Oxide layer boron leads to reduced symmetry spin filtering magnetic tunnel junctions

Experimental studies of FeCoB/MgO/FeCoB tunnel junctions indicate that boron diffuses into MgO during rf-sputtering and forms polycrystalline Mg-B-O regions. These tunnel junctions provide high tunneling magnetoresistance values and low RA products. However the crystal structure of the Mg-B-O region remains unknown. Using density functional techniques, I examine three potential Mg(B) oxides including Mg$_{2}$B$_{2}$O$_{5}$ (monoclinic and triclinic) and the orthorhombic mineral Kotoite (Mg$_3$B$_2$O$_6$). Kotoite is the best candidate for formation in magnetic tunnel junctions. The (100) surface of Kotoite has a good lattice match with (001) MgO and could template neighboring FeCo into bcc layers during annealing. Complex band structure analysis of Kotoite shows that the C$_{2v}$ $\tilde{\Delta}_1$ band has a much smaller imaginary k component than the C$_{2v}$ $\tilde{\Delta}_4$ band. Based on symmetry analysis, the majority spin $\Delta_1$ band in FeCo should couple well with the Kotoite $\tilde{\Delta}_1$ band, while the minority FeCo $\Delta_5$ will couple partially with the $\tilde{\Delta}_4$ band. Kotoite provides a new route to high tunneling magnetoresistance based on spin filtering by a lower symmetry oxide region.