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

Abstract

The combination of a low tunnelling barrier height and a large tunnelling magnetoresistance (TMR) ratio in MgO-class magnetic tunnel junctions 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 (PAW) method. These point defects can generate barrier heights as low as 0.4V for FeCo electrodes irrespective of the defect's spatial position within the 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 (P) 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 would an ideal MTJ, only with a much lower barrier height. 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.

Consolidated picture of tunnelling spintronics across oxygen vacancy states in MgO

“…unoccupied, above E F ), and the dominant 0.4 eV barrier height for T(K) > 250 to the M center's anti-bonding ground state. In contrast, theory [9,10] suggests that, for comparable FeCo electrodes, these three barriers could correspond to the M center's bonding ground state M 1 (at 1.1 eV), to the F center's ground state (at 0.8 eV), and to the M center's anti-bonding ground state M 2 (at 0.4 eV), i.e. that tunnelling is proceeding relative to occupied states.…”

“…Although the presence of structural defects such as oxygen vacancies has been historically [7] perceived as degrading spintronic performance, we have conducted [5,[8][9][10] recent experiments and theory suggesting that the electronic properties of oxygen vacancies in fact explain this combination of high spintronic performance and lowered barrier heights needed for STT. In simple terms, the charge density of an oxygen vacancy, whether single (denoted F center) or paired (denoted M center), exhibits spherical symmetry [9,10]. This is precisely the (Δ 1 ) symmetry of electrons with high spin polarization in the FM electrodes that is invoked to account for high spintronic performance.…”

“…This is precisely the (Δ 1 ) symmetry of electrons with high spin polarization in the FM electrodes that is invoked to account for high spintronic performance. Some of us have proposed a new ab initio theoretical description of tunnelling spintronic performance in the presence of oxygen vacancies [10].…”