Theory of resonant spin-dependent tunneling in an Fe/Ag/MgO/Fe(001) junction

Abstract

Calculation of the tunneling magnetoresistance (TMR) of an Fe/Ag/MgO/Fe(001) magnetic junction is reported. The magnetoresistance is determined without any approximations from the real-space Kubo formula using tight-binding bands fitted to an ab initio band structure. It is shown that the calculated TMR oscillates as a function of Ag interlayer thickness between positive values in excess of 2000% and negative values of the order of ?100%. The oscillation period is determined by the spanning vector of the Ag Fermi surface. The large positive TMR and the changes in its sign are due to resonant enhancement of the tunneling conductance of majority-spin carriers in the ferromagnetic configuration and of the conductance of carriers tunneling in the antiferromagnetic configuration from the minority-spin channel in the Fe electrode adjacent to the Ag layer to the majority-spin channel in the other Fe electrode. The resonant enhancement occurs because the Ag interlayer creates potential steps for electrons in both the ferromagnetic and antiferromagnetic configurations of the junction. This mechanism, which results in a very large TMR, is quite different from the mechanism that causes large TMR in the standard Fe/MgO/Fe(001). It offers the possibility of tuning the magnitude and sign of the TMR by the choice of the interlayer thickness. A Lateral supercell method was also used to investigate the effect of interfacial roughness on the resonant tunneling in an Fe/Ag/MgO/Fe(001) junction. It is found that, in contrast to the Fe/MgO/Fe(001) junction whose TMR is reduced drastically by disorder, the junction with a silver interlayer is much less affected by interfacial roughness.