Elastic Origin of the Unsymmetrical Thermal Hysteresis in Spin Crossover Materials: Evidence of Symmetry Breaking

• Published in: Symmetry (Basel) Vol. 13; no. 5; p. 828
• Main Authors: Ndiaye, Mamadou, Belmouri, Nour El Islam, Linares, Jorge, Boukheddaden, Kamel
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 2021; MDPI
• Subjects: Broken symmetry; Condensed Matter; Crystallography; electro-elastic modeling; first-order transitions; Heating; Hysteresis loops; Information storage; Ising-like model; Ligands; Monte Carlo simulations; Phase transitions; Physics; Spin transition; unsymmetrical thermal hysteresis; unsymmetrical thermal hysteresis; Monte Carlo simulations; spin crossover materials; symmetry breaking; electro-elastic modeling; first-order transitions; Ising-like model; spin transition
• ISSN: 2073-8994; 2073-8994
• DOI: 10.3390/sym13050828

The jungle of experimental behaviors of spin-crossover materials contains a tremendous number of unexpected behaviors, among which, the unsymmetrical hysteresis loops having different shapes on heating and cooling, that we often encounter in literature. Excluding an extra effect of crystallographic phase transitions, we study here these phenomena from the point of view of elastic modeling and we demonstrate that a simple model accounting for the bond lengths misfits between the high-spin and low-spin states is sufficient to describe the situation of unsymmetrical hysteresis showing plateaus at the transition only on cooling or on heating branches. The idea behind this effect relates to the existence of a discriminant elastic frustration in the lattice, which expresses only along the high-spin to low-spin transition or in the opposite side. The obtained two-step transitions showed characteristics of self-organization of the spin states under the form of stripes, which we explain as an emergence process of antagonist directional elastic interactions inside the lattice. The analysis of the spin state transformation inside the plateau on cooling in terms of two sublattices demonstrated that the elastic-driven self-organization of the spin states is accompanied with a symmetry breaking.