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

TL;DR

This study provides experimental evidence that the fluctuation-dissipation theorem (FDT) is violated in a superspin glass phase of magnetic nanoparticles, showing an increase in effective temperature relative to the bath temperature at low temperatures.

Contribution

The paper reports the first experimental observation of FDT violation in a superspin glass, measuring magnetic noise and susceptibility to quantify effective temperature deviations.

Findings

FDT violation observed in superspin glass at low temperatures

Effective temperature exceeds bath temperature significantly

Results align with behaviors seen in atomic and Heisenberg spin glasses

Abstract

We present the experimental observation of the fluctuation-dissipation theorem (FDT) 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 $T_{\rm eff}$ 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.