DOI: 10.1051/jp1:1995165
J. Phys. I France 5 (1995) 747-760

A Simple Ising-Like Model for Spin Conversion Including Molecular Vibrations

A. Bousseksou¹, H. Constant-Machado² and F. Varret<sup>3

1</sup>  Laboratoire de Chimie de Coordination du C.N.R.S., 205 Route de Narbonne, 31077 Toulouse Cedex, France
²  Département de Recherches Physiques (CNRS URA n$^{\circ}\,71$), Université Pierre et Marie Curie, 75252 Paris Cedex 05, France
³  Laboratoire de Magnétisme et d'Optique (CNRS URA n$^{\circ}\,1531$), Université de Versailles Saint-Quentin, 78035 Versailles Cedex, France

(Received 6 June 1994, revised and accepted 9 January 1995)

Abstract
An Ising-type model for spin conversion, explicitly accounting for intramolecular vibrations has been studied. Each two level system is associated with p harmonic oscillators having two possible frequencies . The major advantage of this model is that it provides an excellent agreement with both the conversion curve and calorimetric data, in particular the entropy change upon spin conversion. With the help of Arrhenius plots, the predictions of the model are accurately compared to the literature experimental data on the spin equilibruim curves. A very accurate Mössbauer investigation by Jacobi, Spiering and Gütlich, provides evidence for a small effect typical for vibrations. A novel example is given where the conversion curve is essentially monitored by vibrations; this originates from the extremely small value of the energy gap between HS and LS electrovibrational groundstates. However, in most cases, as a first approach, the low frequency approximation of the model can be used. Then the model reduces to a simple two-level model with additional degeneracies implicitely accounting for intramolecular vibrations.

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