  Efficient conversion of a spin signal into an electric voltage in mainstream
semiconductors is one of the grand challenges of spintronics. This process is
commonly achieved via a ferromagnetic tunnel barrier where non-linear electric
transport occurs. In this work, we demonstrate that non-linearity may lead to a
spin-to-charge conversion efficiency larger than 10 times the spin polarization
of the tunnel barrier when the latter is under bias of a few mV. We identify
the underlying mechanisms responsible for this remarkably efficient spin
detection as the tunnel barrier deformation and the conduction band shift
resulting from a change of applied voltage. In addition, we derive an
approximate analytical expression for the detector spin sensitivity
$P_{\textrm{det}}(V)$. Calculations performed for different barrier shapes show
that this enhancement is present in oxide barriers as well as in Schottky
tunnel barriers even if the dominant mechanisms differs with the barrier type.
Moreover, although the spin signal is reduced at high temperatures, it remains
superior to the value predicted by the linear model. Our findings shed light
into the interpretation and understanding of electrical spin detection
experiments and open new paths to optimize the performance of spin transport
devices.
