Aging dynamics in a reentrant ferromagnet:Cu0.2Co0.8Cl2−FeCl3graphite bi-intercalation compound

Abstract

The aging dynamics of a reentrant ferromagnet ${\mathrm{Cu}}_{0.2}{\mathrm{Co}}_{0.8}{\mathrm{Cl}}_{2}\ensuremath{-}\mathrm{Fe}{\mathrm{Cl}}_{3}$ graphite bi-intercalation compound has been studied using ac and dc magnetic susceptibility. This compound undergoes successive transitions at the transition temperatures ${T}_{c}$ $(=9.7\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and ${T}_{\mathit{RSG}}$ $(=3.5\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. The relaxation rate $S(t)$ $[=\mathrm{d}{\ensuremath{\chi}}_{\mathit{ZFC}}(t)∕\mathrm{d}\phantom{\rule{0.2em}{0ex}}\mathrm{ln}\phantom{\rule{0.2em}{0ex}}t]$ exhibits a characteristic peak at ${t}_{\mathit{cr}}$ close to a wait time ${t}_{W}$ below ${T}_{c}$, indicating that the aging phenomena occur in both the reentrant spin glass phase below ${T}_{\mathit{RSG}}$ and the ferromagnetic (FM) phase between ${T}_{\mathit{RSG}}$ and ${T}_{c}$. The aging state in the FM phase is fragile against a weak magnetic-field perturbation. The time $(t)$ dependence of ${\ensuremath{\chi}}_{\mathit{ZFC}}(t)$ around $t\ensuremath{\approx}{t}_{\mathit{cr}}$ is well approximated by a stretched exponential relaxation, ${\ensuremath{\chi}}_{\mathit{ZFC}}(t)\ensuremath{\approx}\mathrm{exp}[\ensuremath{-}{(t∕\ensuremath{\tau})}^{1\ensuremath{-}n}]$, around $t={t}_{W}$. The exponent $n$ depends on ${t}_{W}$, $T$, and $H$. The relaxation time $\ensuremath{\tau}$ $(\ensuremath{\approx}{t}_{\mathit{cr}})$ exhibits a local maximum around $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, reflecting a frustrated nature of the FM phase. It drastically increases with decreasing temperature below ${T}_{\mathit{RSG}}$.