Intermixing of iron and cobalt with oxygen-rich magnesium oxide in CoFeB/MgO/CoFeB magnetic tunneling junctions.

Atomic-scale spectroscopic imaging of sputtered magnetic tunnel junction structures with a thick oxygen-rich MgO barrier reveals the diffusion of iron and cobalt into the MgO barrier from CoFeB electrodes. First principles calculations are performed to (1) confirm that Fe diffusion through Mg vacancies is energetically favorable, (2) quantify the reduction of interfacial perpendicular magnetic anisotropy due to Fe diffusion into MgO, and (3) predict that the presence of Fe impurities in MgO causes an increased leakage and a tunneling magnetoresistance decrease. Through the chemical shift of the Fe L₃ edge and the peak ratio Fe L₃/Fe L₂ measured by electron energy loss spectroscopy, we suggest that, within MgO, iron with mixed oxidation state Fe²⁺ and Fe³⁺ or higher is found in the as-grown structure, which is reduced by annealing to Fe²⁺. These results indicate that the stoichiometry of as-deposited MgO barrier layers plays an important role in controlling the microstructure and optimizing the performance of magnetic tunnel junctions. [ABSTRACT FROM AUTHOR]