Asymmetric Nonlinear Differential Resistance of Mesoscopic AuFe Spin-Glass Wires

We have measured the differential resistance $R=dV/dI$ of mesoscopic AuFe wires as a function of temperature $T$ and dc current bias $I$. $R\left(T\right)$ shows a maximum at a temperature $T_m\sim 1–4\mathrm{K}$, consistent with the onset of spin-glass order in these films. At temperatures $T<\mathrm{}{T}_m$, $R\left(I\right)$ also shows a maximum; however, $R\left(I\right)$ is asymmetric in $I$, the asymmetry increasing with decreasing temperature. The asymmetry is sample specific, sensitive to the four terminal measurement configuration, and is associated with the presence of magnetic impurities in the samples.