Nanocomposite Fe1xO=Fe3O4, Fe=Fe1xO thin films prepared by RF sputtering and revealed by magnetic coupling effects

Magnetic and semi-conducting nanocomposite iron oxide thin films have been prepared under bias polarization, by radio-frequency sputtering of a magnetite target. The nature of the phases obtained in the thin films depends on the bias power density. The increase in power density, from 0 to 110mW=cm2, allows the preparation of magnetite, magnetite/wustite and wustite/a-iron nanocomposites successively. Magnetic measurements at low temperature show exchange bias for two-phases films even though the minor phase is not detected by grazing angle X-ray diffraction. The exchange bias can reach very high values of about 4300 Oe. Electrical properties at room temperature are interpreted taking into account both the modifications of the film compactness, and the nature of the phases from which they are made

Nanostructured cobalt manganese ferrite thin films for gas sensor application

Ferrite compounds are very important because of their optical, electrical or magnetic properties. Moreover, many papers relate to their development as possible gas sensor. In this study, we were interested in using cobalt-manganese-ferrite as sensitive layer for CO2 sensor devices. Such an application required a high surface activity, and consequently a small crystallite size and a large surface area. The physical vapor deposition (RF-sputtering) is widely used for thin film synthesis. In this work, porous thin films were obtained from a Co1Mn0.65Fe1 3504 target sputtered under pure argon plasma, by optimizing the deposition parameters (gas pressure, power). The deposition time was adjusted in order to obtain an average thickness of 300 nm. Structural (G-XRD) and microstructural (SEM-FEG, gas adsorption, electron microprobe) analyses were carried out on these thin films. The chemical composition was found to be homogeneous on the whole surface of the samples. The grain size ranged from 10 to 25 nm. The surface enhancement factor (SEF) was about 100 m2/m2, which is equivalent to a specific surface area of 76 m2/g for the ferrite layer. In conclusion, these nanostructured cobalt-manganese-ferrite films appear to be quite suitable for an application as gas sensors

Patterned ferrimagnetic thin films of spinel ferrites obtained directly by laser irradiation

Some spinel ferrites can be oxidized or transformed at moderate temperatures. Such modifications werecarried out on thin films of mixed cobalt copper ferrites and maghemite, by heating small regions with alow-power laser spot applied for about 100 ns. The very simple laser heating process, which can be donedirectly with a conventional photolithographic machine, made it possible to generate two-dimensionalmagnetization heterogeneities in ferrimagnetic films. Such periodic structures could display the specificproperties of magneto-photonic or magnonic crystals

Thin films preparation by rf-sputtering of copper/iron ceramic targets with Cu/Fe=1: From nanocomposites to delafossite compounds

In the Cu–Fe–O phase diagram, delafossite CuFeO2 is obtained for the CuI oxidation state and for the Cu/Fe=1 ratio. By decreasing the oxygen content, copper/spinel oxide composite can be obtained because of the reduction and the disproponation of cuprous ions. Many physical properties as for instance, electrical, optical, catalytic properties can then be affected by the control of the oxygen stoichiometry. In rf-sputtering technique, the bombardment energies on the substrate can be controlled by the deposition conditions leading to different oxygen stoichiometry in the growing layers. By this technique, thin films have been prepared from two ceramic targets: CuFeO2 and CuO+CuFe2O4. We thus synthesized either Cu0/ CuxFe1−xO4 nanocomposites thin films with various Cu0 quantities or CuFeO2-based thin films. Two-probes conductivity measurements were permitted to comparatively evaluate the Cu0 content, while optical microscopy evidenced a selfassembly phenomenon during thermal annealing

Magnetic and semi-conducting nano-composite films of spinel ferrite and cubic zinc oxide

Magnetic and semi-conducting nano-composite films have been prepared under bias polarization, by radio-frequency sputtering of a pure zinc ferrite target. These composite thin films are made of cubic Zn1 − yFeyO monoxide islands inside a spinel ferrite matrix. The relative proportion of each phase depends on the substrate polarization (i.e. bias power). When no bias is applied the films solely display the diffraction pattern of a spinel phase even if some islands inside the film can be observed by electron microscopy. When the bias power is increased, the spinel phase disappears progressively as enhanced formation of islands takes place in such a manner that the cubic Zn1 − yFeyO monoxide is solely revealed by X-ray diffraction for a bias power higher than 5 W. From bibliographical data and calculated phase diagrams, it can be inferred that these phases would require very low oxygen partial pressure, high temperature and mechanical pressure, to be obtained simultaneously by a conventional ceramic process. This underlines the strong potential of radio-frequency sputtering of oxide targets to prepare original oxides or composite materials

CO2 sensing properties of semiconducting copper oxide and spinel ferrite nanocomposite thin film

