Cristian Enachescu

Examples of molecular switching in inorganic solids, due to temperature, light, pressure, and magnetic field

We describe various molecular switching processes occurring in several types of inorganic solids: spin cross-over (SC) compounds, photomagnetic Prussian blue analogs (PBAs), and valence-tautomeric system. Their thermo-, photo-, piezo-, and magneto-chromic properties are illustrated by recent examples. A common description of their static properties by a two-level model is given.

Comparison of static and light-induced thermal hystereses of a spin-crossover solid, in a mean-field approach†

We investigate the relationship between the two mean-field models which have been proposed independently for characterizing the static and the light-induced thermal hystereses of spin-transition solids, respectively involving an interaction parameter J and a relaxation self-acceleration parameter α. We unify these two models into a unique - static and dynamic - mean-field description based on Arrhenius thermal activation processes. The model is used to analyse the experimental hysteresis loops, the relaxation curves in the dark, and the light-induced instability in the spin-crossover system [FexCo1-x(btr)2(NCS)2]H2O. A linear relationship between J and αT is obtained, which is in qualitative agreement with the available theories.

Direct access to the photo-excitation and relaxation terms in photo-switchable solids : non-linear aspects

Abstract We investigate the photo-excitation process and the relaxation of the photo-excited state in photo-switchable solids, such as spin transitions systems. We follow the competition between the relaxation and the photo-excitation (up and eventually down) processes. The kinetic experimental curves are treated so as to derive the various contributions to the master equation, in terms of a single macroscopic variable: the population of the photo-excited state. The experimental examples are taken from [Fe x M 1− x (btr) 2 (NCS) 2 ].H 2 0, with M=Co, Ni, Zn. They give evidence for a non-linear character of the relaxation at all temperatures, including the tunnelling regime, and for non-linear behaviours of the photo-excitation rates.

Monte Carlo entropic sampling applied to spin crossover solids: the squareness of the thermal hysteresis loop

Abstract The Monte Carlo entropic sampling method previously presented in [ I. Shteto, J. Linares, F. Varret, Phys. Rev. E 56 (1997) 5128 ] is adapted here to an Ising-like system with short- and long-range interactions. Such model is suited to spin crossover solids [ [2] , [3] ] where the long interaction is due to elastic coupling mediated by the lattice, while the short-range interaction originates from the bonding between the spin crossover units [J. Linares, H. Spiering, F. Varret, Eur. J. Phys. B 10 (1999) 271]. Taking into account the different degeneracies gHS for high-spin (HS) and gLS for low-spin (LS) states, the Ising Hamiltonian associated with fictitious spins is written: H=−h∑σ i −J∑σ i σ j with h=− Δ 2 + k B T ln g HS g LS 2 +G〈σ〉 where J and G are the short- and long-range interactions, respectively, and Δ the energy gap of ligand field such that the LS state is the ground state. The numerical method has been tested successfully by comparison to the exact solution for a 1D system: [Fe(Htrz)2(trz)](BF4)2 [ J. Linares, H. Spiering, F. Varret, Eur. J. Phys. B 10 (1999) 271; J. Krober, J.P. Audiere, R. Claude, O. Kahn, J. Hassnoot, F. Groliere, C. Jay, A. Bousseksou, J. Linares, F. Varret, A. Gonthier-Vassal, Chem. Mater. 6 (1994) 1404 ]. We describe here the results obtained for 2D systems, and show that the squareness of the thermal hystersis loop, associated with the spin-transition, can be correlated to the strength of short-range interactions.

Static and light induced hysteresis in spin-crossover compounds: experimental data and application of Preisach-type models

In this paper we focus on the different types of hysteresis exhibited by spin transition compounds: thermal hysteresis, pressure hysteresis and light induced thermal hysteresis. We analyse the manner in which different Preisach-type models can be applied to reproduce major and minor hysteresis loops.

A two-sublattice model for light-induced hysteresis in spin-crossover solids: symmetry breaking and kinetic effects

The recently developed macroscopic model of light-induced equilibrium of spin- crossover solids is extended to the case of two equivalent sublattices coupled antiferro- magnetically. To do this, we use an Ising-like model involving two coupling parameters of opposite signs for the intra- and inter-sublattice interactions, previously introduced to describe the two-step static spin transition, and we have adapted the macroscopic master equation describing the time evolution of the system accordingly. We mostly report on the quasi-static solutions of the system, under constant light irradiation and for slow temperature variations. Light-induced thermal hysteresis loops are obtained and have typical two-step shapes for suitable sets of parameter values. The model predicts, in analogy with the static transition, a possible light-induced symmetry breaking between the structurally equivalent sublattices. Kinetic effects are briefly described.