A SIMPLE ISING-LIKE MODEL FOR SPIN CONVERSION INCLUDING MOLECULAR VIBRATIONS

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 omega(LS)((i)), omega(HS)((i)). 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 equilibrium curves. A very accurate Mossbauer investigation by Jacobi, Spiering and Gutlich, 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.