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Light-induced thermal hysteresis and high-spin low-spin domain formation evidenced by optical microscopy in a spin-crossover single crystal

Abstract : The low-temperature photoinduced effects of the spin-crossover [{Fe(2−pytrz)2[Pd(CN)4]}].3H2O single crystal have been investigated by means of a cryogenic optical microscopy technique down to 10 K from which the imaging and quantitative analysis of the spatiotemporal transformation are derived. The magnetic investigations revealed that this compound exhibits an incomplete spin transition between a full high-spin (HS) state at high temperature and an intermediate HS and low-spin (LS) state, where HS and LS species coexist, as a result of the existence of an elastic frustration at the molecular scale, most likely caused by the rigidity of the interconnected [Pd(CN)4] [Fe(II)/Pd(II)] two-dimensional network. At low temperature (10 K), thanks to reverse light-induced excited spin-state trapping effect, we could switch the system from the intermediate HS-LS state to the fully photoinduced LS state by irradiating the sample in near-infrared region, revealed by photomagnetic and optical microscopy studies. Optical microscopy images showed monotonous and homogeneous transformation of the crystal color along this process, corresponding to a gradual change of the spin state under light. In contrast, the thermal relaxation in the dark of this photoinduced LS-LS state shows a transition to the intermediate HS-LS state at ∼90 K with domain formation, which is characteristic of a first-order transition at equilibrium. Interestingly, the same behavior is also obtained during the heating process of the reverse light-induced thermal hysteresis cycle with a heating branch almost unchanged, confirming that light does not act on this transition. It is then concluded that the transition from the full LS to the intermediate HS-LS transition is of first order and therefore the LS state reached by light is a hidden stable state of the system until ∼90 K. These experimental results are modeled using an adapted version of the electroelastic model including photoexcitation effects and solved by Monte Carlo method. Both thermal equilibrium and light-induced thermal hysteresis are reproduced, in fair qualitative agreement with the experimental data of photomagnetism and optical microscopy.
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Submitted on : Tuesday, June 7, 2022 - 10:30:29 AM
Last modification on : Tuesday, September 27, 2022 - 4:28:29 AM
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Houcem Fourati, Mamadou Ndiaye, Mouhamadou Sy, Smail Triki, Guillaume Chastanet, et al.. Light-induced thermal hysteresis and high-spin low-spin domain formation evidenced by optical microscopy in a spin-crossover single crystal. Physical Review B, American Physical Society, 2022, 105, 174436 (16 p.). ⟨10.1103/physrevb.105.174436⟩. ⟨hal-03689350⟩

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