Effects of Ta diffusion on the perpendicular magnetic anisotropy (PMA) of Ta/Co 0.6Fe0.2 B0.2 (12.5 Å)/MgO stacks were investigated. We found that Ta initially mixed with Co0.6Fe 0.2B0.2 and its diffusion became prominent above 300 °C. Initial diffusion of Ta into the Co0.6 Fe0.2B 0.2 was not detrimental to the PMA, but a significant PMA reduction occurred when Ta migrated to the Co 0.6Fe0.2 B 0.2/MgO interface. This accounts for the fact that optimizing the PMA of Ta/CoFeB/MgO requires the tuning of both CoFeB thickness and annealing temperature. © 2010-2012 IEEE.