Spatiotemporal Investigations on Light-Driven High-Spin–Low-Spin Interface Dynamics in the Thermal Hysteresis Region of a Spin-Crossover Single Crystal

Abstract

We have investigated by optical microscopy the thermal spin transition of a single crystal of the spin-crossover compound [{Fe­(NCSe)­(py)2}2(m-bpypz)] under various shining intensities, far from the light-induced spin-state trapping region. We found evidence of photoheating on the thermally induced hysteretic response of the crystal, leading to the control of the transition temperature and the hysteresis width as a function of the light intensity. The inspections of the spatiotemporal behaviors of the spin-crossover transition, on heating and cooling, have also evidenced a significant dependence of the propagation speed of the high-spin–low-spin interface on the intensity of light. In particular, for strong shining intensities, a slowing of the interface speed at the transition is obtained, and an unprecedented dynamical two-step-like transition was observed in the thermal hysteresis. These results are analyzed theoretically using a spatiotemporal approach based on reaction–diffusion equations including the spin-state propagation and the heat transfer between the crystal and the thermal bath. The obtained results are in good agreement with experimental observations and lead to identification of the key factors governing the interface velocity and the thermal hysteresis behaviors under the light excitation in spin-crossover materials.

Unraveling the Environment Influence in Bistable Spin-Crossover Particles Using Magnetometric and Calorimetric First-Order Reverse Curves

“…As previously, we represent here the distribution in the two half planes of the Preisach plane, separated by the T R = T line, with the heating distribution below and the cooling distribution above this axis. The long shape of the main peaks is consistent with the progressive widening of the hysteresis loop and could be also connected with possible avalanche-type switchings 18,25. Regarding the different FORC distributions obtained for glycerol when cycling up to 190 and 250 K (Figs.…”

Evidence of Photo-Thermal Effects on the First-Order Thermo-Induced Spin Transition of [{Fe(NCSe)(py)2}2(m-bpypz)] Spin-Crossover Material

“…This behavior is attributed to the photo-heating effects produced by light absorption, which obviously favors the HS state. It is worth noting that this behavior is different from that arising from the temperature scan rate kinetics which affect equally both branches of the thermal hysteresis and keeps the middle of the thermal hysteresis invariant [53,[75][76][77].…”

“…It is important here to consider that the crystal is in contact with a thermal bath whose temperature, T B , is monitored at a temperature sweep rate. The crystal is maintained under light intensity that causes its photo-heating [48,53,75,80], with a constant rate. The photo-heating effect is assumed to be proportional to the light intensity, the optical absorption of the SCO material and must also depend on the crystal heat capacity [35,81].…”

“…In addition, OM investigations showed that although the first-order thermal transitions are sharp on one single crystal, the transition temperatures and so the thermal hysteresis loops are crystal-dependent and significantly depend on the crystal sizes and shapes [50]. Recent OM investigations [53] on the effect of light on the thermally-induced HS to LS transitions far from the Light-Induced Excited Spin-State Trapping (LIESST) effect [14] region, showed that under relatively strong light intensities the measured thermal hysteresis shifts to lower temperatures and the velocities of the HS-LS interfaces are also substantially affected. In the present work, we report on original photo-magnetic experiments conducted on a set of several single crystals of the [{Fe(NCSe)(py) 2 } 2 (m-bpypz)] spin-crossover compound under various shining intensities.…”

Spatio-temporal Investigations of the Incomplete Spin Transition in a Single Crystal of [Fe(2-pytrz)2{Pt(CN)4}]·3H2O: Experiment and Theory

“…It is worth mentioning that this enhanced brittleness of the crystals around the interface region which stores the excess of elastic energy, is quite well explained in the theoretical electro-elastic models [37,[39][40][41][42] describing the elastic properties of the SCO solids. Finally, more recent OM studies, performed on the resilient SCO single crystal [Fe(NCSe)(py) 2 (µ-bpypz)] (py = pyridine and bpypz = 3,5-bis(2-pyridyl)-pyrazolate) [37,[42][43][44], allowed for the first-time to monitor the motion of the HS-LS interface on both cooling and heating processes, whose corresponding velocities were estimated in the range 4-6 µm•s −1 . In the latter case, due to the particularly regular shape of the crystals, made of rectangular platelets along the direction of the interface propagation, the shape of the HS-LS interface remained unchanged during its motion.…”