第3回　理論グループ共通セミナー　（講師：Dr. William Nicolazzi）

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modelisation in spin crossover phenomena”
概　要：
Spin Crossover (SC) compounds are transition metallic cations complexed by six ligands forming an octaedral ligand field giving rise to the existence of two bistable magnetic
molecular states. The same order of magnitude of the electron pairing energy and the ligand field leads to the possibility for the molecule to switch from a High Spin (HS) to Low Spin (LS) state with the application of an external stimuli like temperature, pressure, magnetic field. It is well known that the characteristic of the spin state change, gradual, abrupt with or without hysteresis, are not only dependent on intra-molecular properties (ligand field, intramolecular vibration) but rather on the elastic intermolecular interactions, usually called cooperativity. The bistability related to hysteresis phenomenon existence confers to this material a great potential for future industrial applications in computing data storage. Moreover, the discover of the possibility to populate a metastable HS state at low temperature with a light irradiation, the so called LIESST effect (Light Induced Excited Spin State Trapping) and reverse effect (reverse LIESST) in the 80’s has generated a growing and strong scientific interest for those bistable solids.
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.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.
In order 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. Neighboring spins are coupled with anharmonic spin dependent potentials, describing the difference in term of volume between the two spin states. 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. We discuss the conditions for the observation of the coexistence of HS and LS Bragg peaks, corresponding to the signature of a structural phase separation usually attributed to strong elastic coupling in the system. In the case of weak cooperativity in the crystal packing, a continuous displacement in the reciprocal space is observed, corresponding to a homogeneous and uniform expansion or contraction of the lattice due to a process mainly dominated by nucleation of the stable or metastable phase.