Equilibrium, Metastability, and Hysteresis in a Model Spin-crossover Material with Nearest-neighbor Antiferromagnetic-like and Long-range Ferromagnetic-like Interactions

TL;DR

This study investigates the complex phase behavior and hysteresis phenomena in a simplified spin-crossover model with competing local antiferromagnetic-like and long-range ferromagnetic-like interactions, revealing novel metastability features.

Contribution

It introduces a combined Monte Carlo and mean-field analysis of a spin-crossover model with competing interactions, highlighting new metastability and critical phenomena not previously characterized.

Findings

Identification of horn-shaped metastability regions near critical points.

Observation of hysteresis loops similar to experimental two-step transitions.

Demonstration of complex phase diagram features due to interaction competition.

Abstract

Phase diagrams and hysteresis loops were obtained by Monte Carlo simulations and a mean-field method for a simplified model of a spin-crossover material with a two-step transition between the high-spin and low-spin states. This model is a mapping onto a square-lattice $S=1/2$ Ising model with antiferromagnetic nearest-neighbor and ferromagnetic Husimi-Temperley (equivalent-neighbor) long-range interactions. Phase diagrams obtained by the two methods for weak and strong long-range interactions are found to be similar. However, for intermediate-strength long-range interactions, the Monte Carlo simulations show that tricritical points decompose into pairs of critical endpoints and mean-field critical points surrounded by horn-shaped regions of metastability. Hysteresis loops along paths traversing the horn regions are strongly reminiscent of thermal two-step transition loops with hysteresis, recently observed experimentally in several spin-crossover materials. We believe analogous phenomena should be observable in experiments and simulations for many systems that exhibit competition between local antiferromagnetic-like interactions and long-range ferromagnetic-like interactions caused by elastic distortions.