Spatially resolved electron energy-loss spectroscopy of electron-beam grown and sputtered CoFeB∕MgO∕CoFeB magnetic tunnel junctions

Electron energy-loss spectroscopy at subnanometer resolution is used to investigate CoFeB∕MgO∕CoFeB magnetic tunnel junctions grown by electron-beam evaporation and radio frequency (rf) sputtering before and after annealing. Gap states were observed in the MgO layer for both growth methods although the rf-sputtered MgO layer showed significantly more gap states. Asymmetry in oxygen bonding between the top and bottom CoFeB∕MgO interfaces was also observed. Moreover, significant amounts of diffused B as BOx were observed in the rf-sputtered MgO layer. A Mg underlayer between the MgO layer and the bottom electrode greatly reduced BOx formation in the barrier upon annealing.