Fluctuation-dissipation relation in a spin glass in the non-stationary regime

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 corresponding response, the magnetic susceptibility chi(, t), is made possible by the use of a new experimental setup allowing both measurements in the same conditions. Clearly, we observe a non-linear 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 on. A scaling procedure allows us to extrapolate to the limit of long waiting times. (C) 2003 Elsevier Science B.V. All rights reserved.