Ageing dynamics of ferromagnetic and re-entrant spin glass phases in a stage-2 Cu_0.80Co_0.20Cl₂ graphite intercalation compound

Ageing dynamics of a re-entrant ferromagnet stage-2 Cu_0.8Co _0.2Cl₂ graphite intercalation compound has been studied using DC magnetization measurements. This compound undergoes successive transitions at the transition temperatures T_c (≈8.7K) and T _RSG (≈3.3K). The relaxation rate S_ZFC(t) exhibits a characteristic peak at t_cr below T_c. The peak time t _cr at constant t_w shows a local maximum around 5.5K, indicating a slow dynamics arising from a frustrated nature of the ferromagnetic phase. It drastically increases with decreasing temperature below T _RSG. The spin configuration imprinted at the stop and wait process at a stop temperature T_s (<T_c) during the field-cooled ageing protocol becomes frozen on further cooling. On reheating, the memory of the ageing at T_s is retrieved as an anomaly of the thermoremnant magnetization at T_s. These results indicate the occurrence of the ageing phenomena in the ferromagnetic phase (T_RSG<T<T _c) as well as in the re-entrant spin glass phase (T<T _RSG).