Velocity of the High-Spin Low-Spin Interface Inside the Thermal Hysteresis Loop of a Spin-Crossover Crystal, via Photothermal Control of the Interface Motion

Abstract

We investigated by optical microscopy the thermal transition of the spin-crossover dinuclear iron(II) compound [(Fe(NCSe)(py)(2))(2)(m-bpypz)]. In a high-quality crystal the high-spin (HS) low-spin (LS) thermal transition took place with a sizable hysteresis, at ~108 K and ~116 K on cooling and heating, respectively, through the growth of a single macroscopic domain with a straight LS and HS interface. The interface orientation was almost constant and its propagation velocity was close to ~6 and 26 μ m s(-1) for the on-cooling and on-heating processes, respectively. We found that the motion of the interface was sensitive to the intensity of the irradiation beam of the microscope, through a photothermal effect. By fine-tuning the intensity we could stop and even reverse the interface motion. This way we stabilized a biphasic state of the crystal, and we followed the spontaneous motion of the interface at different temperatures inside the thermal hysteresis loop. This experiment gives access for the first time to an accurate determination of the equilibrium temperature in the case of thermal hysteresis--which was not accessible by the usual quasistatic investigations. The temperature dependence of the propagation velocity inside the hysteretic interval was revealed to be highly nonlinear, and it was quantitatively reproduced by a dynamical mean-field theory, which made possible an estimate of the macroscopic energy barrier.

Shape effects on the cluster spreading process of spin-crossover compounds analyzed within an elastic model with Eden and Kawasaki dynamics

“…These results were obtained, either in the framework of Molecular Dynamics approaches 13 or using Monte Carlo Arrhenius, 25 Metropolis dynamics, 26 as well as by combining Molecular Dynamics and Monte Carlo method. 27 This general behavior was also supported by simple energetic considerations. 27,28 It has also been shown that in the framework of elastic models with Molecular Dynamics, 13 the nucleation is a macroscopic process, with the size of the critical nucleus proportional to the total system size 29 and the importance of the shape effects has been underlined.…”

“…27 This general behavior was also supported by simple energetic considerations. 27,28 It has also been shown that in the framework of elastic models with Molecular Dynamics, 13 the nucleation is a macroscopic process, with the size of the critical nucleus proportional to the total system size 29 and the importance of the shape effects has been underlined.…”

“…Indeed, experimental studies of the shape and the orientation of the interface between the high-spin and low spin phases have recently been performed 27,28 , with the aim of controlling the interface movement by fine-tuning of the relevant factors influencing the stability of the competing elastic domains, such as temperature. The experimental investigations are currently developping towards the control of local conditions, such as crystal shape or the curvature of the surface.…”

“…3 left), with a similar kinetic and quasi-constant velocity as described in a previous paper and observed in experiments. 27,28 For smaller values of the major axis, the evolution of a cluster will in most cases prevent the spreading of another cluster from the opposite vertex (Fig. 3 right).…”

“…35 Elongated crystals, also, provided text-book example for the propagation of HS/LS interfaces, with quasi-1D behavior. 27,36 Therefore, it is important to study clustering in highly anisotropic shapes, such as ellipses, used also as sample shapes in other magnetic materials, such as nanopillars. 37 Moreover, the study of ellipses is useful as different points on their surface are characterized by different radii of curvature, with the smallest ones at vertices corresponding to the corners of hexagonal/ rectangular systems and the largest ones at co-vertices, approaching infinity for elongated ellipses and then corresponding to linear edges.…”