Perpendicular magnetic anisotropy in as-deposited CoFeB/MgO thin films

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 °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 obtain the as-deposited PMA. Interestingly, contrary to the previous understanding, post-annealing does not significantly influence the well-established as-deposited PMA 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 PMA degradation at high temperatures. These results provide a practical approach to build spin-orbitronic devices based on various high-efficient SOT materials.