Theoretical and experimental study of shape effects on magnetic nano-particles using simulation-assisted Lorentz microscopy

Characterization usually involves analysis of experimental data, often images. While image interpretation is usually simple for optical microscopy, it is in general non-trivial for transmission electron microscopy. A thorough analysis of experimental images involves working backwards to develop a model of the microstructure, such that the image computed from the model agrees with the observed image. Such a procedure can then be used to fine-tune the microstructure model. This is often an iterative and time-consuming process.; This characterization cycle is discussed in detail for Lorentz microscopy, a tool to map the magnetic microstructure of materials. This thesis shows how real-time characterization of magnetic materials can be made more accurate and robust by the development and deployment of a phase extraction tool (Transport-of-Intensity formalism) and a mathematical technique to model this phase (Fourier space approach).; An important component of the characterization process is the characterization tool itself. For Lorentz microscopy, this is an in-situ magnetizing stage, a modified sample stage that generates a uniform in-plane magnetic field around the sample to enable a dynamic study of magnetic domain wall behavior. Based on high temperature superconductors, a novel design for a machine tool to aid experimental Lorentz microscopy is proposed.; One important factor in determining the properties of nanoparticles is the particle shape. This aspect has been least studied so far due to the complexity in mathematically accounting for shape effects. We have studied shape effects using Fourier space formalism. In doing so, we have solved the demagnetization tensor field of an arbitrary shaped particle at any point in 3D space. An analytical expression is worked out for a cylindrical particle. Knowledge of this tensor field is required to predict the field distribution around a uniformly magnetized particle.; Based on the Fourier space formalism, we have also worked out an expression for magnetostatic interactions between particles, and have used it to generate phase diagrams to study how two cylindrical shaped particles behave in close proximity. We have derived analytical expressions for the magnetostatic energy for nano-rings, and constructed phase diagrams describing the energetically stable magnetization state for given material and shape parameters.