Observation of an asymmetry in the thermal hysteresis loop at the scale of a single spin-crossover particle

We have studied the spin-crossover phenomenon of single isolated micro-particles using a Differential Interference Microscope (DIC) operating in transmission mode. For that we have selected the [Fe(NH2-trz)3]Br2-2.5H2O coordination polymer complex well-known to exhibit, at room temperature, a spin transition with a large thermal hysteresis loop. The comparison of the thermal hysteresis loop of several isolated micro-particles demonstrates some heterogeneity and also an obvious asymmetry of the cycle. The warming branch clearly appears more abrupt than the cooling branch. We also report some correlations between the threshold temperature and the size of the particles.