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

The low-temperature photoinduced effects of the spin-crossover $[{\mathrm{Fe}{(2\text{\ensuremath{-}}\mathrm{pytrz})}_{2}[\mathrm{Pd}{(\mathrm{CN})}_{4}]}].3{\mathrm{H}}_{2}\mathrm{O}$ single crystal have been investigated by means a cryogenic optical microscopy technique down to 10 K from which imaging and quantitative analysis spatiotemporal transformation are derived. magnetic investigations revealed that this compound exhibits an incomplete spin transition between full high-spin (HS) state at high temperature intermediate HS low-spin (LS) state, where LS species coexist, as result existence elastic frustration molecular scale, most likely caused rigidity interconnected $[\mathrm{Pd}{(\mathrm{CN})}_{4}$] [Fe(II)/Pd(II)] two-dimensional network. At low (10 K), thanks reverse light-induced excited spin-state trapping effect, we could switch system HS-LS fully irradiating sample in near-infrared region, photomagnetic studies. Optical images showed monotonous homogeneous color along process, corresponding gradual change under light. In contrast, thermal relaxation dark LS-LS shows $\ensuremath{\sim}90$ with domain formation, is characteristic first-order equilibrium. Interestingly, same behavior also obtained during heating process hysteresis cycle branch almost unchanged, confirming light does not act on transition. It then concluded first order therefore reached hidden stable until K. These experimental results modeled using adapted version electroelastic model including photoexcitation solved Monte Carlo method. Both equilibrium reproduced, fair qualitative agreement data photomagnetism microscopy.