Magnetite (Fe3O4) is of both scientific and technological interest because of its fascinating magnetic properties. It has a high Curie temperature of 860 K and a theoretical 100% spin polarization at the Fermi level. There are a variety of deposition techniques to form thin films of magnetite, such as molecular beam epitaxy (MBE), pulsed laser deposition (PLD), iron oxidation, sputtering and so on. In comparison with other deposition methods mentioned above, electrodeposition has a key advantage of relatively low processing temperature. The intention of this work was to investigate magnetite (Fe3O4) thin films grown via an electrochemical route by using various kinds of characterization techniques, especially on morphology, chemical composition, structure and magnetic properties. Fe3O4 thin films were obtained by using a galvanostatic or potentiostatic deposition from simple aqueous solutions of ferrous salts. Iron oxide thin films have been grown at different current densities and temperatures onto polycrystalline copper substrates. XRD results indicate that Fe3O4 is formed at 90 oC at an applied current density of 0.05 mA·cm-2. Lower growth temperatures can cause the formation of another phase, a-FeOOH at a certain concentration of Fe2+ and pH buffer. Time-dependent growth of the iron oxides exhibits nucleation and coalescence. In order to obtain uniform Fe3O4 film surface, longer deposition times are needed. The influence of applied potential on the characteristics of the deposited iron oxide was examined. The formation of Fe3O4 in a low potential regime (< 100 mV) vs. gold reference electrode while iron oxyhydroxides such as goethite (a-FeOOH) and lepidocrocite (?-FeOOH) are favoured for E > 100 mV. The magnetic properties of the films were found to be strongly dependent on the deposition potential. The multi-layer structure of Fe3O4/a-FeOOH/Fe3O4 onto NiO/Ni substrates has been demonstrated via successive deposition. A TEM cross-section image shows a-FeOOH is coherently formed between two ferromagnetic layers. ADF-STEM micrographs show that Fe3O4 has a columnar structure and has less composition variation compared to that grown onto a polycrystalline copper substrate. Synchrotron techniques, i.e. x-ray absorption near edge structure (XANES) and x-ray magnetic circular dichroism (XMCD), were performed to examine the iron oxide film. Fe K-edge x-ray absorption spectra demonstrate that the films grown at low potential regime (< 100 mV) have a comparable valency state with the standard Fe3O4 sample. The identification of the iron oxide was further confirmed by using XMCD technique. The calculation of the asymmetry ratio suggests that the total magnetic moment increased with decreasing applied potential. In addition, vibrating sample magnetometer (VSM) data show that the magnetic response is somewhat slower for the iron oxide grown at higher potential regime. A change of pH in the electrolyte does not change the lattice constant and film morphology or texture but does affect particle sizes in Fe3O4 thin films. This decrease with the pH is due to the reaction of FeOH+ ions with molecular oxygen in electrolyte.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:503164 |
| Date | January 2008 |
| Creators | Teng, Chien-Lung |
| Contributors | Ryan, Mary |
| Publisher | Imperial College London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10044/1/4260 |
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