Oxygen adsorption on pristine MnBi and Ta-capped MnBi thin-films: Ab-initio study of structural and electronic properties, and magnetic anisotropy

We have studied the structural and electronic properties, as well as magnetic anisotropy in oxidized MnBi thin film with and without tantalum (Ta) atom as a capping layer. Spin -polarized density -functional theory calculations with the generalized gradient approximation (GGA) including U+J Hubbard correction, i.e., DFT+U+J have been employed. We found that the oxygen (O) atom binds more strongly to the MnBi surface when co -present with the Ta atom. We postulate that the strong binding prevents the O atoms from diffusing into the MnBi film layer. Thus, the Ta layer prevents surface oxidation by inducing strong binding of the O atoms to the Mn surface. Furthermore, in cases where molecular O2 is adsorbed on the Ta-capped MnBi, the O2 molecule fragments into two atomic components such that each of the O atoms relaxes to a different lattice sites on the MnBi film. Electronic structure analysis performed with this structure shows that the Ta atom on which one of the O atom is directly attached is weakly binded to the Mn. This makes it less feasible for the O atom to attach to MnBi surface. We conclude therefore, that two different mechanisms contrive to prevent MnBi surface oxidation. These are the strong binding of the O atom to the Mn which prevents the O diffusion into the MnBi film and the weak electronic bonding between the Ta and O which prevent the O atom from binding to the MnBi surface. Furthermore, we found that the presence of either Ta or O, or both, largely preserves the lattice structure of the MnBi film. With respect to the magnetic anisotropy energy (MAE), the presence of Ta capping layer restores the in -plane parallel magnetization of the pristine MnBi thin film after the latter's oxidation. Where the direction of MAE is reversed from the in -plane to perpendicular, the perpendicular MAE is order of magnitude smaller than the in -plane MAE. This affirms the tendency of Ta capping layer to preserve either strongly or weakly, the direction of magnetization in oxidized MnBi. Our work provides the missing but highly crucial atomic level insights into an experimental study (M.Y. Sun et.al., Journal of Alloys and Compounds 672 (2016) 59-63) and has implications for the modeling of oxidation prevention via capping of magnetic thin film with external metallic atoms.