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

An Ising-type model for spin conversion, explicitly accounting for int ramolecular vibrations has been studied. Each two level system is asso ciated with p harmonic oscillators having two possible frequencies ome ga(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 convers ion. With the help of Arrhenius plots, the predictions of the model ar e 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 fo r vibrations. A novel example is given where the conversion curve is e ssentially monitored by vibrations; this originates from the extremely small value of the energy gap between HS and LS electrovibrational gr oundstates. However, in most cases, as a first approach, the low frequ ency approximation of the model can be used. Then the model reduces to a simple two-level model with additional degeneracies implicitely acc ounting for intramolecular vibrations.