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

摘要

We present phase diagrams, free-energy landscapes, and order-parameterdistributions for a model spin-crossover material with a two-step transitionbetween the high-spin and low-spin states (a square-lattice Ising model withantiferromagnetic-like nearest-neighbor and ferromagnetic-like long-rangeinteractions) [P. A. Rikvold et al., Phys. Rev. B 93, 064109 (2016)]. Theresults are obtained by a recently introduced, macroscopically constrainedWang-Landau Monte Carlo simulation method [C. H. Chan, G. Brown, and P. A.Rikvold, Phys. Rev. E 95, 053302 (2017)]. The method's computational efficiencyenables calculation of thermodynamic quantities for a wide range oftemperatures, applied fields, and long-range interaction strengths. Forlong-range interactions of intermediate strength, tricritical points in thephase diagrams are replaced by pairs of critical end points and mean-fieldcritical points that give rise to horn-shaped regions of metastability. Thecorresponding free-energy landscapes offer insights into the nature ofasymmetric, multiple hysteresis loops that have been experimentally observed inspin-crossover materials characterized by competing short-range interactionsand long-range elastic interactions.