Nature and mechanism of the photoinduced spin transition in [Fe(PM-BiA)2(NCS)2]

We report the investigation of a prototype cooperative photoinduced spin transition in [Fe(PM-BiA)(2)(NCS)(2)] by x-ray diffraction. The structural rearrangement accompanying the change of an electronic state between the low spin and the photoinduced out-of-equilibrium high spin phases is similar to the one occurring around the phase transition at thermal equilibrium. The cooperative behavior of the light-induced optical hysteresis and light-induced thermal hysteresis phenomena investigated here show a strong similarity with thermal hysteresis around first-order phase transitions, since they are driven by a domain nucleation process.