  To study the impact of oxygen vacancies 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. 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 magnetic tunnel junctions (MTJ)'s
parallel (P) magnetic state that mainly accounts for an order-of-magnitude drop
in TMR, from about 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 $\uparrow$ transmission away from the $\Gamma$ point.
This scattering lowers TMR to 145$\%$. In contrast, the M center merely
broadens this transmission around $\Gamma$, thereby boosting TMR to 315$\%$.
Rotating a M center so as to partly point along the transmission direction
sharpens transmission around $\Gamma$, further increasing TMR to 1423$\%$. When
these defects are placed at the MTJ interface, the transmission and ensuing
TMR, which reaches $\approx$ 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, 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.
