Origin of Perpendicular Magnetic Anisotropy Enhancement in Co/Ni Bilayer Due to Plasma Oxidation

• Published in: Physica status solidi. PSS-RRL. Rapid research letters 2022-02, Vol.16 (2), p.n/a
• Main Authors: Anastaziak, Błażej, Głowiński, Hubert, Urbaniak, Maciej, Frąckowiak, Łukasz, Stobiecki, Feliks, Kuświk, Piotr
• Format: Article
• Language: English
• Subjects: antiferromagnetic oxides; Co/Ni thin films; exchange bias; perpendicular magnetic anisotropy; plasma oxidation
• ISSN: 1862-6254; 1862-6270
• DOI: 10.1002/pssr.202100450

In spintronics and magnonics, materials that offer tunable perpendicular magnetic anisotropy (PMA) along with low Gilbert damping and high spin polarization are particularly important. Therefore, a lot of studies are performed on Co/Ni films, that aim at satisfying all these requirements. Moreover, the Dzyaloshinskii−Moriya interaction is induced in them by surrounding the magnetic layers with heavy metal or oxide layers. For this reason, the study of the oxidation of Co/Ni is of particular interest. Therefore, the magnetic properties of Co/Ni bilayers after plasma oxidation (PO) are investigated. It is shown that the magnetic anisotropy of these bilayers can be tuned, not only by the thicknesses of Co and Ni layers, but also by oxidation time varied in the range between 15 and 220 s. After PO, the effective thickness of ferromagnetic Ni is reduced, but it does not oxidize more than ≈2 nm, even for longer oxidation. However, the contribution to PMA increases for the entire range of oxidation times. This indicates that the thickness reduction of the Ni layer is not the only source of PMA enhancement. This additional contribution is attributed to the exchange bias coupling between the ferromagnetic (Co/Ni) and the antiferromagnetic NiO layers. How to use plasma oxidation to tailor magnetic properties of Co/Ni bilayer by Ni oxidation is described. This type of oxidation forms NiO, which stabilizes exchange bias coupling, inducing an additional surface contribution to the effective anisotropy favoring the perpendicular alignment of the Co/Ni magnetization.