temperature drops, the universality class deviates from the three-dimensional Heisenberg class, and the transition eventually changes to first order. A heat-bath algorithm that grows or shrinks low-spin and high-spin domains is developed to handle the first-order transition. When the ground state has low spin, a reentrant magnetic transition is found in a broad parameter range. We also observe a percolation transition of the high spins, which branches off the first-order magnetic transition.