Dance of Interference Patterns during the Spin Transition of a Single Crystal

Abstract

Optical microscopy investigations have allowed the study of the spatiotemporal dynamics of the nucleation, growth, and propagation along with first-order phase transitions in cooperative spin-crossover (SCO) single crystals. They made possible the monitoring of the high-spin (HS)/low-spin (LS) interface and the control of its orientation and its velocity along the transformation process. Up to now, and especially for spin transition materials, all information obtained from this technique was restricted to the 2D area of the crystal's surface, imaged by a camera. In this paper, we present a breakthrough method which allows us to follow the dynamics of the 3D single-crystal deformation during the spin transition. To pursue this objective, we adapted an optical interferometry technique under a microscope, to capture in real-time the images for first-order thermally induced SCO transition on a unique single crystal. The optical interferometry data showed that along the cooling regime, the spin transition proceeds through a nucleation and propagation of a single domain process with an average interface velocity of 0.8 μm/s. In contrast, on the heating regime, the transformation started from the two opposite sides of the crystal and the analysis of their mutual dynamics indicated a clear interplay between them, thus confirming the long-range elastic nature of the spin transition phenomenon. Taking advantage of the high sensitivity of the interferometry technique to the crystal's deformation, we could observe the deformation field of the SCO crystal by monitoring the interface propagation which resulted in an impressive dance of the interference patterns, through their global shift and their distortion. Moreover, optical interference technique was able to detect precursor phenomena of stress-driven long-range effects in the HS and LS phases immediately before the transition, even though that optical microscopy technique was silent in these regions. To understand these observations, a spatiotemporal model has been developed in which we have dissected the complex behavior of the interference fringes, resulting from the volume change (expansion and/or compression) of the crystal in the course of the spin transition.