Eur. Phys. J. B (2013) 86: 271
https://doi.org/10.1140/epjb/e2013-30801-5

Regular Article

Interplay between spin-crossover and magnetic interactions in a BEG model

¹ Département de Physique (FAST) et Institut des Mathématiques et de Sciences Physiques (IMSP), Universitéd’Abomey-Calavi, 01 BP 613, Porto-Novo, Benin
² Groupe d’Etudes de la matière condensée, Université de Versailles/St. Quentin en Yvelines-CNRS, 45 Avenue des Etats Unis, 78035 Versailles Cedex, France

^a e-mail: fhontinfinde@yahoo.fr

Received: 3 September 2012
Received in final form: 2 March 2013
Published online: 17 June 2013

Abstract

A two-dimensional Blume-Emery-Griffiths spin-1 model with spin-phonon interaction is introduced to investigate the thermodynamic properties of Prussian Blue Analogs and Spin-crossover materials. The quadrupolar interaction parameter is assumed to depend on the temperature in the form K = αk_BT while the crystal-field depends both on the ligand-field strength and the degeneracy ratio between high spin (HS) and low spin (LS) states as in some previous works. The model is solved by means of two statistical-mechanical methods: kinetic Monte Carlo simulations and corrective effective field theory calculations. Our calculations indicate that by tuning α, the spin-crossover transition changes to a sharp first order transition where the HS fraction, n_HS changes discontinuously. Second order transitions are observed in the presence of magnetic ordering when the nearest-neighbor coupling constant J exceeds some critical value J_c which depends on α and other model parameters. Below J_c, simple spin-transition occurs at an equilibrium temperature T_eq that is very sensitive to the values of the degenaracy ratio and the ligand-field. Competition between model parameters lead to interesting phase diagrams. Some of them are displayed for varying values of the coupling J and also in the specific case where J and K are of the same order of magnitude. Thermal hysteresis loops have been calculated by Monte Carlo simulations and also by using the self-consistent equations in the case of long-lived metastable states showing strong dependence on model parameters.