Reentrant magnetic ordering and percolation in a spin-crossover system

TL;DR

This study uses Monte Carlo simulations to explore how exchange interactions influence magnetic ordering and phase transitions in spin-crossover compounds, revealing reentrant behavior and percolation phenomena.

Contribution

It introduces a comprehensive 3D spin-crossover model with a novel heat-bath algorithm to analyze phase transitions from high to low spin states.

Findings

Curie temperature decreases with low-spin favorability

Transition changes from second to first order as low-spin state dominates

Reentrant magnetic transition occurs in broad parameter range

Abstract

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 non-magnetic low-spin ions, which applies to Fe(II) compounds. 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.