Organized by the Institute of Experimental Physics, University of Bialystok and the Institute of Atomic Energy, Swierk under the auspices of the European Crystallographic Association; Committee of Crystallography, the Polish Academy of Sciences; and the Polish Neutron Scattering Society The School is financially supported in part by the International Union of Crystallography; the European Crystallographic Association; the European Office of Aerospace Research & Development; the Ministry of Science and Information Society Technologies; the Warsaw University of Technology; and the University of Bialystok.
		The Ising-like model applied to spin-Crossover transition: equilibrium and non-equilibrium properties under various excitations. Kamel Boukheddaden, F. Varret, E. Codjovi, J. Linares, D. Loutete, I. Maurin. Laboratoire de Magnétisme et d'Optique, Université de Versailles Saint-Quentin-en-Yvelines, France Topics The spin-crossover (SC) phenomenon is well described by the two-states Ising-like model proposed for the first-time by Wajnflasz and Pick. We will present a general review of this model applied to several situations of SC systems: exact treatment for 1D case, including short and long range interactions, Monte Carlo simulations for the two-step SC behaviour, ... Recently, the non-equilibrium properties of SC systems attracted more interest. In particular, the relaxations of photo-induced high spin state and the effect of a continuous irradiation, combined with a temperature change, have been deeply studied by our group. Here, we will introduce the concept of a microscopic stochastic treatment, based on the master equation approach, which allows us to determine the relaxation of the HS fraction at low temperature. Two different levels of approaches will be discussed: (i) a dynamical mean-field theory and (ii) dynamical extension of Bethe-Peierls approach which takes into account short range correlations between nearest-neighbouring molecules. A quantitative comparison of the obtained results by these two treatments to the experimental data of relaxation extracted from magnetic measurements will be also made. In the next section, we will explain how we introduce the effect of photo-excitation (light) in the model, and we will show the variety of photo-induced bistabilities we obtain in the non-equilibrium regime, such as light induced thermal hysteresis, light-induced optical hysteresis, etc ... The Ising-like model succeeds as well to describe qualitatively the effect of a pulse magnetic field on SC systems. This point will be also briefly reviewed. Finally, in the last section, we will present a new three states model [X], based on Blume-Emery-Griffiths Hamiltonian, which unifies the description of SC with magnetic interaction and Prussian blue analogues. There, will present the synergy between magnetic interaction responsible for the magnetic ordering and the elastic interaction responsible for the SC transition. Non-equilibrium properties of this system will be also presented. [1] J. Wajnflasz, R. Pick J. Phys. (1971) ; [2] K. Boukheddaden et al. Phys. Rev. B. I. (2000); [3] K. Boukheddaden et al. Phys. Rev. B II (2000); [4] B. Hôo, K. Boukheddaden, .Eur. Phys. J. B. (2000); [5] J. Linares, H. Spiering, F. Varret, Eur. Phys. J. B. (1999); [6] K. Boukheddaden, J. Linares, H. Spiering, F. Varret (2001); [7] M. Nishino, K. Boukheddaden, S. Miyashita, F. Varret, Phys. Rev. B. (2003); [8] K. Boukheddaden, Prog. Theor. Phys., 112, 205, (2004); [9] M. Nishino, K. Boukheddaden, S. Miyashita, F. Varret, Phys. Rev. B. (2005, in press).