We present the first experimental determination of the time autocorrelation
C(t′,t) of magnetization in the non-stationary regime of a spin glass.
Quantitative comparison with the response, the magnetic susceptibility
χ(t′,t), is made using a new experimental setup allowing both measurements
in the same conditions. Clearly, we observe a non-linear
fluctuation-dissipation relation between C and χ, depending weakly on
the waiting time t′. Following theoretical developments on mean-field models,
and lately on short range models, it is predicted that in the limit of long
times, the χ(C) relationship should become independent on t′. A scaling
procedure allows us to extrapolate to the limit of long waiting times.Comment: 4 pages, 3 figure