  Fabrication of perpendicularly magnetized ferromagnetic films on various
buffer layers, especially on numerous newly discovered spin-orbit torque (SOT)
materials to construct energy-efficient spin-orbitronic devices, is a
long-standing challenge. Even for the widely used CoFeB/MgO structures,
perpendicular magnetic anisotropy (PMA) can only be established on limited
buffer layers through post-annealing above 300 {\deg}C. Here, we report that
the PMA of CoFeB/MgO films can be established reliably on various buffer layers
in the absence of post-annealing. Further results show that precise control of
MgO thickness, which determines oxygen diffusion in the underneath CoFeB layer,
is the key to obtaining the as-deposited PMA. Interestingly, contrary to
previous understanding, post-annealing does not influence the well-established
as-deposited PMA significantly but indeed enhances unsaturated PMA with a thick
MgO layer by modulating oxygen distributions, rather than crystallinity or Co-
and Fe-O bonding. Moreover, our results indicate that oxygen diffusion also
plays a critical role in the PMA degradation at high temperature. These results
provide a practical approach to build spin-orbitronic devices based on various
high-efficient SOT materials.
