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Title: Nanoanalytical electron microscopy of cobalt ferrite thin films
Author: Spillane, Liam Jonathan
ISNI:       0000 0004 2699 6387
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2011
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Electron energy­‐loss spectroscopy (EELS) is a powerful method for providing detailed information on the bonding, chemical structure and electronic structure of materials. In this work, EELS has been used to correlate variations in magnetic properties of cobalt ferrite films with film thickness and post‐processing conditions. Magnetometry performed on as‐deposited and oxygen post­‐annealed films has shown saturation magnetization (Ms) to be strongly affected by post processing. This has been attributed to an enhancement in superexchange by reoxidation and cation ordering processes during post­‐anneal. To date this has not been confirmed using nanoanalytical techniques. This work addresses this issue. In particular, it is of interest to determine local changes in the degree of inversion of the ferrite spinel in order to link local chemical changes to bulk magnetic properties. Two sample preparation techniques were used to produce electron transparent sections – conventional ion beam milling and focussed ion beam (FIB) milling using a dual beam system. The suitability of each technique is discussed in terms of, sample damage, thickness, reproducibility and reliability. Aberration corrected HRTEM was used to investigate the microstructure of the thin films. Lattice strain and defect strain were quantified at increasing distance from the substrate/interface in as‐deposited and oxygen post­‐annealed cobalt ferrite films and structural defects responsible for misfit accommodation were characterised. Local variation in cation valence and coordination cobalt in an oxygen post­‐annealed film was investigated by monochromated EELS of the iron and cobalt L2,3­‐edges in the electron energy­‐loss spectrum. A method to determine the spinel degree of inversion (λ) by multiple linear least squares fitting was developed using data acquired from reference materials. A commercially available full multiple scattering code (FEFF 8.2) was used to aid interpretation of reference spectra and the fitting technique used to determine λ was applied to the cobalt ferrite thin film in order to identify variations in λ.
Supervisor: McComb, David Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral