Ageing, Rejuvenation and Memory: The Example of Spin-Glasses

Abstract

In this introductory talk, we shall present spin glasses and review the general experimental features of their glassy dynamics [1]. Spin glasses are model magnetic systems in which the interactions are disordered and frustrated, due to a random dilution of magnetic ions. Below their glass temperature Tg , their response to a magnetic excitation is a slow relaxation of the magnetization, which strongly depends on the time spent below Tg (« aging » phenomenon). In contrast with structural and polymer glasses, aging is here hardly influenced by the cooling rate. During cooling, each further step down tends to restart some aging processes (« rejuvenation »), while the memory of previous aging periods, achieved at different temperatures during cooling, can be retrieved when re-heating [1,2]. It is possible to store independently several memories, which corresponds to imprinting the trace of various spin arrangements at well separated length scales that are selected by temperature [3,4]. Similar aging, rejuvenation and memory phenomena can be observed in various other glassy systems [1], making the spin glasses conceptually simple examples for the study of out-of-equilibrium dynamics in disordered systems.