Seminaria

Dnia 2023-04-25 o godzinie 13:15 odbędzie się wykład online, na którym dr hab. Piotr Kuświk, prof.IFMPAN z Instytutu Fizyki Molekularnej PAN z Poznania wygłosi wykład pt:

The influence of plasma oxidation on magnetic properties of Co/Ni thin films

Serdecznie zapraszamy

Andrzej Maziewski

Jerzy Przeszowski

The influence of plasma oxidation on magnetic properties of Co/Ni thin films

Piotr Kuświk

Institute of Molecular Physics, Polish Academy of Sciences

Nowadays, spintronics research is focused on finding materials that offer tunable perpendicular magnetic anisotropy (PMA) together with low Gilbert damping and high spin polarization. Co/Ni films satisfy all these requirements [1]. Moreover, new applications are expected because Dzyaloshinskii-Moriya interaction (DMI) is activated in them by surrounding with heavy metal or oxide layers [2, 3]. For this reason, we study magnetic anisotropy and Exchange Bias (EB) coupling of Co/Ni bilayers after plasma oxidation (PO). The oxidation process was controlled using different oxidation times (tOx.). We show that the magnetic anisotropy of these bilayers can be tuned by the thickness of Co (tCo) and Ni (tNi) and exposures times, tOx, between 15 and 220 seconds [4]. For the systems in the as-deposited state, as well as after each step of the PO process, the magnetization process was registered at room temperature with polar magneto-optical Kerr effect magnetometer. It was shown that for properly adjusted thicknesses of Co and Ni, oxidation of the Ni layer leads to a transition from easy-plane anisotropy to PMA. After PO, magnetization process measurements at low-temperature (from nitrogen temperatures up to room temperature) revealed the presence of EB coupling, indicating the formation of antiferromagnetic NiO, which increases PMA. The presence of a NiO layer was confirmed by X-ray photoelectron spectroscopy. This has shown that during PO, there is a process of reducing the thickness of metallic Ni and forming a stoichiometric NiO layer. Changes in anisotropy and its sources were studied in Co/Au/Co/Ni multilayer systems using magnetoresistance measurements. Based on these results, it was possible to determine changes in the basic parameters characterizing the magnetic properties of the Co layer and the Co/Ni bilayer after PO, in particular changes of magnetic anisotropy. These data reveal that Ni thickness reduction would not be the only source of anisotropy changes. It was shown that the contribution to PMA increases in the range of PO times for which Ni thickness is not further reduced. This change was correlated with the formation of the NiO layer. Moreover, using an atomic force microscope, it was shown that plasma oxidation does not significantly affect surface roughness and grain size [5]. Also, magnetic domain structure at different magnetic field as a function of temperature were carried out. They allows to determine the chirality of the domain walls. It was also shown that after PO there is a DMI favoring right-handed chirality, which does not change as a function of temperature. Based on this results, a method for local modification of magnetic properties, in particular magnetic anisotropy, was proposed. Using optical lithography the masks made from positive resist were produced. Their thickness was adjusted to ensure that the areas (matrix) covered by the resist were protected from plasma. This techniques allowed fabrication of 2D heterostructures characterized by four different magnetic states of the matrix and modified areas (I) squares with a non-magnetic state embedded in matrix with PMA,(II) squares and matrix with PMA with significantly different coercivities, (III) squares with PMA embedded in matrix with easy-plane,(IV) both squares and matrix have easy-plane anisotropy with different saturation fields [5].

This work was supported by the National Science Centre, Poland under OPUS 17 funding (Grant No. 2019/33/B/ST5/02013).

[1] S. Andrieu, et al., Phys. Rev. Materials. 2, 064410, (2018)

[2] G. Chen, et al., Nature Com., 4, 2671, (2013)

[3] G. Chen, et al., Sci. Adv., 6, eaba4924, (2020)

[4] B. Anastaziak, et al., Phys. Status Solidi RRL, 16 (2022) 2100450

[5] B. Anastaziak, et al, Scientific Reports 12 (2022) 22060