Micromagnetic Investigations of Magnetic Multilayers and Ferrites by Transmission Electron Microscopy

[摘要] The work described in this thesis is a study of domain structures in Co/Pt multilayers and MnZn-ferrites using Lorentz microscopy. The vast majority of the work is concerned with the multilayers which are produced for use as future magneto-optical media. The basic ideas of ferromagnetism and domain structures together with an outline of the concepts of magneto-optical recording are given in chapter 1. Throughout this thesis, domains are observed using the Foucault, Fresnel and differential phase contrast (DPC) modes of Lorentz microscopy. Chapter 2 explains the techniques used and how they are applied to the materials in question. Also given, is a brief discussion of image formation in a scanning transmission electron microscope (STEM) for DPC imaging which allows induction distributions within thin magnetic films to be mapped. Chapter 3 discusses the deposition and initial characterisation of the multilayers and also reports on the preliminary results achieved from the electron microscopy of these films. Optimum conditions for all the Lorentz modes are given. The main body of results from the Co/Pt multilayers is presented in chapters 4-7. Chapter 4 deals with all the in-situ magnetising results using DPC microscopy. Domain patterns in various remanent states were imaged. Such observations provided great insight into the mechanisms (nucleation and reversal) that determine the shape of the bulk magnetising loop. Remanence loops were constructed from the MDPC images and their shapes agreed well with those measured on the AGFM. An investigation of thermomagnetically written domains (marks) was carried out in chapter 5. Marks written with different parameters were imaged by MDPC and their most important features investigated. It was found that the size of the marks was only weakly dependent on the applied bias field, but increased linearly with increasing laser power. Reverse domains were present in the written mark until the bias field exceeded 30kA.m-1 but above this value the domain boundary became more ragged. A quantitative analysis of the electron signal from a series of marks written with different bias fields was undertaken and the shape of the curve agreed well with actual recording results. In chapter 6 domain wall calculations were carried out and the domain periods modelled for a series of multilayers with increasing numbers of bilayers. The theoretical results agreed best with the experimental measurements when a magnetic characteristic length of 6.7nm was used in the model. Chapter 7 deals with the work carried out during a placement at Philips Research Laboratories. The domain structure of the multilayers was investigated with scanning magnetic force microscopy and the images and results compared with those achieved from Lorentz microscopy. It was found that the greatest spatial resolution and reproducibility was achieved with MDPC microscopy, although SMFM was most useful when imaging written tracks because no specimen preparation was required. Chapter 8 concerns all the work carried out on thinned samples MnZn-ferrite. Domain structures in an ac-demagnetised state were examined for films with different grain sizes. In general it was established that most grains were multidomain. Local variations in the atomic composition across grain boundaries were investigated using energy dispersive x-ray analysis (EDX) and substantial Zn depletion was found in the vicinity of grain boundaries.. Chapter 9 contains conclusions drawn from the observations of chapters 4-8 along with suggestions for the continuation of the work.

[发布机构] University:University of Glasgow

[关键词] Electromagnetics, Condensed matter physics, Applied physics