Degree Date

2020

Degree

Doctor of Philosophy (PhD)

Department

Physics

Abstract

Magnetic skyrmions, topologically stable spin textures that can be driven by electric currents efficiently, have both rich physics and potential applications in future technology such as spintronic devices for data storage. Magnetic skyrmions in multilayers with interfacial Dzyaloshinskii-Moriya interaction (DMI) were investigated in three aspects: stabilization, current-driven motion and antiferromagnetically coupled skyrmion pairs. The formation and stabilization of magnetic skyrmions without external magnetic fields are determined by the competition of the Heisenberg exchange interaction, dipolar interaction, magnetic anisotropy, and interfacial DMI. In the [Pt/Co/HM]n (HM = W, Mn, Ir, Au, n = 1, 3, 5, 8, 10, 12) multilayers, DMI and the dipolar interaction can be tuned by changing the heavy metal (HM) and the repetition number n, respectively. The size of the stabilized skyrmions decreases as the DMI or the dipolar interaction increases. Current-driven skyrmion motion was investigated in a Ta/CoFeB/TaOx multilayer and the skyrmion Hall effect was observed. The transverse motion of the skyrmions, driven by an electric current in a perpendicular magnetic field, can be quantized by the skyrmion Hall angle, the angle between the current direction and the skyrmion motion direction. A significant current/velocity dependence of the skyrmion Hall angle was observed. Combining the experimental results and the theoretic derivation based on the Thiele equation, we attribute the skyrmion Hall effect to the topological Magnus force and random pinning potentials in the materials.

To avoid the undesired skyrmions motion towards the device edge, antiferromagnetically coupled skyrmion pairs were realized and investigated in [Co/Gd/Pt]10 multilayers. These skyrmion pairs are stable over a broad temperature range from about 300 K to 55 K. Below 50 K, the magnetization of the core area of the skyrmion pairs starts to rotate from the original direction of out-of-plane to in-plane. However, the domain wall regions are preserved during this spin reorientation transition (SRT). After the temperature is increased back to above 50 K, skyrmions recover to their initial state before the SRT. The recovery of the skyrmions can be attributed to the persisted chiral domain walls due to the strong topological protection by DMI, confirmed by micromagnetic simulations.

Comments

Part of this dissertation has been published as Wang, X., et al. 2022. “Topological spin memory of antiferromagnetically coupled skyrmion pairs in Co/Gd/Pt multilayers.” Phys. Rev. Materials 6: 084412.

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