Off-equilibrium fluctuation-dissipation relation in a spin glass - An experimental test for mean-field predictions

We report new experimental results obtained on the insulating spin glass CdCr 1.7 In 0.3 S 4 . Our experimental setup allows a quantitative comparison between the thermo-remanent magnetisation and the autocorrelation of spontaneous fluctuations of magnetisation, yielding a complete determination of the fluctuation-dissipation relation. The dynamics can be studied both in the quasi-equilibrium regime, where the fluctuation-dissipation theorem holds, and in the deeply ageing regime. The limit of separation of time-scales, as used in analytical calculations, can be approached by use of a scaling procedure.