Dynamic response of the spin-crossover solidCo(H2(fsa)

Abstract

The spin transition between the paramagnetic high-spin ͑HS͒ and low-spin ͑LS͒ electronic states of Co II ͑H 2 ͑fsa͒ 2 en͒͑py͒ 2 , ͓H 2 ͑fsa͒ 2 enϭN,NЈ-ethylene bis͑3-carboxysalicylaldimine͒, pyϭpyridine͔ under a pulsed and intense magnetic field ͑32 T͒ has been probed by optical reflectivity. In the absence of perturbation, a thermal hysteresis loop (⌬Tϭ19.4 K,T 1/2 ϭ126 K) was detected. Applying a magnetic field of 32 T to the solid, the initial state of which belongs to the ascending branch of the hysteresis loop, leads to an irreversible and quasicomplete Sϭ1/2→Sϭ3/2 transition. The experimental evidence of this crossover triggered by a pulsed magnetic field is reported here in conjunction with an adequate Ising-like model. This two-level model, used as a first approach, allows us to reproduce quantitatively the Ϫ4.9 K shift in transition temperature under the effect of the 32 T static magnetic field. The dynamic aspects of the previous model are also explored, leading to a description of the main features of the phenomenon. Indeed, an irreversible ͑reversible͒ jump of the high-spin fraction is predicted when the pulsed magnetic field is applied in the metastable LS ͑HS͒ phase.

Examples of molecular switching in inorganic solids, due to temperature, light, pressure, and magnetic field

“…Recent experiments [38,39] under pulsed magnetic field up to 35 Tesla, using reflectivity detection, have shown the partial or complete triggering of the SC transition by the magnetic field, see Fig. 7.…”

“…Major conclusions of the experiment are the following: (i) the transition can be (partially) triggered only when the initial state of the system belongs to the metastable ascending branch of the thermal hysteresis loop; this was adequately discussed in ref. [38]; (ii) the out-of-phase response of the sample to the magnetic excitation shows the kinetic character of the process. This kinetic character is thought to be responsible for the incompleteness of the transition.…”

“…This kinetic character is thought to be responsible for the incompleteness of the transition. Further experiment on a Co-based SC solid (faster relaxation) provided a complete triggering of the transition [39]. The simple equation was derived for the isothermal process [38], in the high-temperature approximation (paramagnetic Curie law).…”

“…Therefore, the incompleteness of the triggering was interpreted as due to the kinetic aspect of the pulsed-field experiment. These aspects were recently modeled through dynamic treatments of the Ising-like model [39,40]. …”

Structure of a spin-crossover Fe(II)-1,2,4-triazole polymer complex gel in toluene. Small angle neutron scattering and viscoelastic studies

“…This spin transition is usually accompanied by changes in the optical and other physico-chemical properties [1][2][3][4][5][6][7][8][9]. This behaviour has attracted a great deal of interest due to the potential applications of these systems in the design of multi functional materials with interesting thermal, optical, magnetic and electronic properties [10][11][12][13][14].…”