Optical hysteresis in a spin-crossover complex

Photoexcitation effects on the spin states were investigated for a prototypical spin-crossover complex [Fe(ptz)(6)](BF4)(2) (ptz=1-propyltetrazole) by the pump and probe technique at 77 K. We have found (i) superlinear increase of density n(HS) of the photoexcited high-spin molecules when the excitation power density P exceeded a critical value (=120 mW/cm(2)) and (ii) optical hysteresis between the P-increasing and the P-decreasing runs. These characteristic features are well reproduced by the Monte Carlo simulation based on the Ising model, in which the spin state of the respective Fe2+ ion is represented by the pseudospins.