Multistep spin-crossover transitions induced by the interplay between short- and long-range interactions with frustration on a triangular lattice

Abstract

We investigate the phase diagram of an elastic interaction model for spin crossover (SC) materials on a triangular lattice, in which the long-range (LR) interaction of elastic origin, antiferromagnetic-like nearest-neighbor (NN) interaction ${J}_{1}$, and ferromagnetic (F)-like next-NN interaction ${J}_{2}$ interplay nontrivially. We find that for relatively weak elastic interactions in the model, two critical lines with the three-state Potts universality and the Berezinskii-Kosterlitz-Thouless (BKT) phase line merge at the BKT high temperature end point at zero effective field $H=0$. In contrast, for relatively large elastic interactions, the two critical lines and the coexistence line between the ferrimagnetic (FR)-like phases merge at a critical end point at $H=0$. The phase diagrams are also characterized by tricritical points, at which F-like spinodal, FR-like spinodal, and critical lines merge, and the metastable regions of the F-like and FR-like phases expand for stronger elasticity. We also find that, in the model with only the NN interaction ${J}_{1}$, for relatively weak elastic interactions, the critical end point is located at a very low temperature, the disordered phase appears at low temperatures, and the metastable regions of the F-like and FR-like phases are very small. The SR interactions are relevant for the critical lines, while the spinodal lines are caused by the LR interaction of elastic origin, which is independent of the strength of elasticity. Making use of these phase diagrams with the different characteristics, we show a wide variety of SC transitions with one to four steps, in which a second-order (continuous), first-order (discontinuous), or BKT (continuous) transition can occur at each step.