Impact of single and double oxygen vacancies on electronic transport in Fe/MgO/Fe magnetic tunnel junctions

Abstract

The combination of a low tunneling barrier height and a large tunneling magnetoresistance (TMR) ratio in MgO-class magnetic tunnel junctions (MTJs) has enabled next-generation information storage and bio-inspired computing solutions thanks to the spin transfer torque effect. Recent literature has proposed that this synergistic combination arises from the electronic properties of oxygen vacancies. To explicitly understand their impact on spin-polarized transport, we have computed the electronic and transport properties of single (F centers) and paired (M centers) oxygen vacancies using density functional theory and the projector augmented wave method. These point defects can generate energy level positions of 0.4 eV with respect to the Fermi level for FeCo electrodes irrespective of the defect’s spatial position within the MgO barrier and of the orientation of the M center. These defects promote a strong decrease in the conductance of the spin up channel in the MTJ’s parallel magnetic state that mainly accounts for an order-of-magnitude drop in TMR from ≈10000% in the ideal case toward values more in line with experiment. When placed in the middle layer of the MgO barrier, the F center introduces additional P ↑ transmission away from the Γ point. This scattering lowers TMR to 145%. In contrast, the M center merely broadens this transmission around Γ, thereby boosting TMR to 315%. Rotating a M center so as to partly point along the transmission direction sharpens transmission around Γ, further increasing TMR to 1423%. When these defects are placed at the MTJ interface, the transmission and ensuing TMR, which reaches ≈4000%, suggest that such junctions behave as an ideal MTJ only with a much lower TMR. Our results, thus, theoretically reconcile the concurrent observations of high TMR and low barrier heights in line with experimental preparation techniques such as post-deposition oxidation of metallic Mg, which can generate oxygen vacancies at the lower MTJ interface, and annealing which can promote M centers over F centers. Our theory is also in line with an origin of perpendicular magnetic anisotropy in terms of oxygen vacancies at MTJ interfaces. The effective size of these vacancies sets a limit for both the barrier thickness, in line with experiment, as well as for the MTJ’s lateral dimension. Our work provides a much-needed theoretical basis to move beyond the mostly unsuspected, fortuitous defect engineering of spintronic performance that has, thus, far propelled MgO-based spintronics and its applications.