  Linear and nonlinear dynamic properties of a reentrant ferromagnet
Cu$_{0.2}$Co$_{0.8}$Cl$_{2}$-FeCl$_{3}$ graphite bi-intercalation compound are
studied using AC and DC magnetic susceptibility. This compound undergoes
successive phase transitions at the transition temperatures $T_{h}$ (= 16 K),
$T_{c}$ (= 9.7 K), and $T_{RSG}$ (= 3.5 K). The static and dynamic behaviors of
the reentrant spin glass phase below $T_{RSG}$ are characterized by those of
normal spin glass phase with critical exponent $\beta$ = 0.57 $\pm$ 0.10, a
dynamic critical exponent $x$ = 8.5 $\pm$ 1.8, and an exponent $p$ (= 1.55
$\pm$ 0.13) for the de Almeida -Thouless line. A prominent nonlinear
susceptibility is observed between $T_{RSG}$ and $T_{c}$ and around $T_{h}$,
suggesting a chaotic nature of the ferromagnetic phase ($T_{RSG} \leq T \leq
T_{c}$) and the helical spin ordered phase ($T_{c} \leq T \leq T_{h}$). The
aging phenomena are observed both in the RSG and FM phases, with the same
qualitative features as in normal spin glasses. The aging of zero-field cooled
magnetization indicates a drastic change of relaxation mechanism below and
above $T_{RSG}$. The time dependence of the absorption $\chi^{\prime \prime}$
is described by a power law form ($\approx t^{-b^{\prime \prime}}$) in the
ferromagnetic phase, where $b^{\prime \prime} \approx 0.074 \pm 0.016$ at $f$ =
0.05 Hz and $T$ = 7 K. No $\omega t$-scaling law for $\chi^{\prime \prime}$
[$\approx (\omega t)^{-b^{\prime \prime}}$] is observed.
