Papers by Author: Vitor S. Amaral

Abstract: Elemental powder mixtures of Fe and Si were mechanically alloyed with a ball-mill. Mössbauer spectroscopy and X-ray diffraction were used to characterise the microstructural changes of these mixtures which are induced by high-energy ball-milling. Mössbauer spectra are discussed in terms of two main spectral components (corresponding to FeSi alloy and α-Fe) and the time dependence of the alloy formation follows a Johnson-Mehl type law. Calorimetry measurements show that the formed alloy is stable up to 800°C as no crystallisation or phase transformation peaks are observed. From X-ray diffraction, a crystallite size of 9 nm is obtained. Magnetic measurements at low temperature were carried out on the final alloy and the saturation magnetic moment at 4.2K is 0.44 μB/Fe.

Abstract: Superconducting fibres of Bi-Sr-Ca-Cu-O system were grown by the conventional Laser Floating Zone (LFZ) process and by the new technique of Electrical Assisted Laser Floating Zone (EALFZ). The presence of an electric field during solidification process significantly changes the fibre microstructure with Bi2Sr2Ca2Cu3O10 nominal composition. The higher cristallinity of the asgrown EALFZ fibres, resulting from the current application during solidification, leads to a transformation rate enhancement during subsequent annealing. A strong correlation between the transport properties and the final microstructure of the annealed fibres is observed. The highest critical current density of the EALFZ fibres is obtained after heat treatment at 845°C while for the LFZ ones similar values are obtained only after annealing at 860°C.

Abstract: Bulk MgB2 samples were synthesized by hot isostatic pressing under pressures up to 200MPa at 950°C. In these conditions, full densification of samples was obtained (~98% of theoretical density). SEM, EDS and XRD analysis on final dense bodies were used to evaluate samples, and show increasingly better control over the amounts of secondary MgO (down to ~10%) and complete prevention of formation of MgB4 by using simple glass encapsulation techniques and addition of Mg(s) to the capsule. The samples display superconducting properties, including a narrow critical transition in electrical properties (Tc ~36-38K). Magnetic studies were performed, allowing the determination of the superconducting fraction and critical current density Jc of the materials. Contrary to the Tc, the Jc is quite sensitive to the processing and microstructure and values from 0.3 to 0.6x106 A/cm2 are obtained at 10K. The reduction of Jc with the applied magnetic field requires further improvements to reduce weak links.

Abstract: A comprehensive study of CMR manganite related phases, particularly those that develop under the effects of Ca and Mn-excess co-doping is undertaken. The relationships between phase composition, processing and observed crystalline structure are investigated for co-precipitated powders of composition La1-xCaxMn1+zO3+δ, thermally treated at 800 and 1000 °C in air for Ca contents of x=0.10 and x=0.15 and Mn content from z=0 up to z=0.88. A relevant structural transition from orthorhombic to rhombohedric symmetry is observed as a function of Mn excess near 25% A site vacancies. This solid solution is in equilibrium with the conjugated phases, either LaMn7O12 at 800°C, or Mn3O4 at 1000°C, respectively. Results suggest that at the polymorphic phase transition the solid solution is stable enough to slow down Mn diffusion into the perovskite cell. Ca-doping introduces stability in the perovskite structure and broadens the domain for phase formation reaction. The solid solution limit of the Ca-manganite solid solution is set above 50% Mn excess.

Abstract: Ferromagnetism persisting above 375 K and anisotropic ferromagnetic resonance (FMR) spectra have been detected for the first time in Si co-implanted with Mn and As and annealed under appropriate conditions. For comparison, semi-insulating GaAs samples have been implanted with the same ions and subsequently annealed. They also exhibit ferromagnetism with a Curie temperature well in excess of 375 K. High-resolution transmission electron microscopy (TEM) performed on the samples with the best magnetic characteristics has shown the presence of nanoclusters due to the segregation of the implanted species in both Si and GaAs. The angular dependence of the FMR spectra also reveals the existence of magnetic clusters with the hard magnetization axis aligned along the four equivalent <111> crystal axes. The spectra are very similar in Si and GaAs, indicating that the clusters in both materials probably consist of hexagonal MnAs.

Abstract: Nanometric ferrihydrite, maghemite and magnetite particles formed within an organicinorganic hybrid matrix were obtained by the sol-gel process. In contrast to precipitation techniques, sol-gel process appears as suitable way to achieve size-controlled nanoscopic magnetic particles anchored in a hybrid structure. The hybrid matrix here reported, named di-ureasil, is composed of poly(oxyethylene) chains grafted to siloxane groups by means of urea cross-linkages. The formation of ferrihydrite particles was achieved incorporating iron nitrate during the sol-gel process, at low pH. The formation of maghemite takes place after the incorporation of a mixture of Fe3+ and Fe2+ ions and treatment with an ammonia solution, after the sol-gel process. Magnetite nanoparticles are formed after the incorporation of Fe2+ ions and treatment with ammonia at 80°C. The AC magnetic susceptibility shows thermal irreversibility with a blocking temperature TB≈13K and ≈25K depending on frequency for the ferrihydrite and maghemite particles, respectively. The magnetite nanoparticles are blocked at room temperature. Above the irreversibility the magnetization of ferrihydrite and maghemite follows a Langevin function modified with a linear term, as found in antiferromagnetic and ferrimagnetic particles.