Investigation of Electrodeposited Magnetite Films: Formation and Characterization
Author(s)
Teng, Chien-Lung
Type
Thesis
Abstract
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, α-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 (α-FeOOH) and
lepidocrocite (γ-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/α-FeOOH/Fe3O4 onto NiO/Ni substrates has been
demonstrated via successive deposition. A TEM cross-section image shows α-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.
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, α-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 (α-FeOOH) and
lepidocrocite (γ-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/α-FeOOH/Fe3O4 onto NiO/Ni substrates has been
demonstrated via successive deposition. A TEM cross-section image shows α-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.
Date Issued
2008
Date Awarded
2008-11
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Ryan, Mary
Creator
Teng, Chien-Lung
Publisher Department
Department of Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)