Spatiotemporal Observation and Modeling of Remarkable Temperature Scan Rate Effects on the Thermal Hysteresis in a Spin-Crossover Single Crystal

The direct observation by optical microscopy of the thermally induced spin transition and the subsequent low-spin high-spin interface propagation inside the thermal hysteresis is a fascinating problem. The kinetic aspects of the spin crossover (SCO) transition investigated under various scan rates was mostly studied so far on powder sample which then average the kinetic effects of the thermal hysteresis. This problem is studied here on the robust spin-crossover single crystal [{Fe(NCSe)(py)2}2(m-bpypz)] for which we followed the dynamics of the thermal hysteresis at several controlled temperature scan rates. In addition to the expected shifts of the switching temperatures, sizable changes in the velocity of the high-spin (HS) low-spin (LS) interface dynamics are observed for the first time. Theoretical developments based on a spatiotemporal description of the SCO phenomenon using reaction diffusion equations including the heat balance between the crystal and its immediate environment are provided. We identified the competition between the heat transfer between the crystal and the thermal bath with the temperature scan rate kinetics as the key parameter governing the interface velocity, at the origin of the kinetic features of the hysteresis. The obtained theoretical data are in excellent agreement with experimental results.