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

We investigate the phase diagram of an elastic interaction model for spin crossover materials with antiferromagnetic (AF)-like short-range (SR) interactions [1]. In this model, the interplay between the SR interaction and the long-range (LR) interaction of elastic origin causes complex phase transitions. For relatively weak elastic interactions, the phase diagram is characterized by tricritical points, at which 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. Similar structures of the phase diagram are found in the Ising AF magnet with infinite-range F interactions [2], and we find universal features caused by the interplay between the competing SR and LR interactions. The LR interaction of elastic origin is irrelevant (inessential) for the critical line. In contrast, those spinodal lines result from the LR interaction of elastic origin.

[1] M. Nishino, S. Miyashita, and P. A. Rikvold, Phys. Rev. B 96, 144425 (2017).
[2] P. A. Rikvold, G. Brown, S. Miyashita, C. Omand, and M. Nishino, Phys. Rev. B 93, 064109 (2016).