Micromagentic simulation and Lorentz microscopy of curved ferromagnetic nanowires

Gergov, Hristo (2024) Micromagentic simulation and Lorentz microscopy of curved ferromagnetic nanowires. PhD thesis, University of Glasgow.

Abstract

Ferromagnetic domain walls can be used to encode and propagate information in novel spintronic devices. However, to realize these devices, different materials and geometries that support stable and mobile domain walls need to be investigated. This thesis contributes to the effort by examining domain walls in nanowires with curved profiles using micromagnetic simulations and Lorentz microscopy.

Domain walls in head-to-head cylindrical segment nanowires were explored using micromagnetic simulations. The energy density (a measure of stability) of the resulting domain walls was recorded for each combination of nanowire geometry (half versus quarter cylinder) and nanowire composition (Fe vs. Co). A domain wall with a potential for the field of spintronics due to its narrow extent and strong out-of-plane component was found to stabilise in a wide range of nanowire sizes in both Co and Fe nanowires. Additionally, the effect of the interfacial Dzyaloshinskii-Moriya interaction (iDMI), on the domain wall formation in quarter cylinder nanowires was examined. That resulted in the stabilisation of another promising Néel-like domain wall with a simple one-dimensional rotation of the magnetisation - the 1D wall. Throughout the entire simulation space, the extent of the different domain walls was measured and recorded. Simulated experimental Fresnel mode and differential phase contrast (DPC) images were calculated and used in the quantification of the domain walls comparing magnetisation profiles to those from integrated induction.

Finally, the effect of electrostatic phase on DPC and Fresnel image contrast was examined with respect to domain wall characterisation and classification. Focused electron beam induced deposition (FEBID) was used to deposit physical Co and Fe nanowires of varying width and thickness with a profile similar to the one studied with simulations. The deposition conditions were optimised to produce defined magnetic nanowires with as high saturation magnetisation as possible. Domain walls were successfully nucleated in said nanowires with the application of an external magnetic field and identified using Fresnel mode microscopy. Differential phase contrast (DPC) imaging was then used to obtain accurate maps of the magnetic induction of the nanowires showing a more clear picture of the structure of the different domain walls. Finally, a combination of transmission electron microscopy (TEM), atomic force microscopy (AFM) and DPC imaging was used to obtain a measure of the saturation induction of the FEBID structures and thus infer their elemental purity.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: Q Science > QC Physics
• Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
• Supervisor's Name: McVitie, Professor Stephen
• Date of Award: 2024
• Depositing User: Theses Team
• Unique ID: glathesis:2024-84588
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 27 Sep 2024 09:55
• Last Modified: 27 Sep 2024 09:55
• Thesis DOI: 10.5525/gla.thesis.84588
• URI: https://theses.gla.ac.uk/id/eprint/84588