An Investigation of the Magnetic Structure in Small Regularly Shaped Particles Using Transmission Electron Microscopy

[摘要] The work described in this thesis is a study of the domain structures in small regularly shaped particles of a soft magnetic material using the techniques of Lorentz microscopy. By using such a material the observed domain structures are primarily determined by the shape of the particle. The basic concepts of ferromagnetism and a discussion of the theoretical framework of micromagnetics is given in chapter 1. Electron microscopy is a powerful tool for the observation of the magnetisation distributions within magnetic materials and in chapter 2 the most important techniques are reviewed in context with the two modes used throughout this work namely the Fresnel and differential phase contrast (DPC) mode. Also briefly discussed in chapter 2 is the theory of image formation in a scanning transmission electron microscope (STEM) for DPC imaging which allows the induction distributions within thin film magnetic objects to be mapped. Previous studies of regularly shaped magnetic particles are reviewed in chapter 3 along with a discussion of the different types of domain walls encountered in magnetic thin films. The shapes to be studied in this project are introduced in this chapter. Fabrication of the particles was performed using the technique of electron beam lithography and the implementation of this process is described in chapter 4. The main body of results in this thesis is presented in chapters 5-7. Chapter 5 deals with preliminary studies of the square and rectangular shapes (PAT1) using the Fresnel mode which was useful in identifying the domain geometries within the particles. The regular domain structures observed in these particles were categorised and the variation of the domain structure with the precise shape of the particle was noted. Further investigation of these structures was made using the DPC mode and described in chapter 6. This mode allowed confirmation of the domain structures observed in chapter 5 to be made as well as providing direct observation of stray fields emanating from particles which did not possess flux closure domain configurations. The techniques used to obtain quantitative information on the domain wall widths are also given in chapter 6. Variation of the particle shape and its influence on the observed domain structures is the subject of chapter 7 where the diamond (PAT2) and triangular (PAT3) shapes are investigated. Chapter 8 contains conclusions drawn from the observations of chapters 5-7 along with suggestions for possible continuation of this work.

[发布机构] University:University of Glasgow

[关键词] Condensed matter physics