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

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 shortrange 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 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. PACS numbers: 75.30.Wx 64.60.-i 75.60.-d 64.60.De

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

“…(Very recently, horn regions and asymmetric, two-step hysteresis loops, analogous to those seen in the model studied here, have also been observed for a model with antiferromagneticlike nearest-neighbor interactions and genuine elastic interactions [78].) The horn structure gives rise to asymmetric, two-step hysteresis loops (see example in Fig.…”

Order-disorder or phase-separation transition: Analysis of the Au-Pd system by the effective site energy model

“…where the Ising term is expressed as follows using Eqs. 7and (8): The triplets term is expressed as a function of the two curvatures A (c) and B (c) (see Appendix C for more details):…”

“…This analysis provides a better understanding of the interplay between competing short-range order (SRO) and long-range order interactions which can be at the origin of complex phase diagrams. Such effects are also the topic of numerous studies in the field of spin-crossover materials [6][7][8][9][10][11][12][13][14][15].…”

Dynamical Properties of Spin‐Crossover Solids Within the Kinetic Spin‐1 BEG Model in the Presence of a Time‐Dependent Magnetic Field

“…The thermally induced spin transition leads to both electric and structural changes, often observed as a color and magnetic moment changes . When the interactions between molecules are weak, the HS fraction changes smoothly with the temperature, whereas when it becomes strong, the system exhibits cooperative phenomena, which manifest through the existence of first‐order phase transitions accompanied with thermal hysteresis. An interesting example is that of the , which exhibits a spin transition with a hysteresis loop near the room temperature region .…”

“…Of course, the interaction in SCO solids is dominated by the variations of unit‐cell volume and bond length, that are considerably larger in the HS state. These result, in addition to the larger electronic degeneracy, also in a larger phonon density for this state . At the atomic scale and in the case of , the SCO phenomenon is the result of the redistribution of the electrons between the bonding and the antibonding orbitals.…”