By first-principles analysis, we investigate the effect of thermal annealing on structural stability of CoFeB/MgO(thin)/CoFeB magnetic tunnel junctions. The calculated phonon dispersion indicates that Mg${}_3$B${}_2$O${}_6$ (kotoite) is a stable spacer after annealing due to B diffusion into MgO. The calculated tunneling magnetoresistance (TMR) of CoFe/kotoite/CoFe is 210$\%$, which is in good agreement with the available experimental value and 2 orders of magnitude lower than the predicted values of CoFe/MgO/CoFe junctions. The physics of this more realistic TMR value is the change in symmetry from C${}_{4v}$ of MgO to C${}_{2v}$ of kotoite. Such symmetry reduction induces scattering and weakens the tunneling transmission of the ${\Delta }_1$-like Bloch states. Our calculations also reveal that the tunneling transmission is sensitive to the electrode/spacer interfacial chemical bonding. Residual boron, localized at the interface due to insufficient annealing temperature, can further reduce the TMR.

• Received 6 November 2012

DOI:https://doi.org/10.1103/PhysRevB.87.014114