Fluctuation-dissipation ratio of a spin glass in the aging regime.

We present the first experimental determination of the time autocorrelation C(t',t) of magnetization in the nonstationary regime of a spin glass. Quantitative comparison with the response, the magnetic susceptibility chi(t',t), is made by using a new experimental setup allowing both measurements in the same conditions. Clearly, we observe a nonlinear fluctuation-dissipation relation between C and chi, depending weakly on the waiting time t'. Following theoretical developments on mean-field models, and lately on short range ones, it is predicted that in the limit of long times the chi(C) relationship should become independent of t'. A scaling procedure allows us to extrapolate to the limit of long waiting times.