Influence of parasitic phases on the properties of BiFeO3 epitaxial thin films

We have explored the influence of deposition pressure and temperature on the growth of BiFeO3 thin films by pulsed laser deposition onto (001)-oriented SrTiO3 substrates. Single-phase BiFeO3 films are obtained in a region close to 10-2 mbar and 580C. In non-optimal conditions, X-ray diffraction reveals the presence of Fe oxides or of Bi2O3. We address the influence of these parasitic phases on the magnetic and electrical properties of the films and show that films with Fe2O3 systematically exhibit a ferromagnetic behaviour, while single-phase films have a low bulk-like magnetic moment. Conductive-tip atomic force microscopy mappings also indicate that Bi2O3 conductive outgrowths create shortcuts through the BiFeO3 films, thus preventing their practical use as ferroelectric elements in functional heterostructures.Comment: sumbitted to Appl. Phys. Let

Fundamental principles for the development of new functional nanocomposite materials based on oxide 3D nanostructures for applications in THz optics and information transfer

International audienceThe purpose of this work is development of the solid-state materials possessing a small absorption combined with sufficient refraction index in the THz range. The main criteria, which were used for the material selection, were quite simple: inorganic compound with strong chemical bonds, stoichiometric composition, small concentration of delocalized charge carriers and sufficient polarizability over THz frequency range. It is obvious, that inorganic materials match these criteria in the greatest degree and, first of all, stable oxides with a high fusion temperature: Al2O3 , SiO2. In this work we studied monolithic nanofibrous alumina for the THz frequency range, which synthesis process was previously developed

Synthesis and photoluminescence properties of nanostructured mullite/α- Al2O3Al_2O_3

Phase transformation of α-Al2O3 to mullite was observed by correlated X-ray diffraction and low-temperature (7 K) time- and energy-resolved photoluminescence methods. The aluminosilicate solids with different Si:Al ratios were prepared after thermal treatment (1400 °C) of ultraporous alumina (UPA) monoliths impregnated with vapour of silica precursors at room temperature. The initial dispersion of Si is homogeneous on a size-scale of 5 nm raw UPA fibres, which grow in size during the thermal treatment stage up to ∼50–100 nm. Three phases were resolved in the considered SiO2/Al2O3 system: silica, α-Al2O3 and their interaction product 2:1 mullite (2Al2O3:SiO2). The 2:1 mullite mass increases and α-Al2O3 mass decreases with an increase of the initial silica content. At the SiO2 content in the initial system above 20 mol.%, the phase transformation to 2:1 mullite is complete and no α-Al2O3 was observed. No mullite phases with either lower or higher Si content were observed. The fundamental absorption onset energy 7.55 eV of 2:1 mullite was measured

Heteroepitaxial growth of ZrO

We have studied Zr1−XCeXO2 film growth on (001) Si by Pulsed Laser Deposition using sputtering of metallic alloy targets and sintered ceramic targets. The conditions of the epitaxial growth have been found and optimized. The epitaxial oxide film growth (001) [100]||(001) [100] Si was obtained for a range of CeO2 content in ZrO2: from 4.5 up to 14% mol in ZrO2. The oxide film structure corresponds to a tetragonal phase with strong preference for c-axis orientation normal to the growth surface. The results obtained by RHEED, XRD and AFM methods have confirmed the high quality of heteroepitaxial Zr1−XCeXO2 layers, and the difference in crystallinities for the films grown from metallic alloy targets and ceramic targets was evaluated. The XRD results show the absence of any reflection distinct from (00l) and (l00) for films grown from alloy targets, and, in contrast with this, the film structure contains some random oriented inclusions in the case of oxide target deposition. Use of metallic alloy Zr-12% Ce targets and low oxygen pressure during deposition provide the best film quality with the minimum of surface microrelief (Rrms < 0.3 nm for 1 × 1 µm2 surface area was achieved)

