Experimental investigation of the fluctuation dissipation relation in a spin glass

Abstract

We present the first experimental determination of the time auto-correlation of magnetization in the non-stationary regime of a spin glass, and its quantitative comparison with the corresponding response, the magnetic relaxation function. These measurements were performed in a new experimental setup working as an absolute thermometer. Clearly, we observe a non-linear fluctuation-dissipation relation between correlation and relaxation: at large correlation (short observation times) fluctuation dissipation theorem is obeyed, while the fluctuation dissipation relation is driven by an effective temperature higher than the bath temperature in the aging regime (small correlation and large observation times). In the aging regime, the relaxation vs. correlation curves depend weakly on the waiting time. According to theoretical developments on mean field models, and lately on short range ones, in the limit of very large waiting times, the relation between relaxation and correlation in the aging regime becomes temperature independent for a given system. A scaling procedure allows us to extrapolate to the limit of long waiting times by separating stationary and non-stationary regimes and to check the validity of the temperature independence of the fluctuation dissipation relation in the non-stationary regime.