Strong perpendicular magnetic anisotropy in an MgO/CoFeB/Pd unit structure with a thick CoFeB layer

A strong perpendicular magnetic anisotropy (PMA) is formed in an MgO/CoFeB/Pd unit structure for an MgO-based magnetic tunnel junction. The most important factors for a strong PMA are the composition and the thickness of the CoFeB layer. A strong PMA is observed for the samples fabricated using the CoFeB target with a high Co/Fe ratio and annealed at 300 °C for 1 or 2 h. The PMA is formed up to a CoFeB layer thickness as thick as 2.5 nm, although the strongest PMA, with an out-of-plane coercivity of 1068 Oe and a PMA energy density of 2.7×10⁶ erg/cc, is seen at a CoFeB thickness of 2.0 nm. The systematic study indicates that the PMA is attributed not to the interface effects but rather to the bulk effect of forming a Pd-rich, Co–Pd alloy, as confirmed by x-ray photoelectron spectroscopy depth profile and x-ray diffraction experiments. The thick CoFeB layer is expected to reduce the template effect from the Pd layer during the annealing, and therefore increase the tunneling magnetoresistance of the MgO-based magnetic tunnel junction.