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Magnetic Domain Wall Skyrmions: Investigating Chiral Spin Textures in Pt/Co/Ni/Ir Multi-Layers using Lorentz Transmission Electron Microscopy

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thesis
posted on 11.11.2020, 16:33 by Maxwell Li
The work presented in this thesis document seeks to confirm the experimental existance of domain wall substructures in the presence of the Dzyaloshinskii-Moriya interaction that we call magnetic domain wall (DW) skyrmions. These DW skyrmions are characterized as a 360 [degree]winding of magnitization along a chiral N'eel wall and have broad implications in the design of future spintronic devices. This is done primarily using Lorentz transmission electron microscopy (LTEM) which offers high resolution imaging of magnetic features.
First, a model materials system of [Pt/(Co/Ni)M /Ir]N based multi-layers was developed to allow for tuning of relevant magnetic parameters that can support the formation of DW skyrmions. The stabilization of chiral N'eel walls is confirmed with magnetic contrast only appearing in the presence of sample tilt. Various magnetic fields were applied in situ and ex situ which formed isolated and arrays of skyrmions, respectively. DW skyrmions were not observed in these thicker films with large N. Symmetric [Co/Ni]M multi-layers were also examined which were not expected to display DMI; these samples readily display magnetic contrast in the absence of sample tilt. A large number of 2 pi vertical Bloch lines were observed in low M symmetric samples as well. In order to promote the frequency of vertical Bloch lines and DW skyrmions, asymmetric samples with low N were examined. Samples with M = 4-10, N=1 displayed a significant Bloch component in domain walls which diminished with decreasing M. A large number of vertical Bloch lines were observed giving confidence that DW skyrmions would be observed if DMI were increased further. Indeed, dipole-like constrast consistant with DW skyrmions were observed in M=3, N=2 samples which also displayed chiral N'eel walls. These DW skyrmions were observed to also pin domain wall motion and were annihilated by further increasing field strength. Using these experimental results, a magnetic phase diagram is qualitatively forumlated with emphasis on sample thickness and DMI strength in relation to the appearance of 1-pi vertical Bloch lines, n-pi vertical Bloch lines, chiral N'eel walls, and DW skyrmions. The dynamical response of DW skyrmions was examined using micromagentic simulations to be used as a framework for future in situ studies. The current-induced movement of the DW skyrmion up and down a DW is influenced by the direction of the current and the topological charge/winding number of the DW skyrmion. The presence of grains in these simulations exhibited faster motion than seen in the absence of grains. This is explained through pinning of portions of the DW effectively reducing the DW length and increasing DW skyrmion motion. Next, field- and current-driven nucleation of DW skyrmions were examined. DW skyrmions were observed to form from an edge in the simulation via field-driven nucleation. However, stabilization of DW skyrmions following nucelation via electric current remains a challenge.
Last, a procedure for fabricating nanowires via focused-ion beam to study DW skyrmion via in situ LTEM is discussed. Careful consideration of ion implantation/irradiation as well as sample integrity are important in using this method to prepare devices. Preliminary results indicate DWs containing VBLs and DW skyrmions span across these nanowires demonstrating its feasibility as use for devices.

History

Date

23/07/2020

Degree Type

Dissertation

Department

Materials Science and Engineering

Degree Name

  • Doctor of Philosophy (PhD)

Advisor(s)

Vincent Sokalski Marc De Graef

Exports