An optical microscope study of photo-switching and relaxation in single crystals of the spin transition solid [Fe(ptz)6](BF4)2, with image processing

We report on the use of optical microscopy for investigating quantitatively the thermal and optical switching of spin states. The sample was [Fe(ptz)6](BF4)2 single crystal in the thermally quenched structure (rhombohedral). We followed the photo-excitation from the stable LS state to the metastable HS state, the subsequent thermally activated relaxation and the light-induced thermal hysteresis. We evidenced the presence of temperature gradients, with larger differences between the crystal support and the cold finger of the cryostat, and observed the development of cracks in the crystal at crucial stages of the spin state transformations, associated with an irreversible increase in diffusion of light. We used the green and red optical densities for monitoring, respectively, the variations of the spin fractions and of diffusion of light. We processed the images so as to blur out the digital character of the data and make possible establishing a temperature map of the crystal in the photo-stationary state. We also discussed the conditions required for visualising the recently observed light-induced phase separation into like-spin domains, which are far form reached at the present state of art.