Nonlinear magnetic susceptibility and aging phenomena in a reentrant ferromagnet: ${\mathrm{Cu}}_{0.2}{\mathrm{Co}}_{0.8}{\mathrm{Cl}}_2-{\mathrm{FeCl}}_3$ graphite bi-intercalation compound

Linear and nonlinear dynamic properties of a reentrant ferromagnet ${\mathrm{Cu}}_{0.2}{\mathrm{Co}}_{0.8}{\mathrm{Cl}}_2-{\mathrm{FeCl}}_3$ graphite bi-intercalation compound are studied using ac and dc magnetic susceptibility. This compound undergoes successive phase transitions at the transition temperatures $T_h(=16\mathrm{}\mathrm{K}),$ $T_c(=9.7\mathrm{}\mathrm{K}),$ and $T_{\mathrm{RSG}}(=3.5\mathrm{}\mathrm{K}).\mathrm{}$ The static and dynamic behaviors of the reentrant spin-glass phase below $T_{\mathrm{RSG}}$ are characterized by those of normal spin-glass phase with critical exponent $\beta =0.57\mathrm{}\pm 0.10$ and a dynamic critical exponent $x=8.5\mathrm{}\pm \mathrm{1.8.}$ In the ferromagnetic phase ${(T}_{\mathrm{RSG}}<~T<~T_c),$ a prominent nonlinear magnetic susceptibility is observed. The relaxation of the absorption ${\chi }^{″}(\omega ,t)$ with time t exhibits a weak power-law decay: ${\chi }^{″}(\omega ,t)\approx t^{-b^{″}}$ where $b^{″}\approx \mathrm{0.07.}$ These results suggest a chaotic nature of ferromagnetic phase.