  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 including a magnetic
interaction. We found that this new model is equivalent to
Blume-Emery-Griffiths (BEG) Hamiltonian with degenerate levels. This model
provides a unified description of the thermodynamic properties associated with
various types of systems, such as spin-crossover (SC) solids and Prussian blue
analogues (PBA). 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 we
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 re-entrant magnetic and first-order SC transitions.
