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

Raghunathan, Arun 2010. Growth, characterisation and modelling of novel magnetic thin films for engineering applications. PhD Thesis, Cardiff University.

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Abstract

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.

Item Type:	Thesis (PhD)
Status:	Unpublished
Schools:	Engineering
Subjects:	T Technology > TK Electrical engineering. Electronics Nuclear engineering
ISBN:	9781303218651
Date of First Compliant Deposit:	30 March 2016
Last Modified:	10 Jan 2018 04:18
URI:	https://orca.cardiff.ac.uk/id/eprint/55011

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