Hematite Films Research Articles

Characterization and gas-sensing properties of nanocrystalline iron(III) oxide films prepared by ultrasonic spray pyrolysis on silicon

Thin films of nanocrystalline hematite (α-Fe 2O 3) are deposited on silicon substrates by nebulization of a 0.01 M solution of iron acetylacetonate in an ethanol/water mixture, followed by pyrolysis and annealing in flowing air. The resulting films are characterized by X-ray diffraction, scanning electron microscopy, atomic force microscope, IR transmission spectrum and Raman Spectra, and the gas-sensing properties of as-deposited films are investigated as well. The resulting films consist of spherical particles and porous, columnar order structure for Si(111) with several nm order of magnitude of crystal grain size, and highly crystalline α-Fe 2O 3 films with (104) orientation are obtained.

Effects of temperature and atmosphere on the epitaxial growth of hematite (α–Fe 2O 3) films on the R-, A- and C-planes of sapphire (α–Al 2O 3) by coating-pyrolysis process

Epitaxially grown hematite (α-Fe 2O 3) films were obtained on the R-, A- and C-planes of sapphire (α-Al 2O 3) substrates by a coating-pyrolysis process in air and in an argon atmosphere. The X-ray diffraction θ–2 θ scanning and pole-figure analysis showed that the films have grown epitaxially to the substrate surfaces after heat treatment at 500°C and higher. The crystallinity of the films was evaluated in terms of a full width at half maximum (FWHM) of the α-Fe 2O 3 104 peak in the φ scans. The crystallinity of the films increased after annealing in air up to 1000°C, while the highest epitaxy was obtained at 600°C in argon. The degraded crystallinity of the films prepared in argon at higher temperatures is attributed to the reaction between the film and the substrate due to a decrease in local oxygen partial pressure caused by an organic residue in the prefired films.

Nucleation and Growth of Phosphate on Metal Oxide Thin Films

The kinetics of phosphate uptake by hematite, titania, and alumina were examined by exposing freshly prepared thin films to phosphate solutions at incremental times and subsequently analyzing the surfaces by Auger electron spectroscopy (AES), temperature programmed desorption (TPD), and atomic force microscopy (AFM). Thin film hematite exposed to a sodium phosphate solution demonstrated initially rapid phosphate chemisorption during the first 10 min of solution exposure, followed by island growth. For titania and alumina exposed to a calcium phosphate solution, the initially rapid reaction was completed after 1 and 3 h, respectively. Subsequent rapid three-dimensional growth occurred after 3−4 h for titania and 25−30 h for alumina. The trend in the rate of formation of phosphate film on titania and alumina was comparable to those of in-situ studies using the same solutions. AFM data on the 10−20-μm scale did not show distinct changes between clean and phosphate-covered alumina and iron oxide surfaces; how...

Formation of Nickel Ferrite Films by Solid-State Reaction

Nickel ferrite (NiFe2O4) thin films are of potential interest for magnetic applications. In the present study, the production of NiFe2O4 by solid-state reaction between thin films of hematite (α-Fe2O3) and nickel oxide (NiO) on (0001) sapphire (α-Fe2O3) substrates has been examined. The NiFe2O4 thin films were prepared by two different methods. In the first case the NiFe2O4film was grown in situ in the deposition system, while in the second case the NiFe2O4 film was formed ex situ by reacting at elevated temperatures in air. These two methods of reaction lead to interesting morphological differences in the ferrite layers.Epilayers of α-Fe2O3 followed by NiO were deposited onto (0001) α-Al2O3 by pulsed-laser deposition (PLD) in 6 mTorr O2. NiFe2O4 films were obtained by reacting the starting films in two different ways: in situ (during film growth) and ex situ.. In both cases, the α-Fe2O3 films were grown under the same conditions while those for the deposition of the NiO layers were different.

Preparation and characterization of iron(III) oxide (α-Fe 2O 3) thin films hydrothermally

Thin films (0.12μm) of hematite (α-Fe 2O 3) were prepared on Si(111) and Si(100) substrates by hydrothermal treatment of a solution of iron nitrate containing urea in the temperature range of 100–200 °C for 4–24h. The films consisted of spherical grains for Si (111) and columnar grains for Si(100) with a typical grain size 10nm and 30nm × 5nm, respectively. Both films deposited on Si (111) and Si(100) have (101) preferred orientation.

Use of Thin‐Film Substrates to Study Enhanced Solid‐State Phase Transformations

A thin‐film substrate geometry is described for the study of enhanced solid‐state phase transformations. A thin film of the seed material is grown on a suitable bulk substrate forming the thin‐film substrate. A sol of the material for which seeding is to be studied is then deposited and the sample heated. Cross sections are prepared to study both the extent of the transformation and its crystallography, using electron microscopy. The special feature of this approach is that the position of the original seed/coating interface is known precisely and the thickness of the transformed layer can be measured directly at different stages of the transformation. As an example of this approach, thin films of hematite have been used as substrates for the study of the seeded phase transformation of a boehmite‐derived transition‐alumina to α‐Al2O3

Photoelectrochemical Studies on Nanocrystalline Hematite Films

Nanocrystalline hematite films supported on conducting glass have been prepared from colloidal Fe{sub 2}O{sub 3} particles, giving a roughness factor of about 30. The current-potential characteristic showed an abnormal drop in photocurrent under reverse polarization. Photocurrent yields are low over the entire potential range. In the presence of iodide the abnormal drop was removed and the photocurrent yield increased. Action spectra were determined by illumination from the electrolyte side and the conducting glass support, respectively. The incident photon-to-current conversion efficiencies (IPCE) is a factor of 100 higher in the former compared to the latter case if water is used as a hole scavenger. In the presence of iodide this ratio is diminished 4-fold due to an increase in the IPCE value measured by illumination from the electrolyte side. The poor photocurrent yields observed are due to charge carrier recombination. Recombination at surface sites can be partially intercepted by adsorbed iodide. 9 refs., 9 figs.

The use of thin Film Substrates to Study Enhanced Solid-State Phase Transformations

AbstractA thin-film substrate geometry is described for the study of enhanced or seeded solid-state phase transformations. As an example of this approach, thin films of hematite have been used as substrates for the study of the seeded phase transformation of a boehmite-derived transition-alumina to α-A12O3. The hematite films were grown on bulk (0001) α-A12O3 single crystal substrates by pulsed-laser ablation. A layer of a boehmite sol was then spin-coated onto the thin film. The assemblages were then heated to 950°C, or 1000°C in order to induce the phase transformation. Specimens were imaged in cross section by transmission electron microscopy. No transformation was observed for specimens heated to 950°C. In specimens heated to 1000°C, the transition alumina was found to transform to alpha-alumina, starting at the surface of the hematite film, via solid-state heteroepitaxy. In this case, islands, growing out from the hematite film into the transition alumina layer, were observed.

Growth of hematite on (0001) and {1102} sapphire substrates

Defects in thin films are often introduced at the substrate-film interface during the early stages of growth. The interface structures of semiconductor heterojunctions have been extensively studied because of the electrical activity of defects in these materials. Much less attention has been paid to the structure of oxide-oxide heterojunctions. In this study, the structures of the interfaces formed between hematite (α-Fe2O3) and two orientations of sapphire (α-Al2O3) are examined in relationship to the defects introduced into the hematite film. In such heterojunctions, the oxygen sublattice is expected to have a strong influence on the epitaxy; however, defects which involve only the cation sublattice may be introduced at the interface with little increase in interface energy.Oxide heterojunctions were produced by depositing small quantities of hematite directly onto electrontransparent sapphire substrates using low-pressure chemical vapor deposition. Prior to deposition, the ionthinned substrates were chemically cleaned and annealed at 1400°C to give “clean”, crystalline surfaces. Hematite was formed by the reaction of FeCl3 vapor with water vapor at 1150°C and 1-2 Torr. The growth of the hematite and the interface structures formed on (0001) and {102} substrates have been studied by bright-field, strong- and weak-beam dark-field imaging techniques.

Formation of ferrite thin films by vacuum evaporation with annealing process

Cobalt ferrite thin films were prepared in successive processes by vacuum evaporation of cobalt on hematite (αFe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> ) films and annealing at relatively low temperature. The surfaces of these films were observed by transmission electron micrograph(TEM) and scanning electron micrograph(SEM), also their structures were identified using transmission electron diffraction(TED) and reflection electron diffraction(RED) methods. Then, a diffusion profile of depth distribution of cobalt ions into (αFe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> ) layer was investigated by Auger electron spectroscopy(AES) analysis. Magnetic properties were measured with a vibrating sample magnetometer. Results showed that cobalt ions migrated easily into a (αFe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> ) layer at relatively low temperature and formed ferrite thin films in the form of Co <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3-x</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> . Their magnetic properties, such as high coercive force, large residual magnetization and smooth surface, indicate considerable possibilities to obtain excellent magnetic medium for high density recording disk.

Ferrite thin films for high recording density

Ferrite thin films with superior mechanical and magnetic properties for a magnetic disk medium have been obtained using reactive sputtering. Recording characteristics of these films considerably exceed those of a conventional γ-Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> coating medium. Hematite thin films were produced on an aluminium alloy substrate by sputtering Fe-Al-Co alloy target in an atmosphere of mixed argon and oxygen. Hematite films were transformed to ferrite films by reduction in hydrogen atmosphere for 2-4 hours at 300°C. The substrate used was covered with a thin alumite layer formed by anodic oxidation to improve surface hardness.

Ferrite Film Recording Surfaces for Disk Recording

Ferrite thin films have been prepared using a chemical deposition process; hematite and substituted hematite films were formed by spin coating a diluted solution of the metal nitrates on a substrate and subsequently heating the substrate in air to 300°C to crystallize the film. Magnetic ferrite films were formed by reducing the films in a wet hydrogen atmosphere. Process parameters, which have evolved from studies on spin coating and reduction on 7.62- and 35.56-cm substrates, have been determined that result in desirable magnetic properties. Experimental studies of film composition and morphology are reported. It has been determined both theoretically and experimentally that film thickness near 0.125 µm is optimum for high-density recording with heads with gap lengths of approximately 1 µm spaced about 0.5 µm from the film. A TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> undercoat (0.125 µm) on the Al-Mg alloy substrate was prepared by chemical vapor deposition and resulted in improved magnetic properties. Magnetic properties of the films and magnetic recording performance of disks using Ti and Al substrates with the TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> undercoat are reported.

The Annealing of Oxide Films on Iron

The annealing reaction in vacuum of thin films of magnetite and hematite on iron has been studied at 463°C. The gravimetric curves obtained for growth of magnetite and decay of hematite have been compared with the curves of growth of both during oxidation at 5.75 cm pressure of oxygen. The flow of iron through the magnetite during annealing is one fourth its value during the oxidation.