Pulsed laser deposition of CeO

SrTiO3 had been often tentatively used as an insulating barrier for HT superconductor/insulator heterostructures. Unfortunately, the deposition of SrTiO3 on the YBa2Cu3O7 inverse interface results in a poor epitaxial regrowth producing a high roughness dislocated titanate layer. Taking into account the good matching with YBa2Cu3O7 and LaAlO3, CeO2 and Ce1−xMxO2 (M = La, Zr), epitaxial layers were grown by pulsed laser deposition on LaAlO3 substrates and introduced into YBa2Cu3O7 based heterostructures as insulating barrier. After adjusting the growth parameters from RHEED oscillations, epitaxial growth is achieved, the oxide crystal axes being rotated by 45° from those of the substrate. The surface roughness of 250 nm thick films is very low with a rms value lower than 0.5 nm over 1 μm2. The YBa2Cu3O7 layers of a YBa2Cu3O7/CeO2 /YBa2Cu3O7 heterostructures grown using these optimized parameters show an independent resistive transition, when the thickness is larger than 25 nm, respectively at Tc1 = 89.6K and Tc2 = 91.4K

Porous monoliths consisting of aluminum oxyhydroxide nanofibrils: 3D structure, chemical composition, and phase transformations in the temperature range 25–1700 °C

We present a study on the chemical and structural transformations in highly porous monolitic materials consisting of the nanofibrils of aluminum oxyhydroxides (NOA, Al2O3·nH2O) in the temperature range 20–1700 °C. A remarkable property of the NOA material is the preservation of the monolithic state during annealing over the entire temperature range, although the density of the monolith increases from ~0.02 up to ~3 g/cm3, the total porosity decreases from 99.3 to 25% and remains open up to 4 h annealing at the temperature ~1300 °C. The physical parameters of NOA monoliths such as density, porosity, specific area were studied and a simple physical model describing these parameters as the function of the average size of NOA fibrils—the basic element of 3D structure—was proposed. The observed thermally induced changes in composition and structure of NOA were successfully described and two mechanisms of mass transport in NOA materials were revealed. (i) At moderate temperatures (T ≤ 800 °C), the mass transport occurs along a surface of amorphous single fibril, which results in a weak decrease of the length-to-diameter aspect ratio from the initial value ~24 till ~20; the corresponding NOA porosity change is also small: from initial ~99.5 to 98.5%. (ii) At high temperatures (T > 800 °C), the mass transport occurs in the volume of fibrils, that results in changes of fibrils shape to elliptical and strong decrease of the aspect ratio down to ≤ 2; the porosity of NOA decreases to 25%. These two regimes are characterized by activation energies of 28 and 61 kJ/mol respectively, and the transition temperature corresponds to the beginning of γ-phase crystallization at 870 °C

Controlling high-mobility conduction in SrTiO3 by oxide thin film deposition

International audienceSrTiO3 becomes a high-mobility metallic conductor when doped with oxygen vacancies at low concentrations (≥1016 cm−3). We show that the vacancy concentration in the SrTiO3 single crystal substrates could be controllably tuned by changing the thickness of oxide films (deposited by pulsed laser deposition at high temperature and low oxygen pressure). The obtained variation in the carrier density strongly influences the transport properties. The quantitative analysis of the experimental results leads toward new and accurate strategies for the design of multifunctional oxide heterostructures for electronics and spintronics

Structural Analysis of Aluminum Oxyhydroxide Aerogel by Small Angle X-Ray Scattering

The work presents studies on the microstructure and mesostructure of nanostructured aluminum oxyhydroxide formed as a high porous monolithic material through the surface oxidation of aluminum liquidmetal solution in mercury in a temperature- and humidity-controlled air atmosphere. The methods of X-ray diffraction analysis, thermal analysis, the low temperature adsorption of nitrogen vapors, transmission electron microscopy, small-angle and very small-angle neutron scattering, and small-angle X-ray scattering are used for comprehensive investigation of the samples synthesized at 25°С as well as that annealed at temperatures up to 1150°C. It is found that the structure of the monolithic samples can be described within the framework of a three-level model involving primary heterogeneities (typical length scale of rc ≈ 9–19 Å), forming fibrils (cross-sectional radius R ≈ 36–43 Å and length L ≈ 3200–3300 Å) or lamellae (thickness T ≈ 110 Å and width W ≈ 3050 Å) which, in turn, are integrated into large-scale aggregates (typical size R c ≈ 1.25–1.4 μm) with an insignificant surface roughness. It is shown that a high specific surface (~200 m2/g) typical for the initial sample is maintained upon its thermal annealing up to 900°С, and it decreases to 100 m2/g after heat treatment at 1150°С due to fibrillary agglomeration