Memory and aging effect in hierarchical spin orderings of the stage-2 Co Cl2 graphite intercalation compound

Stage-2 Co Cl2 graphite intercalation compound undergoes two magnetic phase transitions at Tcl (=7.0 K) and Tcu (=8.9 K). The aging dynamics of this compound is studied near Tcl and Tcu. The intermediate state between Tcl and Tcu is characterized by a spin-glass phase extending over ferromagnetic islands. A genuine thermoremnant magnetization (TRM) measurement indicates that the memory of the specific spin configurations imprinted at temperatures between Tcl and Tcu during the field-cooled (FC) aging protocol can be recalled when the system is reheated at a constant heating rate. The zero-field cooled (ZFC) and TRM magnetization is examined in a series of heating and reheating processes. The magnetization shows both characteristic memory and rejuvenation effects. The time (t) dependence of the relaxation rate SZFC (t) = (1H) d MZFC (t) d ln t after the ZFC aging protocol with a wait time tw exhibits peaks at two different characteristic times in the intermediate state between Tcl and Tcu. This result indicates the coexistence of two types of ordered domains. The observed aging and memory effects are discussed in terms of the droplet model.