  A mesoscopic spin valve is used to determine the effective spin polarization
of electrons tunneling from and into ferromagnetic transition metals at finite
voltages. The tunneling spin polarization from the ferromagnet (FM) slowly
decreases with bias, but drops faster and even inverts with voltage when
electrons tunnel into it. A bias-dependent free electron model shows that in
the former case electrons originate near the Fermi level of the FM with large
polarization whereas in the latter, electrons tunnel into hot electron states
for which the polarization is significantly reduced. The change in sign is
ascribed to the detailed matching of the electron wave function through the
tunnel barrier.