A new active layer for CO2 sensing based on semiconducting CuO–CuxFe3−xO4 (with 0 ≤ x ≤ 1) nanocomposite was prepared by radiofrequency sputtering from a delafossite CuFeO2 target using a specific in situ reduction method followed by post annealing treatment in air. The tenorite–spinel ferrite nanocomposite layer was deposited on a simplified test device and the response in a carbon dioxide atmosphere was measured by varying the concentration up to 5000 ppm, at different working temperatures (130–475 °C) and frequencies (0.5–250 kHz). The results showed a high response of 50% (Rair/RCO2=1.9) at 250 °C and 700 Hz for a CO2 concentration of 5000 ppm

Nanocomposites of metallic copper and spinel ferrite films: Growth and self-assembly of copper particles

Nanocomposites of metallic copper and iron oxides films have been prepared by RF-sputtering of pure CuFeO2 delafossite target. The films are made of copper and spinel ferrite crystallites of less than 10 nm in diameter. The content of metallic copper and the ferrite composition depend on the sputtering conditions. For the shortest substrate-target distances, films are made of copper and copper substituted magnetite with low copper content. The formation of the metallic and spinel phases is due to the loss of a small quantity of oxygen during sputtering. When annealed under inert atmosphere, nanometric copper particles located in the upper part of the film, move on the surface and grow due to coalescence phenomena. The particle motion can be stopped by small grooves allowing the self-assembly of copper particles

Copper and iron based thin film nanocomposites prepared by radio frequency sputtering. Part I: elaboration and characterization of metal/oxide thin film nanocomposites using controlled in situ reduction process

Copper and iron based thin films were prepared on glass substrate by radio-frequency sputtering technique from a delafossite CuFeO2 target. After deposition, the structure and microstructure of the films were examined using grazing incidence X-ray diffraction, Raman spectroscopy, electron probe micro-analysis and transmission electron microscopy coupled with EDS mapping. Target to substrate distance and sputtering gas pressure were varied to obtain films having different amount and distribution of copper nanoparticles and different composition of oxide matrix. The overall reaction process, which starts from CuFeO2 target and ends with the formation of films having different proportion of copper, copper oxide and iron oxide, was described by a combination of balanced chemical reactions. A direct relationship between the composition of the metal/oxide nanocomposite thin film and the sputtering parameters was established. This empirical relationship can further be used to control the composition of the metal/oxide nanocomposite thin films, i.e. the in situ reduction of copper ions in the target

Copper and iron based thin film nanocomposites prepared by radio-frequency sputtering. Part II: elaboration and characterization of oxide/oxide thin film nanocomposites using controlled ex-situ oxidation process

CuO/CuFe2O4 thin films were obtained on glass substrate by ex situ oxidation in air at 450°C for 12 h from various starting metal/oxide nanocomposites by radio-frequency sputtering technique. The structure and microstructure of the films were examined using grazing incidence X-ray diffraction, Raman spectroscopy, scanning and transmission electronmicroscopies, X-ray photoelectron spectroscopy, and electron probe microanalysis. These studies reveal that a selforganized bi-layered microstructure with CuO (surface layer) and CuFe2O4 (heart layer) was systematically obtained. Due to the porosity of the upper layer formed during annealing, an increase in total thickness of the film was observed and is directly correlated to the oxidation of the metallic copper content initially present in the as-deposited sample. A selforganization in twostacked layers CuO/CuFe2O4with various void fractions ranging from 0 to 41 % can be obtained by controlling the as-deposited elaboration step described in the part I of this paper. The highest porosities were observed for films deposited at low argon pressure and low target-to-substrate distance. Due to their specific self-organization in p- and n-type layers associated with their high porosity, such structured films exhibited the best electrical sensitivity to CO2 gas sensing. The obtained results demonstrated the importance of microstructure control to improve the response of sensing layers

Mössbauer characterisations and magnetic properties of iron cobaltites CoxFe3−xO4 (1 ≤ x ≤ 2.46) before and after spinodal decomposition

Iron cobaltite powders CoxFe3_xO4 (1 ≤ x ≤ 2.46) were synthesized with compositions in between the cobalt errite CoFe2 O4 and Co2.46Fe0.54O4. The cationic distribution of pure spinel phases was determined by Mossbauer spectroscopy: as Co content increases in the spinel oxide, Co3+ cations replace Fe3+ cations in the octahedral sites and Co2+ cations migrate from octahedral to tetrahedral sites. Saturation magnetizations MS measured at 5 K by a SQUID magnetometer were consistent with the values calculated from the cationic distribution. MS decreases as diamagnetic Co3+ cations replace strongly magnetic Fe3+ cations. Two spinel phases were formed by spinodal decomposition of Co1.73Fe1.27O4 phase submitted to a subsequent thermal treatment, one with a high amount of iron Co1.16Fe1.84O4 and one other containing mostly cobalt Co2.69Fe0.31O4. Increase of the experimental MS value obtained after the spinodal decomposition is in accordance with the calculated value deduced from the cationic distribution of the two phases