Temporal decoupling of spin and crystallographic phase transitions in $\text{Fe}{\left(\text{ptz}\right)}_6{\left({\text{BF}}_4\right)}_2$

We have studied relaxation dynamics of the thermally and photoinduced metastable states in $\left[\text{Fe}{\left(\text{ptz}\right)}_6\right]{\left({\text{BF}}_4\right)}_2$ near the spin transition temperature using magnetic-susceptibility measurements and optical-absorption spectroscopy as function of the time. We observed two-step relaxations from the photoinduced high-spin to the ground low-spin state. The time-resolved measurements of the spin-relaxation processes allow us to decouple the spin transition and the crystallographic phase transition and evaluate their intrinsic transition temperatures as 122 and 132 K, respectively.

Figure 1

(Color online) (a) Temperature dependence of the HS fraction obtained upon slow temperature change (1 K/min) (open circle) and the HS fraction in the PIHS state (open square) in $\left[\text{Fe}{\left(\text{ptz}\right)}_6\right]{\left({\text{BF}}_4\right)}_2$ compound. (b) The relaxation curves from the HS state at the temperatures indicated by the lower arrows in Fig. 1a: experimental results (open circle) and best-fit curves from Eq. (3) (solid line). (c) The relaxation curves from the PIHS state via the intermediate state in the dark at the temperatures indicated by the upper arrows in Fig. 1a: experimental results (open circle) and best-fit curves from Eq. (3) (solid line).

Figure 2

(Color online) Temperature dependence of the relaxation time in the quenched state (closed square) and the intermediate state (open circle) on a logarithmic scale. The solid line is the best-fit curve according to Eq. (4) with $T_c=132\pm 0.2\text{K}$ and $\Delta =2.0\pm 0.5$.

Figure 3

(Color online) Infrared-absorption spectra in the PIHS state under the laser irradiation (116 K), in the intermediate state (116 and 125 K), in the thermal HS state (140 K), in the slow cooled LS state (116 K), and in the quenched state (116 K).

Figure 4

(Color online) (a) Temperature dependence of the HS fraction in the intermediate state (closed square), in the quenched state (10 K/min) (closed circle), and upon slow temperature change in the dark (1 K/min) (open circle). [(b)–(d)] Proposed contour plot of the potential energy of structure and spin parameters (b) below 122 K, (c) between 122 and 132 K, and (d) above 132 K.