Study of the relationship between magnetic anisotropy and composition ratio of Fe oxide to Fe at CoFeB/MgO film interface

[Display omitted] •As-deposited CoFeB/Mg(O) multilayers shows PMA with reactive sputtering.•The magnetic anisotropy shows close correlation with the composition ratio (ε) of Fe oxide to Fe at interface.•The film exhibits PMA when ε falls in 0.65 between 0.75, beyond it shows IMA. Ta (5)/CoFeB (1)/MgO (2)/Ta (2) (in nm) multilayers were deposited on silicon substrates by magnetron sputtering, and the MgO layer was prepared by two methods at the same argon gas flow rate: the radio frequency (RF) sputtering method of MgO target and the reactive sputtering method of direct current (DC) sputtering Mg target with introducing oxygen at the same time. The sample prepared by the former method was denoted as Sample A, and the latter was denoted as Sample B. The results indicate that as-deposited Sample A shows in-plane magnetic anisotropy (IMA) with Keff = -0.79 × 106erg/cm3. However, the as-deposited Sample B shows perpendicular magnetic anisotropy (PMA) with Keff = 1.69 × 106erg/cm3, and the Keff value increases to 3.17 × 106erg/cm3 after annealing at 250℃. X-ray photoelectron spectroscopy (XPS) results illustrate that the films show PMA when the composition ratio (ε) of Fe oxide to Fe in CoFeB/oxide interface of Sample A and B annealed below 300 ℃, is approximately in the range of 0.65–0.75. When ε is beyond the above range, the film shows IMA. Moderate oxidation state at the interface results in the formation of PMA and it was improved by forming effective orbital hybridization of Fe 3d and O 2p. High resolution transmission electron microscopy (HRTEM) analysis reveals that no crystallization appears in CoFeB and MgO layer of the film before and after annealing, and crystallization has no effect on the interface chemical states in this system.