Nontrivial phase diagram for an elastic interaction model of spin crossover materials with antiferromagnetic-like short-range interactions

TL;DR

This paper explores the complex phase diagram of an elastic interaction model for spin crossover materials, revealing universal features and different domain formation behaviors due to the interplay of short-range and long-range interactions.

Contribution

It introduces a detailed phase diagram for an elastic interaction model with antiferromagnetic-like interactions, highlighting universal features and the impact of elastic origin long-range interactions.

Findings

Presence of tricritical points at weak elastic interactions.

Unusual horn structures at higher temperatures for strong elastic interactions.

Different clustering behaviors in phase transitions due to interaction types.

Abstract

We study the phase diagram of an elastic interaction model for spin crossover (SC) materials with antiferromagnetic-like short-range interactions. In this model, the interplay between the short-range interaction and the long-range interaction of elastic origin causes complex phase transitions. For relatively weak elastic interactions, the phase diagram is characterized by tricritical points, at which antiferromagnetic (AF) -like and ferromagnetic (F) -like spinodal lines and a critical line merge. On the other hand, for relatively strong elastic interactions, unusual "horn structures," which are surrounded by the F-like spinodal lines, disorder (D) spinodal lines, and the critical line, are realized at higher temperatures. These structures are similar to those obtained in our previous study [Phys. Rev. B {\bf 93}, 064109 (2016)] of an Ising antiferromagnet with infinite-range ferromagnetic interactions, and we find universal features caused by the interplay between the competing short-range and long-range interactions. The long-range interaction of elastic origin is irrelevant (inessential) for the critical line. In contrast, the AF-like, F-like, and D spinodal lines result from the long-range interaction of elastic origin. This difference causes qualitatively different features of domain formation or nucleation of the new phase: clustering occurs in the former case, while clustering is absent in the latter. Based on the phase diagrams, we discuss the patterns and clustering features of two-step SC transitions, in which the AF-like phase is realized in the intermediate temperature region.