Fluctuation-Dissipation Ratio of a Spin Glass in the Aging Regime

We present the first experimental determination of the time autocorrelation $C(t^{\prime },t)$ of magnetization in the nonstationary regime of a spin glass. Quantitative comparison with the response, the magnetic susceptibility $\chi (t^{\prime },t)$, 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 $t^{\prime }$. Following theoretical developments on mean-field models, and lately on short range ones, it is predicted that in the limit of long times the $\chi \left(C\right)$ relationship should become independent of $t^{\prime }$. A scaling procedure allows us to extrapolate to the limit of long waiting times.