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

Abstract

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.

Magnetic patterning of Co/Ni layered systems by plasma oxidation

“…It displays three distinct ranges of t Ni with different magnetic properties of the Co/ Ni bilayer (Fig. 5a), which are in good agreement with the data presented in 36 . For the smallest Ni thicknesses (t Ni < 0.25 nm) ϕ ≈ 0 and the system shows non-ferromagnetic behavior (not shown in Fig.…”

“…6 e) at a magnetic field range H C matrix ≤ H ≤ H C sq . Note that this range can be wider for longer oxidation times, because the contribution to the PMA increases with oxidation time 36 .

Figure 6 Magnetic patterning by plasma oxidation of buffer/Co 1.4 nm/Ni- t Ni (wedge 0–5 nm) system: ( a – c ) evolution of magnetic domain structure of matrix and squares after plasma oxidation with τ Ox = 110 s, for t Ni = 0.35, 0.84 and 1.4 nm, respectively.

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“…This shows the ability to independently control the magnetic properties of squares (properties depend on oxidation time and t Ni ) and the matrix (properties of the sample before oxidation tuned by t Ni ). Note that the properties of squares and matrix can also be set by the thickness of Co 36 , which offers even more tailoring opportunities.…”

“…The anisotropy modification mechanism includes the reduction of the Ni thickness and the formation of an antiferromagnetic NiO layer. This layer induces an EB interaction supporting additional contribution to PMA 36 .…”

“…After deposition, the samples were oxidized using a Zepto plasma cleaner by Diener Electronic (Germany), following the procedure described in Ref. 36 .…”