Large magnetoresistance in bcc Co/MgO/Co and FeCo/MgO/FeCo tunneling junctions

TL;DR

This study predicts significantly enhanced magnetoresistance in bcc Co/MgO/Co and FeCo/MgO/FeCo tunneling junctions using first-principles calculations, based on symmetry and electron decay properties at the Fermi level.

Contribution

It introduces a theoretical prediction of large magnetoresistance in specific bcc Co and FeCo junctions, extending understanding of symmetry-dependent tunneling effects.

Findings

Magnetoresistance can be several times larger than in Fe/MgO/Fe systems.

Majority spins have a $\Delta_1$ state that decays slowly in MgO.

Electrons with $\Delta_1$ symmetry undergo total reflection in anti-parallel configurations.

Abstract

By use of first-principles electronic structure calculations, we predict that the magnetoresistance of the bcc Co(100)/MgO(100)/bcc Co(100) and FeCo(100)/MgO(100)/FeCo(100) tunneling junctions can be several times larger than the very large magnetoresistance predicted for the Fe(100)/MgO(100)/Fe(100) system. The origin of this large magnetoresistance can be understood using simple physical arguments by considering the electrons at the Fermi energy travelling perpendicular to the interfaces. For the minority spins there is no state with $\Delta_1$ symmetry whereas for the majority spins there is only a $\Delta_1$ state. The $\Delta_1$ state decays much more slowly than the other states within the MgO barrier. In the absence of scattering which breaks the conservation of momentum parallel to the interfaces, the electrons travelling perpendicular to the interfaces undergo total reflection if the moments of the electrodes are anti-parallel. These arguments apply equally well to systems with other well ordered tunnel barriers and for which the most slowly decaying complex energy band in the barrier has $\Delta_1$ symmetry. Examples include systems with (100) layers constructed from Fe, bcc Co, or bcc FeCo electrodes and Ge, GaAs, or ZnSe barriers.