An Investigation of Magnetic and Semiconductor Materials with Constrained Artificial Superstructures Using Electron Imaging Techniques

[摘要] The work described in this thesis presents an analysis of the magnetic and microstructural properties of small devices fabricated by electron beam lithography. By using this type of specimen preparation process, the shapes of the devices to be studied can be controlled precisely. Two different types of devices were examined, viz. small regularly shaped magnetic particles and secondly, GaAs quantum wires. Although the project was initially two pronged, the amount of data available from the experiments on the magnetic particles shifted the balance firmly in their favour. As a result, most of the chapters are devoted to the experiments on the particles and only chapter 7 is given to the GaAs wires. The first chapter of this thesis reviews the basic concepts of ferromagnetism, domain walls and the theoretical, micromagnetic models relevant to the work. Throughout the thesis, techniques available using electron microscopy are used to examine the properties of the small devices. Chapter 2 describes the basic image formation techniques in the conventional transmission electron microscope, (CTEM), and also the techniques used to reveal the magnetic structure of the particles to be studied. To examine the magnetic structure and behaviour of the small magnetic particles, various experiments were performed using a specially designed magnetising stage which allowed in-situ experiments to be conducted. This stage is also described in chapter 2 as are some theoretical predictions regarding single domain particles. Chapter 3 describes the specimen fabrication technique, and some basic characterisation of the physical and magnetic properties of the particles and films are given in this chapter. The main body of experimental results on the small magnetic Ni 82.5Fe17.5 particles are given in chapters 4 and 5. Different sizes of particles are discussed in each chapter, and a different imaging mode is used. In the latter chapter, hysteresis loops from isolated particles are given for a range of different particles. The previous two chapters described the results on Ni82.5 Fe17.5 particles. The composition of this alloy results in the domain structure of the particles being influenced greatly by the shape of the particle. To study the influence of a different composition of material on the domain structure and magnetic behaviour of the particles, the particles were also fabricated from cobalt. Experiments were conducted on cobalt particles and the results are given in chapter 6. Chapter 7 describes the results obtained from an examination of the microstructural properties of Si doped GaAs quantum wires again fabricated using electron beam lithography. The wires were made using two slightly different processes, and the resultant damage to the wires is described in this chapter. Finally chapter 8 draws some conclusions from the work, and also gives some suggestions for the continuation of the work.

[发布机构] University:University of Glasgow

[关键词] Condensed matter physics