Memory and aging effect in hierarchical spin orderings of the stage-2 CoCl_2 graphite intercalation compound

摘要

Stage-2 CoCl_2 graphite intercalation compound undergoes two magnetic phase transitions at T_(cl) (=7.0 K) and T_(cu) (=8.9 K). The aging dynamics of this compound is studied near T_(cl) and T_(cu). The intermediate state between T_(cl) and T_(cu) 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 T_(cl) and T_(cu) 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 S_(ZFC)(t) = (1/H)dM_(ZFC)(t)/d ln t after the ZFC aging protocol with a wait time t_w exhibits peaks at two different characteristic times in the intermediate state between T_(cl) and T_(cu). 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.