Abstract
The considerable increase of magnetic moment M with increasing temperature T [the spin-crossover (SC) effect] in some transition metal complexes is caused by a predominant occupation of the excited high-spin (HS) state which has a larger statistical weight than the low-spin (LS) ground one. Rather unusual experimentally observed M(T) behavior that provides for molecular electronics applications with such compounds, gives evidence of the importance of cooperative intermolecular interactions. The microscopic theory of cooperative SC transitions is developed. This is based on the phonon-mediated ion-ion interaction and accounts for the possibility of a phase with antiferro-ordered local distortions around the magnetic ions, in addition to the homogeneous ferro-phase that was the only one considered previously. If the parameter ‖J‖ of the antiferro-coupling (J<0) is greater than the energy gap Δ between the LS and HS states, then at T=0 one-half of the ions are in the ‘‘excited’’ HS state while the other half reside in the LS state. A temperature increase causes a M(HS)/2→M(HS) transition to the phase where all the ions are in the HS states. The larger is the value of ‖J‖, the higher is the transition temperature. If ‖J‖<Δ, then at T=0 all the ions are in the LS state because the intermolecular correlations are too small to lead to antiferro-ordering. However, if ‖J‖ is close enough to Δ, then the intermediate ferri-ordered phase appears, resulting in two [M(LS)→M(HS)/2→M(HS)] consecutive transitions (two-step SC change). The closer ‖J‖ is to Δ, the lower the temperature of the first transition is and the wider the temperature interval of the ferri-phase existence is. Hysteresis phenomena and effects of magnetic dilution and external pressure, examined within the framework of the developed microscopic approach, are in a good agreement with experiment.
Published Version
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