Growth, characterisation and modelling of novel magnetic thin films for engineering applications

Magnetic materials, especially thin films, are being exploited today in many engineering applications such as magnetic recording heads and media, magnetic sensors and actuators and even magnetic refrigeration due to their smaller form factor or to thin film effects that do not occur in bulk material. Hence there is a need for optimised growth of thin films to suit the requirements of applications. The aim of this research work is two-fold: 1. Growth and characterisation of optimised magnetic thin films using pulsed-laser deposition and 2. Extension of Jiles-Atherton (JA) theory of hysteresis. A series of magnetoelastic thin films based on cobalt ferrite were deposited on SiO2/Si(100) substrates using pulsed-laser deposition at different substrate temperatures and different reactive oxygen pressures. The crystal structure, composition, magnetic properties, microstructure and magnetic domains of cobalt ferrite thin films were investigated. The optimised growth conditions of poly crystalline spinel cobalt ferrite thin films were determined from characterisation results. The Curie point of the optimised cobalt ferrite thin film was determined from moment vs. temperature measurement. The optimised thin film was magnetically annealed in order to induce an in-plane uniaxial anisotropy. The magnetostriction of the optimised sample was determined in the vibrating sample magnetometer using the inverse measurement technique. A special 3-point bender was designed and built for this purpose. The first successful thin film of Gd5Si2Ge2, a magnetocaloric rare earth intermetallic alloy, was deposited on a polycrystalline AlN substrate. The crystal structure, composition and magnetic phase transformation of Gd5Si2Ge2 thin film were investigated. The preliminary results are furnished in this thesis. The JA model of hysteresis was extended to incorporate thermal dependence of magnetic hysteresis. The extended model was validated against measurements made on substituted cobalt ferrite material. A functional form of anhysteretic magnetisation was derived. The JA theory was also extended to model magnetic two-phase materials. This proposed model was qualitatively compared with measured data published in the literature. The JA theory was applied to magnetoelastic thin films. The cobalt ferrite thin films deposited on SiO2/Si(100) substrates at different substrate temperatures and oxygen pressures have been modelled based on JA theory and were validated against measurements. This model would help in understanding the influence of deposition parameters on properties of thin films. The calculated and measured data were in excellent agreement

Optimization of Nonlinear Switch Cells for Switching Converters

Switch cells consist of an array of power switches and passive components which can replace the main switches alone in many power topologies, allowing reduced switching loss without altering the power topology directly. This thesis discusses the development of a switch cell topology that utilizes a saturable resonant inductor to reduce the size and power loss of the cell. Additionally, the cell transfers energy stored in the inductor into a capacitor for efficient energy storage during the cell\u27s conduction region. This energy is then transferred back to the system when the cell turns off, thus reducing the total switching energy

Modeling and simulation of magnetic components in electric circuits

This thesis demonstrates how by using a variety of model constructions and parameter extraction techniques, a range of magnetic component models can be developed for a wide range of application areas, with different levels of accuracy appropriate for the simulation required. Novel parameter extraction and model optimization methods are developed, including the innovative use of Genetic Algorithms and Metrics, to ensure the accuracy of the material models used. Multiple domain modeling, including the magnetic, thermal and magnetic aspects are applied in integrated simulations to ensure correct and complete dynamic behaviour under a range of environmental conditions. Improvements to the original Jiles-Atherton theory to more accurately model loop closure and dynamic thermal behaviour are proposed, developed and tested against measured results. Magnetic Component modeling techniques are reviewed and applied in practical examples to evaluate the effectiveness of lumped models, 1D and 2D Finite Element Analysis models and coupling Finite Element Analysis with Circuit Simulation. An original approach, linking SPICE with a Finite Element Analysis solver is presented and evaluated. Practical test cases illustrate the effectiveness of the models used in a variety of contexts. A Passive Fault Current Limiter (FCL) was investigated using a saturable inductor with a magnet offset, and the comparison between measured and simulated results allows accurate prediction of the behaviour of the device. A series of broadband hybrid transformers for ADSL were built, tested, modeled and simulated. Results show clearly how the Total Harmonic Distortion (THD), Inter Modulation Distortion (IMD) and Insertion Loss (IL) can be accurately predicted using simulation.A new implementation of ADSL transformers using a planar magnetic structure is presented, with results presented that compare favourably with current wire wound techniques. The inclusion of transformer models in complete ADSL hybrid simulations demonstrate the effectiveness of the models in the context of a complete electrical system in predicting the overall circuit performance

An analysis of pulse transformers

This thesis focuses on developing a comprehensive understanding of the behaviour of the output pulse transformer used in an electric fence energiser, a unique application which has received little attention in the literature. Maxwell’s Equations form the starting point for an investigation into the magnetisation of the magnetic core under high current pulse conditions. An equation is derived which describes the time dependent nature of the penetration of the magnetic field into the core material. The concept of a region of high permeability moving into the lamination as a result of rapid saturation is also developed. Quantitative results are obtained for the area of this high permeability region, both from the pulse magnetisation equation and by analytical methods. The analytical approach is also used to derive expressions for the pulse current, output voltage and power, and inductance. The analytical model of the output transformer provides much insight into the transformer’s behaviour, and into the influence upon performance of the various circuit elements. The description of pulse magnetisation is a key factor in the development of both analytical and electrical circuit models of the output transformer. The final part of the thesis considers the output transformer from the perspective of an electrical circuit model, which is developed as a direct representation of the physical transformer. The inductor-reluctance method is used to develop the model, and methods of calculating each parameter are derived, so that the final model may be obtained directly from the transformer’s specification without the need for experimental measurements. The electrical circuit model is implemented using a SPICE simulator, and an analysis of the transformer highlights the major design parameters. The description of pulse magnetisation, the analytical model and the electrical circuit model combine to provide a comprehensive description of the behaviour of the output transformer, and provide the tools required for design and further investigation of this application

Evaluation of structural integrity of steel components by non-destructive magnetic methods

Magnetic non-destructive methods utilising the Magnetic Flux Leakage (MFL) and Magnetic Barkhausen Noise (MBN) phenomena are widely used in the evaluation of the structural integrity of steel components. The MFL method is effectively applied for in-service flaw monitoring of oil and gas pipelines, fuel storage tank floors and rails; whereas the MBN method, due to high sensitivity of Barkhausen emission to residual and applied stress, has become one of the most popular NDE tools for investigating this condition of steels. Despite the affirming research and successful applications, which helped these methods to gain acceptance as a viable non-destructive tools, there is still a requirement for establishing a quantitative links between magnetic and mechanical properties of steel which would enable their further understanding and optimisation. In this thesis the applications of MFL and MBN methods for flaw and stress detection are analysed via analytical and numerical modelling. A new model relating the MBN amplitude and stress for materials having different magnetostrictive behaviour under load is proposed and validated in the quantitative stress evaluation of different grades of steel. Moreover, a new method for determining depth dependence of stress from measured magnetic Barkhausen signals is presented. A complete set of newly derived equations describing the detected Barkhausen signals in terms of the actual emissions that are generated inside the material and how these appear when they propagate to the surface is given. The results from finite element modelling of magnetic flux leakage signals above unflawed and flawed rails energised in various directions are presented. These results enabled to identify the most effective current injection procedure and optimise the probability of transverse flaw detection in the rail inspection. The agreement between modelled and measured electromagnetic signals indicating presence of transverse rail defects has been justified