Aging dynamics across a dynamic crossover line in the three-dimensional short-range Ising spin-glassCu0.5Co0.5Cl2−FeCl3graphite bi-intercalation compound

Abstract

Cu$_{0.5}$Co$_{0.5}$Cl$_{2}$-FeCl$_{3}$ graphite intercalation compound is a three-dimensional short-range Ising spin glass with a spin freezing temperature $T_{c}$ ($= 3.92 \pm 0.11$ K). The stability of the spin glass phase in the presence of a magnetic field $H$ is examined from (i) the spin freezing temperature $T_{f}(\omega,H)$ at which the differential field-cooled (FC) susceptibility $\partial M_{FC}(T,H)/\partial H$ coincides with the dispersion $\chi^{\prime}(\omega,T,H=0)$ with the angular frequency $\omega$, and (ii) the time dependence of zero-field cooled (ZFC) susceptibility $\chi_{ZFC}$ after a ZFC aging protocol with a wait time $t_{W}$ ($= 1.0 \times 10^{4}$). The relaxation rate $S(t)$ (= d$\chi_{ZFC}$/d$\ln t$) exhibits a local maximum at a characteristic time $t_{cr}$, reflecting non-equilibrium aging dynamics. The peak time $t_{cr}(T,H)$ decreases with increasing $H$ at the fixed temperature $T$ (2.9 K $\leq T<T_{c}$). The spin freezing temperature $T_{f}(\omega,H)$ provides evidence for the instability of the spin glass phase in thermal equilibrium in a finite magnetic field. Both $t_{cr}(T,H)$ and $T_{f}(\omega,H)$ exhibit certain scaling behavior predicted from the droplet picture, suggesting a dynamic crossover from SG dynamics to paramagnetic behavior in the presence of $H$.