  A spin metal-oxide-semiconductor field-effect-transistor (spin MOSFET), which
combines a Schottky-barrier MOSFET with ferromagnetic source and drain
contacts, is a promising device for spintronic logic. Previous simulation
studies predict that this device should display a very high magnetoresistance
(MR) ratio (between the cases of parallel and anti-parallel magnetizations) for
the case of half-metal ferromagnets (HMF). We use the non-equilibrium Green's
function (NEGF) formalism to describe tunneling and carrier transport in this
device and to incorporate spin relaxation at the HMF-semiconductor interfaces.
Spin relaxation at interfaces results in non-ideal spin injection. Minority
spin currents arise and dominate the leakage current for anti-parallel
magnetizations. This reduces the MR ratio and sets a practical limit for spin
MOSFET performance. We found that MR saturates at a lower value for smaller
source-to-drain bias. In addition, spin relaxation at the detector side is
found to be more detrimental to MR than that at the injector side, for drain
bias less than the energy difference of the minority spin edge and the Fermi
level.
