  Aging dynamics of a reentrant ferromagnet
Cu$_{0.2}$Co$_{0.8}$Cl$_{2}$-FeCl$_{3}$ graphite bi-intercalation compound has
been studied using AC and DC magnetic susceptibility. This compound undergoes
successive transitions at the transition temperatures $T_{c}$ ($= 9.7$ K) and
$T_{RSG}$ ($= 3.5$ K). The relaxation rate $S(t)$ exhibits a characteristic
peak at $t_{cr}$ close to a wait time $t_{w}$ below $T_{c}$, indicating that
the aging phenomena occur in both the reentrant spin glass (RSG) phase below
$T_{RSG}$ and the ferromagnetic (FM) phase between $T_{RSG}$ and $T_{c}$. The
relaxation rate $S(t)$ ($=\text{d}\chi_{ZFC}(t)/\text{d}\ln t$) in the FM phase
exhibits two peaks around $t_{w}$ and a time much shorter than $t_{w}$ under
the positive $T$-shift aging, indicating a partial rejuvenation of domains. The
aging state in the FM phase is fragile against a weak magnetic-field
perturbation. The time ($t$) dependence of $\chi_{ZFC}(t)$ around $t \approx
t_{cr}$ is well approximated by a stretched exponential relaxation:
$\chi_{ZFC}(t) \approx \exp[-(t/\tau)^{1-n}]$. The exponent $n$ depends on
$t_{w}$, $T$, and $H$. The relaxation time $\tau$ ($\approx t_{cr}$) exhibits a
local maximum around 5 K, reflecting a chaotic nature of the FM phase. It
drastically increases with decreasing temperature below $T_{RSG}$.
