  Using Fe/GaAs Schottky tunnel barriers as electrical spin detectors, we show
that the magnitude and 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 ten-fold compared to the usual
case of zero-bias spin detection.
