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The Co-Fe-B/MgO system with perpendicular magnetic anisotropy finds extensive application in modern magnetic memories. Critical issues for the practical usage of magnetic tunnel junctions (MTJs) include the limited thermal stability of bit storage, and low switching efficiency. This study elucidates the deterministic influence of Co-Fe-B composition and the MgO interface on an MTJ's fundamental magnetic properties, which affect both thermal stability and switching current. Furthermore, the authors observe an anomalous trend in saturation magnetization, which is attributed to a change in magnetic anisotropy.

Using density-functional theory calculations, we investigate the dominant defects formed by boron (B) and carbon (C) impurities in a CoFe/MgO/CoFe magnetic tunnel junction (MTJ) and their influence on conductivity and tunneling magnetoresistance (TMR). We find that, in the O-poor conditions relevant to experiment, B forms the substitutional defect ${\mathrm{B}}_{\mathrm{Co}}$ and C forms the interstitial site ${\mathrm{C}}_{\mathrm{i}}$ at the CoFe/MgO interface. The C-doped MTJ is predicted to have a significantly higher TMR than the B-doped MTJ. This is due to interface state densities associated with the majority spin ${\mathrm{\ensuremath{\Delta}}}_{1}$-symmetry bands being more heavily suppressed by the ${\mathrm{B}}_{\mathrm{Co}}$ defects than by the ${\mathrm{C}}_{\mathrm{i}}$ defects. Our results indicate that carbon can serve as a viable alternative to boron as a dopant for MTJ fabrication.