Investigation of Magnetic Microstructures Using Novel Transmission Electron Microscopy Techniques

Johnston, Alan Biggar (1995) Investigation of Magnetic Microstructures Using Novel Transmission Electron Microscopy Techniques. PhD thesis, University of Glasgow.

Abstract

The work described in this thesis is concerned with the high spatial resolution magnetic imaging which can be realised on the Philips CM20 transmission electron microscope (TEM). Central to the thesis is the novel TEM technique of coherent Foucault (CF) imaging which is used here to investigate small regularly shaped magnetic elements. In particular details are given relating to studies of the image formation process analytically, by computer simulations and, finally, by implementing CF imaging experimentally on the CM20. In the final chapters we look at Fe3O4 thin films which show a very complex, small scale magnetic microstructure, and we describe a TEM and compositional study of NiFe- Ag thin films. The first chapter begins with an outline of basic ferromagnetism and the energy considerations governing domain configurations in ferromagnetic thin films. There is a brief discussion about the magnetic microstructure found in small elements and the different types of domain walls commonly found in thin films. The second chapter considers the instrumental requirements of a TEM for imaging magnetic microstructure and formally introduces the Philips CM20 TEM at the University of Glasgow. Here we discuss the flexibility of the imaging column, along with the range of modifications which provide a 'micromagnetic laboratory' around the specimen region. We then consider the interaction between the incident electron beam and a ferromagnetic or electrostatic specimen. Finally, we discuss image formation in the TEM and the Lorentz imaging modes which are used to reveal magnetic microstructure. An analytical description of coherent Foucault imaging provides insight into how a fringe pattern relating to the induction distribution within a magnetic specimen is produced. The interferogram can be thought of as an 'in-line hologram' which is capable of providing a quantitative description of the induction distribution across the specimen. In chapter 3 a basic theory shows how a suitable model is created and how the corresponding diffraction pattern and image are calculated. These ideas are expanded by analysing two systems, a semi-infinite uniformly magnetised thin film and a two domain element. Here, we discuss the CF images which are obtained when an opaque aperture is positioned carefully in the back focal plane (BFP). Finally, the advantages of using a phase-shifting aperture, rather than the opaque aperture, are discussed. The one-dimensional computer simulation of CF imaging, presented in chapter 4, goes beyond the simple analytical study and allows us to investigate how fringe visibility and spacing vary with precise positioning of the aperture in the diffraction plane. CF imaging with a small-hole phase-shifting aperture is also studied, with specific attention being paid to how the imaging conditions vary with the size of the small-hole. The magnitude of phase-shift introduced by the phase-shifting apertures is then discussed, to find how important its precise value is for experimental use. Thereafter, calculations are performed in which the angle subtended by the source is extended to show why a FEG source is favoured for CF imaging. These considerations are further discussed in the context of two-dimensional simulations which facilitate direct comparison with experimental images. A simple two domain element and a small element containing four domains in a closure structure are investigated. In chapter 5 we discuss the implementation of CF imaging on the modified Philips CM20 TEM to obtain magnetic interferograms of small permalloy elements. These have been fabricated by electron beam-lithographic and evaporation techniques to create individual elements of thicknesses of 50nm and 30nm with in-plane dimensions between 0.2mum and 4.0mum. Elements studied were rectangular, triangular or rhomboidal in shape, forming a near-ideal model micromagnetic system. Their dimensions are comparable with what might be expected for the next generation magnetoresistive sensors in ultra-high density recording systems. Initially, the different aperture types are discussed to confirm the CF imaging theory discussed in chapters 3 and 4. High visibility fringes with the predicted periodicity are apparent across the magnetic element as a whole and give a direct and immediate picture of the spatial variation of induction. This study also provides insight into the effect of size and shape on the magnetic properties of the elements. Thereafter, the elements are investigated while being magnetised along their easy and hard axes. The effect of increasing the temperature in-situ is considered, the domain structures and the changes in the magnetisation of the elements being analysed as they approach the Curie temperature. Finally, two different samples prepared using different etching techniques are compared, for the purpose of determining how the magnetic properties of material change during the fabrication process. (Abstract shortened by ProQuest.).

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: I N Chapmen
• Keywords: Applied physics, Analytical chemistry, Electromagnetics
• Date of Award: 1995
• Depositing User: Enlighten Team
• Unique ID: glathesis:1995-75625
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Nov 2019 19:15
• Last Modified: 19 Nov 2019 19:15
• URI: https://theses.gla.ac.uk/id/eprint/75625