Theory of tunneling magnetoresistance of an epitaxial Fe/MgO/Fe(001) junction

Calculation of the tunneling magnetoresistance (TMR) of an epitaxial Fe/MgO/Fe(001) junction is reported. The conductances of the junction in its ferromagnetic and antiferromagnetic configurations are determined without any approximations from the real-space Kubo formula using tight-binding bands fitted to an ab initio band structure of iron and MgO. The calculated optimistic TMR ratio is in excess of 1000% for an MgO barrier of $\approx 20$ atomic planes and the spin polarization of the tunneling current is positive for all MgO thicknesses. It is also found that spin-dependent tunneling in an Fe/MgO/Fe(001) junction is not entirely determined by states at the $\Gamma$ point $({\mathbf{k}}_{\Vert }=0)$ even for MgO thicknesses as large as $\approx 20$ atomic planes. All these results are explained qualitatively in terms of the Fe majority- and minority-spin surface spectral densities and the complex MgO Fermi surface.