The study of origin of interfacial perpendicular magnetic anisotropy in ultra-thin CoFeB layer on the top of MgO based magnetic tunnel junction

摘要

A comprehensive microstructure study has been conducted experimentally for identifying the origin or mechanism of perpendicular magnetic anisotropy (PMA) in the ultra-thin (10 A) CoFeB layer on the top of magnetic tunnel junction (MTJ). The high resolution transmission electron microscopy reveals that the feature of crystal structure in 10 A-CoFeB layer is localized in nature at the CoFeB-MgO interface. On the other hand, the strain-relaxed crystalline structure is observed in the thick CoFeB (20 A) layer at the CoFeB-MgO interface, associated with a series of dislocation formations. The electron energy loss spectroscopy further suggests that the local chemical stoichi-ometry of the ultra-thin 10 A-CoFeB layer is notably changed at the CoFeB-MgO interface, compared with an atomic stoichiometry in a thick 20 A-CoFeB layer. The origin of PMA mechanism is therefore identified experimentally as an interface effect, which can be attributed to a change of local atom bonding or lattice constant of the transition metal at the CoFeB-MgO based MTJ interface. Furthermore, such a local interfacial atom bonding change is seemly induced by the localized aniso-tropic strain and consistent with previous theoretical speculations and calculations. The observed experimental findings provide some perspective on microstructure and chemistry on PMA in ultra-thin CoFeB film at the MTJ interface, then deepening our understanding of the mechanism of PMA within MTJ stack and thus facilitating advancement for emerging spintronics technology.