Theory of resonant tunneling in an epitaxial Fe/Au/MgO/Au/Fe(001) junction

Calculation of the magnetoresistance of an epitaxial Fe/Au/MgO/Au/Fe(001) tunneling junction is reported. The tunneling magnetoresistance (TMR) is determined without any approximations from the real-space Kubo formula using tight-binding bands fitted to an ab initio band structure of Fe, Au, and MgO. A very high TMR ratio of approximate to 1000% is predicted for a junction with seven to nine atomic planes of MgO sandwiched between two Au layers, each consisting of eight atomic planes. The insertion of two Au interlayers should prevent oxidation of the Fe electrodes, which is detrimental to TMR in the junction without Au interlayers. It is also found that the TMR oscillates as a function of Au thickness with a period determined by the spanning vector of the Au Fermi surface belly. The underlying physics is explained in terms of a simple parabolic band model of resonant tunneling for a barrier sandwiched between two quantum wells.