  The conversion of spin information into electrical signals is indispensable
for spintronic technologies. Spin-to-charge conversion in ferromagnetic tunnel
contacts is well-described using linear (spin-)transport equations, provided
that there is no applied bias, as in nonlocal spin detection. It is shown here
that in a biased ferromagnetic tunnel contact, spin detection is strongly
nonlinear. As a result, the spin-detection efficiency is not equal to the
tunnel spin polarization. In silicon-based 4-terminal spin-transport devices,
even a small bias (tens of mV) across the Fe/MgO detector contact enhances the
spin-detection efficiency to values up to 140 \% (spin extraction bias) or, for
spin injection bias, reduces it to almost zero, while, parenthetically, the
charge current remains highly spin polarized. Calculations reveal that the
nonlinearity originates from the energy dispersion of the tunnel transmission
and the resulting nonuniform energy distribution of the tunnel current,
offering a route to engineer spin conversion. Taking nonlinear spin detection
into account is also shown to explain a multitude of peculiar and puzzling spin
signals in structures with a biased detector, including two- and three-terminal
devices, and provides a unified, consistent and quantitative description of
spin signals in devices with a biased and unbiased detector.
