Ageing and memory in the relaxation dynamics of collections of two-level subsystems

The relaxation dynamics of collections of interacting, thermally activated, two-level subsystems are shown to be characterized by nonequilibrium ageing and memory effects which are analogous to those observed in magnetic systems where randomness and frustration yield a collectively frozen magnetic state with spin-glass-like correlations. Numerical simulations of these nonequilibrium relaxation effects are presented and compared with measurements performed on assemblies of nanodimensional magnetic particles, and their physical origins, as well as their relationship to genuinely collective nonequilibrium relaxation phenomena, are discussed within a theoretical framework based on the Preisach model of hysteresis.