Abstract
We have studied the relaxation dynamics from the photo-induced state in the spin crossover complex [FeH2L2-Me](ClO4)2 by magnetic susceptibility and X-ray diffraction measurements. Just after turning off the laser irradiation, the photo-induced high-spin state immediately starts relaxing back to the low-spin state, while the crystal lattice shows a nucleation process. This result shows that the spin state and lattice deformation should be treated as individual parameters in the non-equilibrium phase transition from the photo-induced state. Experimental results are qualitatively explained by the two-sublattice Ising-like model.