Stretched-exponential relaxation in the three-dimensional short-range Ising spin-glass Cu_0.5Co_0.5Cl₂-FeCl₃ graphite bi-intercalation compound

Cu_0.5Co_0.5Cl₂-FeCl₃ graphite bi-intercalation compound is a three-dimensional short-range spin glass with a spin freezing temperature T_SG (=3.92±0.11 K). The time evolution of the zero-field-cooled magnetization M_ZFC (t) has been measured under various combinations of wait time (t_w), temperature (T), temperature shift (ΔT), and magnetic field (H). The relaxation rate SZFC (t) [= (1/H) dM_ZFC(t)/dlnt] shows a peak at a peak time t _cr. The shape of S_ZFC(t) in the vicinity of t_cr is well described by stretched exponential relaxation (SER). The SER exponent b and the SER relaxation time τ_SER are determined as a function of t_w, T, H, and ΔT. The value of b at T=T_SG is nearly equal to 0.3. There is a correlation between τ_SER and 1/b, irrespective of the values of t_w, T, H, and ΔT. These features can be well explained in terms of a simple relaxation model for glassy dynamics.