  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.
