A Unified Theoretical Description of the Thermodynamical Properties of Spin Crossover with Magnetic Interactions

After the discovery of the phenomena of light-induced excited spin state trapping (LIESST), the functional properties of metal complexes have been studied intensively. Among them, cooperative phenomena involving low spin-high spin (spin-crossover) transition and magnetic ordering have attracted interests, and it has become necessary to formulate a unified description of both phenomena. In this work, we propose a model in which they can be treated simultaneously by extending the Wajnflasz-Pick model including a magnetic interaction. We found that this new model is equivalent to Blume-Emery-Griffiths (BEG) Hamiltonian with degenerate levels. This model provides a unified description of the thermodynamic properties associated with various types of systems, such as spin-crossover (SC) solids and Prussian blue analogues (PBA). Here, the high spin fraction and the magnetization are the order parameters describing the cooperative phenomena of the model. We present several typical temperature dependences of the order parameters and we determine the phase diagram of the system using the mean-field theory and Monte Carlo simulations. We found that the magnetic interaction drives the SC transition leading to re-entrant magnetic and first-order SC transitions.Comment: 30pages, 11figure

A tentative tracking of the SARS-Cov2 pandemic in France, based on a corrected SIR model including vaccination effects

We developed successive extensions of the SIR model in order to track the dynamics of the SARS-Cov2 disease. The analysis of health system available data is published in a chronicle accessible on the net: https://corona-circule.github.io/lettres

Photo-excitation et structure magnétique photo-induite dans les analogues photo-magnétiques du bleu de Prusse

PARIS-BIUSJ-Thèses (751052125) / SudocCentre Technique Livre Ens. Sup. (774682301) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Effets du couplage spin-orbite sur les propriétés des composés magnétiques ioniques du groupe du fer à température de transition basse

Nous rassemblons ici un certain nombre de résultats relatifs à l'effet du couplage spin-orbite sur le niveau fondamental des ions du groupe du fer dans un cristal. A l'aide de quelques exemples simples, nous montrons l'importance de ces considérations dans les études de structures magnétiques

Emergence of Dynamical Dissipative Structures in a Bistable Spin Crossover Solid Under Light: Experience and Theory

International audienc

Light-induced instability generated by photo-thermal effect in switchable molecular crystals

The photo-thermal effect of thermo-chromic crystals generates a novel kind of light-induced instability. We investigate here a mean-field (zero-dimensional) approach of the problem using basic equations of heat transfer and spin-crossover phenomena. This mean-field approach compares to the well-known Stoner-Wolfarth mechanism for the magnetization reversal of nano-particles, it should be suited to switchable micro-crystals and leads to auto-catalytic oscillations which remain to be observed. The stabilization of the high-spin–low-spin interface by photo-thermal effect is noticed

When T(LIESST) meets thermal hysteresis - a theoretical approach

We used a macroscopic master equation for a two-level system, with simple assumptions, to calculate the kinetic behavior of a spin-crossover system when the equilibrium temperature interplays with the T(LIESST) temperature (LIESST = light-induced excited spin state trapping), that is, the thermal return temperature of the low-temperature metastable state of the system. The leading role of the unstable equilibrium branch is characterized through the crossing of the cooperative energy barrier, which induces a bifurcation of the response of the system in the strongly cooperative case. The results are shown in terms of a kinetic phase diagram, which shows the eventual discontinuities that result from the bifurcation effect. The present study suggests an experimental method for locating the unstable equilibrium branch of the thermal hysteresis, which enables determination of the equilibrium temperature of the hysteretic system, and which is not accessible by usual static measurements