Perpendicular Magnetic Anisotropy in MgO/CoFeB/Nb and a Comparison of the Cap Layer Effect

High perpendicular magnetic anisotropy (PMA) has been recently revealed in CoFeB/MgO/CoFeB tunnel junctions if the thickness of CoFeB is <;1.5 nm. However, PMA has been observed in MgO/CoFeB/Ta but not in MgO/CoFeB/Ru, indicating a metallic cap layer effect. In this paper, we extend the study to MgO/CoFeB(0.8-1.8 nm)/Nb(1.3 nm) by sputtering and find that the magnetic behavior of MgO/CoFeB/Nb resembles the magnetic behavior of MgO/CoFeB/Ta. The interface anisotropy constant of both MgO/CoFeB/Ta and MgO/CoFeB/Nb is ~2.2 erg/cm² while MgO/CoFeB/Ru has a much weaker interface anisotropy constant ~0.68 erg/cm². In addition, the magnetic dead layer (MDL) of MgO/CoFeB/cap (cap = Ta and Nb) is ~0.1 nm while MgO/CoFeB/Ru has a much larger MDL ~0.5 nm. The metallic layer effect on the PMA in CoFeB/MgO-based perpendicular structure is attributed to the interdiffusion during the post-annealing process. The possible correlation between PMA and MDL provides a possible mechanism for the metal layer effect in the CoFeB/MgO-based structures.