Modélisation des processus à l'équilibre et hors équilibre de matériaux à transition de spin : application à la simulation des diagrammes de diffraction des rayons X

Abstract : Modelisation of Equilibrium and non equilibrium processes experimentally observed in spin crossover compounds is essential for a better understanding of mechanisms of the transition existing in those bistable solids which are good candidates for future industrial applications in computing data storage. Measurements following the molecular spin state change (magnetic, optical...) have been successfully reproduced, using various models, in particular Ising-like models. However, structural phase changes, occurring during the spin state change and only accessible with X-ray or neutron diffraction experiments, can't be reproduced with Ising-like models. The aim of this work is to interpret both kinds of measures: magnetic and diffraction. We have also introduced a new microscopic spin model in which lattice degrees of freedom have been explicitly taken into account. This anharmonic model has been solved analytically in the one dimensional case and numerically in higher dimensions using Monte Carlo simulation of equilibrium and non equilibrium processes. This model retrieves most of results or Ising-like models and simulates lattice expansion or contraction during the spin transition. We have coupled this anharmonic model with a soft called DISCUS, in order to simulate diffraction experimental conditions and to calculate the intensity diffracted by the lattice. Diffraction patterns have been reproduced in the different cases (thermal transition, thermal relaxation, photoexcitation...) observed experimentally. Conditions for the observation of the coexistence of HS and LS Bragg peak are discussed and analyzed. The concept of "like spin domains" (LSD) is then defined more precisely by introducing the concept of "molecular like spin domains" (MLSD) and "structural like spin domains" (SLSD).
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William Nicolazzi. Modélisation des processus à l'équilibre et hors équilibre de matériaux à transition de spin : application à la simulation des diagrammes de diffraction des rayons X. Autre [cond-mat.other]. Université Henri Poincaré - Nancy 1, 2008. Français. ⟨NNT : 2008NAN10078⟩. ⟨tel-01748424⟩

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