Experimental Evidence for Violation of the Fluctuation-Dissipation Theorem in a Superspin Glass

We present the experimental observation of the fluctuation-dissipation theorem violation in an assembly of interacting magnetic nanoparticles in the low temperature superspin-glass phase. The magnetic noise is measured with a two-dimension electron gas Hall probe and compared to the out of phase ac susceptibility of the same ferrofluid. For "intermediate'' aging times of the order of 1 h, the ratio of the effective temperature T-eff to the bath temperature T grows from 1 to 6.5 when T is lowered from T-g to 0.3 T-g, regardless of the noise frequency. These values are comparable to those measured in an atomic spin glass as well as those calculated for a Heisenberg spin glass.