Papers by Author: Vitor S. Amaral

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Authors: Armandina M. Lima Lopes, João P. Araújo, A.M. Gomes, Tânia M. Mendonça, Pedro B. Tavares, J.G. Correia, Vitor S. Amaral
Authors: Pedro B. Tavares, Vitor S. Amaral, João P. Araújo, A. Lourenço, J. Bessa Sousa, Joaquim M. Vieira
Authors: Nikolai A. Sobolev, Marcio A. Oliveira, Vitor S. Amaral, Armando Neves, M. Celeste Carmo, Werner Wesch, Oliver Picht, Elke Wendler, Ute Kaiser, J. Heinrich
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.
Authors: João P. Araújo, Pedro B. Tavares, Vitor S. Amaral, A. Lourenço, J. Bessa Sousa, Joaquim M. Vieira
Authors: Nuno João O. Silva, Angel Millan, Vitor S. Amaral, Fernando Palacio, Lianshe Fu, Luís D. Carlos, V. de Zea Bermudez
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.
Authors: João P. Araújo, Armandina M. Lima Lopes, Elisabete Rita, J.G. Correia, Vitor S. Amaral, Ulrich Wahl
Authors: Mário S. Reis, A.M. Gomes, João P. Araújo, Pedro B. Tavares, João S. Amaral, I.S. Oliveira, Vitor S. Amaral
Authors: Luís D. Carlos, Rute A. Sá Ferreira, Nuno João O. Silva, Vitor S. Amaral, M.C. Gonçalves, V. de Zea Bermudez
Authors: D. Marinha, Filipe J. Oliveira, Mário S. Reis, Florinda M. Costa, Maria Fátima Carrasco, João P. Araújo, João Bessa Sousa, Joaquim M. Vieira, Vitor S. Amaral
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.
Authors: Pedro B. Tavares, Vitor S. Amaral, João P. Araújo, Marta C. Oliveira, Joaquim M. Vieira
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