  We investigate the thermoelectric properties of Fe/MgO/Fe(001) magnetic
tunnel junctions (MTJs) by means of the linear-response theory combined with a
first-principles-based Landauer-B\"uttiker approach. We find that the Seebeck
coefficient of Fe/MgO/Fe(001) MTJs strongly depends on the barrier thickness
and the tetragonal distortion. A compressive tetragonal distortion of the
in-plane lattice parameter in the MTJs provides interface resonant states just
above the Fermi energy. This causes resonant tunneling in the MTJs and
significantly enhances the Seebeck coefficient when the thickness of the MgO
barrier is around 1 nm (four or five atomic layers of MgO). Moreover, an
extensive tetragonal distortion of the in-plane lattice parameter pushes the
interface states away from the Fermi energy, leading to a reduction of the
Seebeck coefficient. Furthermore, we find that the interface resonant tunneling
enhances the power factor of the MTJs for the compressive distortion. These
results indicate that control of the barrier thickness and the tetragonal
distortion will be effective for maximizing the thermoelectric properties of
MTJs.
