title: 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.
authors.affiliation: Groupe d'Etude de la Matière Condensée, Université de Versailles, CNRS UMR 8635, 45 Avenue des Etats-Unis, 78035 Versailles cedex, France.
authors.forename: Ahmed
authors.initials: A
authors.lastname: Slimani
authors.forename: François
authors.initials: F
authors.lastname: Varret
authors.forename: Kamel
authors.initials: K
authors.lastname: Boukheddaden
authors.forename: Damien
authors.initials: D
authors.lastname: Garrot
authors.forename: Hassane
authors.initials: H
authors.lastname: Oubouchou
authors.forename: Sumio
authors.initials: S
authors.lastname: Kaizaki
mesh_terms: D056831:Coordination Complexes / Q000737:chemistry*; D005296:Ferrous Compounds / Q000737:chemistry*; D006358:Hot Temperature; D008956:Models, Chemical; D055668:Photochemical Processes; D011720:Pyrazoles / Q000737:chemistry; D011725:Pyridines / Q000737:chemistry; D013499:Surface Properties
chemical_list: D056831:Coordination Complexes; D005296:Ferrous Compounds; D011720:Pyrazoles; D011725:Pyridines