# Phase diagrams and free-energy landscapes for model spin-crossover materials with antiferromagnetic-like nearest-neighbor and ferromagnetic-like long-range interactions

## TL;DR

This paper uses advanced Monte Carlo simulations to explore phase diagrams and free-energy landscapes of a spin-crossover model with competing interactions, revealing complex critical phenomena and hysteresis behaviors relevant to experimental materials.

## Contribution

It introduces an efficient simulation method to analyze thermodynamics of a spin-crossover model with both short- and long-range interactions, uncovering new critical phenomena.

## Key findings

- Identification of tricritical points and critical end points in phase diagrams
- Observation of horn-shaped metastability regions
- Insights into hysteresis loops in spin-crossover materials

## Abstract

We present phase diagrams, free-energy landscapes, and order-parameter distributions for a model spin-crossover material with a two-step transition between the high-spin and low-spin states (a square-lattice Ising model with antiferromagnetic-like nearest-neighbor and ferromagnetic-like long-range interactions) [P. A. Rikvold et al., Phys. Rev. B 93, 064109 (2016)]. The results are obtained by a recently introduced, macroscopically constrained Wang-Landau Monte Carlo simulation method [C. H. Chan, G. Brown, and P. A. Rikvold, Phys. Rev. E 95, 053302 (2017)]. The method's computational efficiency enables calculation of thermodynamic quantities for a wide range of temperatures, applied fields, and long-range interaction strengths. For long-range interactions of intermediate strength, tricritical points in the phase diagrams are replaced by pairs of critical end points and mean-field critical points that give rise to horn-shaped regions of metastability. The corresponding free-energy landscapes offer insights into the nature of asymmetric, multiple hysteresis loops that have been experimentally observed in spin-crossover materials characterized by competing short-range interactions and long-range elastic interactions.