Boron diffusion induced symmetry reduction and scattering in CoFeB/MgO/CoFeB magnetic tunnel junctions

Abstract

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 Mg3B 2O6 (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 C4v of MgO to C2v of kotoite. Such symmetry reduction induces scattering and weakens the tunneling transmission of the Δ1-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. © 2013 American Physical Society.