Contrasting effects of field and temperature variations on ageing in spin glasses

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Abstract
We present in this paper some new experimental results on the effect of temperature and field cyclings on the out-of-phase susceptibility of an insulating spin glass. The temperature-cycling experiments are consistent with a previously derived hierarchical scenario in which the probed metastable states continuously split into new substates as the temperature is lowered. The new results, however, suggest that the states of lowest free energy are not the same at different temperatures, supporting the idea of chaotic behaviour. Changing the magnetic field makes the system flow from one hierarchically organized set of states to another set of states with a different magnetization. An analysis of the field effect on the escape times of the states is proposed. It accounts for the observed behaviour of the a.c. susceptibility. It, moreover, predicts a scaling form for the time decay of the thermoremanent magnetization in terms of the probing field, which is nicely satisfied by the available experimental data.