AGING PHENOMENA IN A SPIN-GLASS - EFFECT OF TEMPERATURE-CHANGES BELOW TG

The magnetic properties of a spin-glass depend on the time spent in the low-temperature phase: this is the so-called ageing phenomenon. We present measurements of the low-frequency ac susceptibility (0. 01 to 0. 1 Hz) and of the relaxation of the thermo-remanent magnetization in the CdCr//1//. //7In//0//. //3S//4 insulating spin-glass; we investigate the influence of temperature variations on the ageing processes in the spin-glass phase, in order to get a better insight into their mechanism. The wide spectrum of relaxation times observed in spin-glass dynamics reflects the existence of many energy valleys in the phase space; a hierarchical organization of these valleys, whose bifurcations appear as the temperature is decreased, is the simplest picture which accounts for our results.