Papers by Author: João P. Araújo

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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: João P. Araújo, Pedro B. Tavares, Vitor S. Amaral, A. Lourenço, J. Bessa Sousa, Joaquim M. Vieira
Authors: João Ventura, A. Pereira, José M. Teixeira, João P. Araújo, Francisco Carpinteiro, Y. Liu, João Bessa Sousa, Z. Zhang, Paulo Freitas
Abstract: To commute between the different resistance states of a magnetic tunnel junction (TJ) one can use a thermally-induced pinned layer switching mechanism. When a sufficiently high electrical current flows through the insulating barrier, local temperatures inside the tunnel junction can increase above the blocking temperature of the antiferromagnetic layer used to pin the magnetization of the adjacent ferromagnet. Then, it is possible to switch the magnetization of the pinned layer with a small magnetic field H and thus revert the magnetic state of the TJ. Here we demonstrate thermally-induced pinned layer switching in thin magnetic tunnel junctions. We further present numerical results that suggest that heating is small when one takes into consideration the uniform current density flowing through the tunnel junction and that one must conclude that nanoconstrictions concentrate most of the current, increasing local current densities and temperature. Simulation of heating and cooling times demonstrates that current-induced pinned layer switching is a competitive mechanism for actual technological applications.
Authors: Anastasiia Moskaltsova, Mariana P. Proenca, Celia T. Sousa, Arlete Apolinário, João Ventura, Gleb N. Kakazei, João P. Araújo
Abstract: Ordered hexagonal arrays of Co (x)Ni (1-x) nanowires (NWs) with different content of cobalt (0.35 < x < 0.75), were successfully fabricated by a potentiostatic electrodeposition technique inside anodic aluminum oxide (AAO) templates. The CoNi NWs were electrodeposited using only one electrolyte bath and tuning the Co% by the applied deposition potential. The systematic study of the morphology, chemical composition and magnetic properties of the CoNi NW arrays was performed. The obtained results unambiguously demonstrated the influence of the Co content on the magnetic properties of CoNi NWs. The Co% was found to decrease when increasing the applied cathodic potential and the coercivity of the CoNi NWs linearly increased with the Co%. The magnetic texture of the fabricated NW arrays was found lower for smaller Co concentrations, which was in good agreement with the squareness results.
Authors: João P. Araújo, Armandina M. Lima Lopes, Elisabete Rita, J.G. Correia, Vitor S. Amaral, Ulrich Wahl
Authors: D.C. Leitão, I.G. Trindade, R. Fermento, João P. Araújo, S. Cardoso, P.P. Freitas, João Bessa Sousa
Abstract: In this work, a study of the sensitivity enhancement of spin valve sensors, when located in close proximity to magnetic flux guides, is presented. The magnetoresistance (MR) of spin-valve sensors, lithographically patterned into stripes with lateral dimensions, (length) l = 500 µm, (width) wsensor = 1, 2, 6 µm and placed near one/two Co93.5Zr2.8Nb3.7 (CZN) magnetic flux guide, is characterized at room temperature. CZN has a high permeability that together with a defined microstructured shape, is able to concentrate the magnetic flux in a small area, leading to an increase in sensor's sensitivity. The magnetic field amplification is estimated by comparison of sensor sensitivity with/without magnetic flux guides, in the linear operation range, and studied as a function of different parameters. Besides an enhancement in sensitivity, sensors also exhibit an important increase in the hard axis coercivity and a shift from MR(H=0) = 0.5, both attributed to the magnetic flux guides. Amplification factors of the order of 20 are observed..
Authors: Mário S. Reis, A.M. Gomes, João P. Araújo, Pedro B. Tavares, João S. Amaral, I.S. Oliveira, Vitor S. Amaral
Authors: João S. Amaral, Mário S. Reis, João P. Araújo, Tânia M. Mendonça, Pedro B. Tavares, Vitor S. Amaral, Joaquim M. Vieira
Abstract: Manganites of general formula ABMnO3 (where A is a trivalent rare-earth ion and B is a divalent dopant) have interesting properties, namely colossal magnetoresistance and their applicability as materials for active magnetic regenerators. La0.70Sr0.30MnO3 (LSMO) is a ferromagnet presenting considerable magnetocaloric effect, with operating temperature TC ~ 370 K and magnetic entropy variation comparable to pure Gadolinium. The high value of TC makes LSMO unsuitable for room-temperature magnetic refrigeration applications, but by substituting La with the high-magnetic moment ion Er, TC is lowered and total magnetic entropy increases. We have found a limit of solid solubility of Er ions in LSMO, in samples prepared by either solid state or sol-gel techniques in previous works [1], in accordance with other authors [2]. We now present a more detailed study of this limit of solubility, with more samples prepared with Er substitution close to the solubility limit and SEM microscopy clearly showing the changes in microstructure caused by the formation of a secondary ErMnO3 phase, in accordance with x-ray diffraction data and TC variation along the series. The magnetocaloric properties of the series are also presented, showing the increase of Relative Cooling Power along the series, in applied magnetic fields up to 1 T.
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.
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