Application and development of advanced Lorentz microscopy techniques for the study of magnetic nanostructures

Beacham, Robert J. (2013) Application and development of advanced Lorentz microscopy techniques for the study of magnetic nanostructures. PhD thesis, University of Glasgow.

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Understanding the magnetic structure and behaviour of domain walls has attracted recent attention in both academia and industry as it is of fundamental importance for the development of magnetic storage media. At the University of Glasgow we specialise in direct magnetic imaging through Lorentz electron microscopy. This PhD project presents an investigation into the development of magnetic imaging methods in the TEM and their application in imaging narrow domain walls in multilayer magnetic structures. Lorentz microscopy techniques are limited in quantitative magnetic imaging as this generally requires using scanning imaging modes which limits the capability of imaging dynamic processes. The first imaging method developed in this study is a phase gradient technique with the aim of producing quantitative magnetic contrast proportional to the magnetic induction of the sample whilst maintaining a live imaging mode. This method uses a specifically engineered, semi-electron-transparent graded wedge aperture to controllably perturb intensity in the back focal plane. The results of this study found that this method could produce magnetic contrast proportional to the sample induction, however the required gradient of the wedge aperture made this contrast close to the noise level with large associated errors. In the second part of this study we investigated the development of a technique aimed at gaining sub-microsecond temporal resolution within TEMs based on streak imaging. We are using ramped pulsed magnetic fields, applied across nanowire samples to both induce magnetic behaviour and detect the electron beam across the detector with respect to time. We are coupling this with a novel pixelated detector on the TEM in the form of a Medipix/Timepix chip capable of microsecond exposure times without adding noise. Running this detector in integral mode and allowing for practical limitations such as experiment time and aperture stability, the resultant streak images were taken in Fresnel, Foucault and low angle diffraction imaging modes. We found that while this method is theoretically viable, the limiting factor was the contrast of the magnetic signal in the streak and therefore the total image counts. Domain walls (DWs) in synthetic antiferromagnetically (SAF) coupled films patterned as nanowires offer exciting possibilities; the domain walls in these multilayers have narrower widths and reduced magnetostatic energy compared to those in single layer nanowires. In this study Co90Fe10/Ru/Co90Fe10 films were used to investigate the existence and structure of these walls. Nanowires were fabricated in these films and the DW structure was studied with respect to both wire width and varying magnetic layer thickness. It was found that while the DW structure does not appear to vary with the range of wire widths used, it changed significantly with varying thickness. The narrow DWs were observed to form only in samples with an unbalanced ratio of 1:25 : 1 or below; 1:75 : 1 and 2 : 1 samples were both observed to form transverse domain walls.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: TEM, Magnetism, Nanostructures, Antiferromagnets, Imaging techniques
Subjects: Q Science > QC Physics
Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
Supervisor's Name: McVitie, Dr. Stephen
Date of Award: 2013
Depositing User: Mr Robert Beacham
Unique ID: glathesis:2013-4763
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 23 Dec 2013 15:13
Last Modified: 23 Dec 2013 15:16

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