Origin of Memory in Spin Glass Dynamics*

The memory effect is one of the most mysterious dynamical processes in spin glasses. It evolves from waiting at a temperature T₁ < T_g for a time t_w, where T_g is the spin glass transition temperature, then reducing the temperature further to T₂ . The spin glass magnetization behaves as though there has been no waiting at T₁ during the temperature drop, and is referred to as "rejuvenation" or "temperature chaos." Yet, when the spin glass is warmed back to T₁ the previous waiting time effect is recovered. This is termed "memory." Equilibrium simulations by Fernandez et al. [Europhys. Lett. 103, 67003 (2013)] have shown that chaos is driven by rare events. We argue that the length scale for chaos takes place within the spin glass correlation length ξ(t_w ,T). This would be true both at equilibirium and in the non-equilibrium conditions of experiment. But if temperature chao is rare, some correlated regions of the spin glass will not experience chaos. Hence, their properties will be reversible, and are responsible for memory. Experimental evidence for this interpretation will be given.