Unified theoretical description of the thermodynamical properties of spin crossover with magnetic interactions

After the discovery of the phenomena of light-induced excited spin-state trapping (LIESST), the functional properties of metal complexes have been studied intensively. Among them, cooperative phenomena involving low spin-high spin (spin-crossover) transition and magnetic ordering have attracted interests, and it has become necessary to formulate a unified description of both phenomena. In this work, we propose a model in which they can be treated simultaneously by extending the Wajnflasz-Pick model to include a magnetic interaction. We found that this model is equivalent to Blume-Emery-Griffiths (BEG) Hamiltonian with degenerate levels. It provides a unified description of the thermodynamic properties associated with various types of systems, such as spin-crossover (SC) solids and Prussian blue analog. Here, the high spin fraction and the magnetization are the order parameters describing the cooperative phenomena of the model. We present several typical temperature dependences of the order parameters and determine the phase diagram of the system using the mean-field theory and Monte Carlo simulations. We found that the magnetic interaction drives the SC transition leading to reentrant magnetic and first-order SC transitions.