Finite size effects in molecular spin crossover materials

In recent years, study of nano-objects with first-order phase transitions has raised new interesting questions. Indeed, the phase stability and the transformation kinetics are highly dependent on the object size. In the case of the spin crossover phenomenon size effects may lead to a loss of the hysteresis (bistability) properties, a shift of the transition temperature or the occurrence of an incomplete transition and also reappearance of the hysteresis upon extreme size reduction (similar to 1-4 nm). Research in this area is growing, but the mechanisms of the spin transition at the nanoscale and in particular the size dependence of the collective behaviour are still not completely understood. The aim of this Focus paper is to point out new issues brought about by the size-reduction effects. We also review size-induced effects for a few selected phase transition phenomena, such as structural or electronic transitions, with the aim of highlighting the possible analogies with the spin crossover phenomenon.