Two-step and multistep spin transitions are frequently observed in switchable cooperative molecular solids. They present the advantage to open the way for three- or several-bit electronics. Despite extensive experimental studies, their theoretical description was to date only phenomenological, based on Ising models including competing ferro- and antiferro-magnetic interactions, even though it is recognized that the elastic interactions are at the heart of the spin transition phenomenon, due to the volume change between the low- and high-temperature phases. To remedy this shortcoming, we designed the first consistent elastic model, taking into account both volume change upon spin transition and elastic frustration. This ingredient was revealed to be powerful, since it was able to obtain all observed experimental configurations in a consistent way. Thus, according to the strength of the elastic frustration, the system may undergo first-order transition with hysteresis, gradual, hysteretic two-step or multistep transitions, and incomplete transitions. Furthermore, the analysis of the spatial organization of the HS and LS species in the plateau regions revealed the emergence of complex antiferro-elastic patterns going from simple antiferro-magnetic-like order to long-range spatial modulations of the high-spin fraction. These results enabled us to identify the elastic frustration as the fundamental mechanism at the origin of the very recent experimental observations showing the existence of organized spatial modulations of the high-spin fraction inside the plateau of two-step spin transitions.

By using a weak modulated laser intensity we have succeeded in reversibly controlling the dynamics of the spin-crossover (SC) single crystal [{Fe(NCSe)(py)2 }2 (m-bpypz)] inside the thermal hysteresis. The experiment could be repeated several times with a reproducible response of the high-spin low-spin interface and without crystal damage. In-depth investigations as a function of the amplitude and frequency of the excitation brought to light the existence of a cut-off frequency ca. 1.5 Hz. The results not only document the applicability of SC materials as actuators, memory devices, or switches, but also open a new avenue for the reversible photo-control of the spin transition inside the thermal hysteresis.

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There has been in recent years a continuous increase in spatiotemporal investigations of the dynamics of the first-order transitions in spin-crossover (SCO) solids. In single crystals, this phenomenon proceeds via a single domain nucleation and propagation, characterized in some systems with the presence of two equivalent and symmetric interface orientations, between the high-spin (HS) and low-spin (LS) phases, due to the anisotropic structural change of the unit cell at the transition. The present investigations bring an experimental evidence of the reversible driving of the translational and rotational degrees of freedom of the HS-LS interface. In addition to its rectilinear displacement, the interface rotates between two stable angles, 60 and 120. It is demonstrated that while the translation motion is accompanied by a crystal's length change, the interface rotation is controlled by the crystal's bending. These results are well-explained in the frame of an elastic theoretical description in which the effect of the crystal bending, on the stability of the interface's orientation, is simulated by applying a moment of forces on the crystal. It is found that the interface orientation becomes unstable beyond a threshold load value, announcing the emergence of a bistability in SCO solids, taking place at constant HS fraction and volume. This work underlines the sensitive character of the interface orientation to any macroscopic crystal bending, an idea that can be used to develop a new generation of robust stress sensors, working at constant volume, thus avoiding deterioration problems due to crystal fatigue.

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Optical microscopy technique is used to investigate the thermal and the spatio-temporal properties of the spin-crossover single crystal [Fe(2-pytrz) 2 {Pt(CN) 4 }]·3H 2 O, which exhibits a first-order spin transition from a full high-spin (HS) state at high temperature to an intermediate, high-spin low-spin (HS-LS) state, below 153 K, where only one of the two crystallographic Fe(II) centers switches from the HS to HS-LS state. In comparison with crystals undergoing a complete spin transition, the present transformation involves smaller volume changes at the transition, which helps to preserving the crystal’s integrity. By analyzing the spatio-temporal properties of this spin transition, we evidenced a direct correlation between the orientation and shape of HS/HS-LS domain wall with the crystal’s shape. Thanks to the small volume change accompanying this spin transition, the analysis of the experimental data by an anisotropic reaction-diffusion model becomes very relevant and leads to an excellent agreement with the experimental observations.

By using aw eak modulated laser intensity we have succeeded in reversibly controlling the dynamics of the spincrossover (SC) single crystal [{Fe(NCSe)(py) 2 } 2 (m-bpypz)] inside the thermal hysteresis.The experiment could be repeated several times with ar eproducible response of the high-spin low-spin interface and without crystal damage.I n-depth investigations as af unction of the amplitude and frequency of the excitation brought to light the existence of ac ut-off frequency ca. 1.5 Hz. The results not only document the applicability of SC materials as actuators,m emory devices,or switches,but also open anew avenue for the reversible photocontrol of the spin transition inside the thermal hysteresis. Thecontrolofthedynamicsoffirst-orderphasetransitionsisavery general and appealing problem in adiversity of fields.It concerns the irreversible character of the processes involved and makes their control of very high importance from the fundamental point of view and also for technological applications.T hus,s ystems showing propagating-interface phenomena include fluid invasion in porous media, flame fronts, cracks,d omain wall in ferroic materials.A lthough very different, their microscopic descriptions share very similar physics at the macroscopic scale and can be described under au nifying framework involving competitions between elasticity and pinning by disordered medium. In cooperative spincrossover solids, [1][2][3][4] the thermally induced first-order transition involves two spin states,n amely the low-spin (LS, diamagnetic) and the high-spin (HS,p aramagnetic) and is accompanied by as izeable volume change and thermal hysteresis loop.F or comparison, in magnetic systems,t he control of the interface between ferromagnetic and paramagnetic domains is made possible thanks to the magnetic field, which induces the domain-wall motion, [5][6][7] which may be reversible under some conditions owing to the presence of the demagnetizing field created by the dipolar magnetic interactions.Such acontrol by afield parameter is no longer possible in spin-crossover solids,s ince the macroscopic interfaces separating the LS and HS states are elastic in nature.A xial pressure might act as an efficient control parameter;h owever its practical realization faces serious challenges,s uch as the brittle nature of the spin-crossover single crystals. [8][9][10] Among the promising multifunctional materials,m ultiferroic systems are of note,t hese are studied as ar oute to harness magneto-electric coupling and enable ar ange of applications whereby magnetism (resp.f erroelectricity) can be controlled by an electric (resp.m agnetic) field. [11][12][13] Interestingly,m ultiferroic Jahn-Teller switches have been already reported for Prussian Blue analogues [14][15][16] demonstrating the important role of the elastic interactions in the control of the electronic properties of SC solids.B ut elasticity is also at the heart of many other types of phase transitions,s uch as in the Mott metal-insulator transition [17] where it has ...

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