Visualization and quantitative analysis of spatiotemporal behavior in a first-order thermal spin transition: A stress-driven multiscale process

We investigated by optical microscopy the propagation of the thermal spin transition in [Fe(btr)₂(NCS)₂]·{}H₂O·{}(btr = bis-triazole) single crystals. Using high-quality fresh crystals embedded in oil we could follow the transformation front of the on-cooling transition. We observed in detail a linear front propagating through the entire crystal at extremely slow velocity, {~}2.5 {μ}m/s on average. The analysis of the temporal dependences of the optical densities (OD) at given positions within a Kolmogorov-Johnson-Mehl-Avrami (KJMA) law led to an exponent {~}0.6, typical for a process governed by planar diffusion. It is inferred that the spin transition is a multiscale process. The agreement with a previous structural investigation, analyzed in terms of an average KJMA law, is shown. We conclude that the stresses generated by the volume change associated with the spin transformation are the driving force for the propagation of the nucleation and growth mechanism. The role of defects is discussed and optical effects specific of the experiment are evidenced.