Effect of Polarized Current on the Magnetic State of an Antiferromagnet

We provide evidence for the effects of spin polarized current on a nanofabricated antiferromagnet incorporated into a spin-valve structure. The signatures of the current-induced effects include bipolar steps in differential resistance, current-induced changes of exchange bias correlated with these steps, and deviations from the statistics expected for thermally activated switching of spin valves. We explain our observations by a combination of spin torque exerted on the interfacial antiferromagnetic moments and electron-magnon scattering in an antiferromagnet.

Figure 1

(a) Hysteresis loop of EB nanopillar acquired at 4.5 K immediately after cooldown at $H=3\mathrm{kOe}$ from 350 K. Reversal fields $H^{+}$ and $H^{-}$ of Py(5) are labeled. (b) Exchange bias $H_E$ vs the hysteresis loop number in EB nanopillar (symbols) and a $5\times 5\mathrm{mm}$ sample with magnetic multilayer structure identical to the EB nanopillar (curve).

Figure 2

$H_E$ of EB sample measured at $I=0.3\mathrm{mA}$ after a 100 ms pulse of current $I_0$ applied at $H_0=-3\mathrm{kOe}$ (a) and $H_0=3\mathrm{kOe}$ (b). Each point is an average of 10 measurements, whose statistics are represented by error bars. $H_0$ and/or $I_0$ were reversed after each group of measurements. The dashed curve in (a) is a guide to the eye.

Figure 3

(a) Switching currents from $P$ to AP state (circles) and from $P$ to AP state (squares) vs $T$ for EB nanopillar (solid symbols) and standard sample (open symbols), at $H=50\mathrm{Oe}$. (b) $dV/dI$ vs $I$ at 4.5 K for standard sample, with $H=50\mathrm{Oe}$ (dashed and solid curves), and 363 Oe (dotted curve). (c) Same as (b), but for the EB sample at the labeled values of $H$. Curves for $H>50\mathrm{Oe}$ are offset for clarity.

Figure 4

(a) $R$ vs $t$ for EB nanopillar at $I=9\mathrm{mA}$, $H=900\mathrm{Oe}$. (b) Average dwell times in the $P$ state ${\tau }_P$ (dashed curve) and the AP state ${\tau }_{\mathrm{AP}}$ (solid curve) vs $I$, at $H=900\mathrm{Oe}$. (c) Distribution of dwell times in the $P$ state at $I=9\mathrm{mA}$ and $H=900\mathrm{Oe}$. Dashed line is exponential fit with characteristic time ${\tau }_0=1.1\mathrm{ms}$. (d) Same as (c), for the AP state. Inset shows the distribution for the shorter time scale.