Bias-controlled sensitivity of ferromagnet/semiconductor electrical spin detectors

Using Fe/GaAs Schottky tunnel barriers as electrical spin detectors, we show that the magnitude and the sign of their spin-detection sensitivities can be widely tuned with the voltage bias applied across the Fe/GaAs interface. Experiments and theory establish that this tunability derives not just simply from the bias dependence of the tunneling conductances $G_{\uparrow ,\downarrow }$ (a property of the interface), but also from the bias dependence of electric fields in the semiconductor which can dramatically enhance or suppress spin-detection sensitivities. Electrons in GaAs with fixed polarization can therefore be made to induce either positive or negative voltage changes at spin detectors, and some detector sensitivities can be enhanced over tenfold compared to the usual case of zero-bias spin detection.

Figure 1

(Color online) (a) Spin-transport device A and the spin-detection experiment. Current $j$ establishes a voltage $V_d$ across a Fe/GaAs detector interface (contact 4). Changes $\left(\Delta V_d\right)$ due to a remote optically injected spin polarization reveal the detector’s spin sensitivity. (b) A $70\times 70\mu {\text{m}}^2$ reflectivity image [white square in (a)], showing device features. (c) A Kerr-rotation image of the optically injected polarization $\left(B_x=0\right)$. (d) Raw spin-detection data at 10 K, $\Delta V_d\left(B_x\right)$, at various $V_d$. 4 and 20 K data are very similar. (e) The corresponding spin-detection sensitivity versus $V_d$ (black points). Squares show similar $\Delta V_d$ measured for injection in the current path ($j$ flowing between contacts 4 and 5).

Figure 2

(Color online) (a) Measuring $P_j$, the tunneling current polarization at contact 4 via spin injection at 10 K. Current $j$ establishes a voltage bias $V_{int}$ across the Fe/GaAs interface. (b) Electrically injected spins are measured via the Hanle-Kerr effect (${\theta }_K$ vs $B_x$), at different $V_{int}$ (curves offset). (c) The peak-to-peak Hanle amplitude, $\Delta {\theta }_K\propto j_{\uparrow }-j_{\downarrow }$, versus $V_{int}$. Inset: $j\left(V_{int}\right)$ for contact 4. (d) Relative polarization, $P_j\propto \left(j_{\uparrow }-j_{\downarrow }\right)/j$, versus $V_{int}$.

Figure 3

(Color online) Spin injection and detection data from sample B at 10 K. (a) The electrically injected spin density, measured by the Kerr effect ($\Delta {\theta }_K$ vs $V_{int}$). (b) Polarization of the tunneling current $P_j$, which now inverts under reverse bias, in contrast to sample A. (c) Raw spin-detection data, $\Delta V_d\left(B_x\right)$ at various detector biases $V_d$ (curves offset). (d) The spin-detection sensitivity $\Delta V_d$ versus $V_d$.

Figure 4

(Color online) (a) Schematic of $n_{\uparrow }-n_{\downarrow }$ near a Fe/GaAs spin detector under forward or reverse bias $V_d$ (solid lines). Both $n_{\uparrow }-n_{\downarrow }$ and its gradient are modified (dashed lines) by a remotely injected polarization $p$. (b) Calculated sensitivity of a spin detector having constant $P_j$. Enhancement and suppression are due to $E$ fields in the GaAs. [(c),(d)] Calculated $\Delta V_d$ for samples A and B, for three electron mobilities $\mu$, using measured $P_j$ (open symbols) as inputs and $p$ injected $20\mu \text{m}$ away.

Figure 5

(Color online) (a) Inset: all-electrical lateral spin-valve setup. Black (red/gray) points show the spin-valve signal $\Delta V_d$ versus detector bias $V_d$ for a forward (reverse) biased spin injector. (b) Raw spin-valve data at three detector biases [arrows in (a)], using a fixed $-1.5\text{mA}$ injector bias (curves offset). Note sign switching and tenfold enhancement of the detected signal.