Advanced Materials Forum III

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Authors: Fábio Figueiras, Joaquim M. Vieira, M.E. Guzik, J. Legendziewicz, Pedro B. Tavares, Vitor S. Amaral
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.
Authors: João S. Amaral, Mário S. Reis, Vitor S. Amaral, Tânia M. Mendonça, João P. Araújo, Pedro B. Tavares, Joaquim M. Vieira
Authors: Bernard Enrico Watts, Daniele Regonini, Fabrizio Leccabue, Francesca Casoli, Franca Albertini, Gabriele Bocelli, David S. Schmool
Abstract: M-type lead hexaferrites (PbFe12O19) have been prepared using sol-gel methods. Lead and iron precursors were dissolved in butoxyethanol and stabilised with acetylacetone to produce a sol with good wetting and surface coating characteristics. Powder samples were investigated to optimise conditions for the growth of thin films. The «spin on» technique was used to provide homogeneous films. X-ray diffraction and thermomagnetic analysis vibrating sample magnetometry have been performed to assess phase distribution and transformation. The results show that the hexaferrite phase can be crystallised above 700 °C and that for the first indication that this phase is metastable in air below 650 °C.
Authors: João A.M. Santos, João Bessa Sousa, João P. Araújo, F. Correia, J.F. Collingwood, S.B. Palmer
Abstract: We measured dilute Gd1-xYx single crystals (x < 0.1) samples using an a.c. magnetoresistance technique. This technique consists in applying a constant current to the sample and measuring the temperature dependent a.c. voltage under a time modulated external magnetic field (f = 333 Hz). The a.c. voltage signal is decomposed in real and imaginary components or, alternatively, in amplitude and phase signals relative to the a.c. magnetic field. This technique proved to be extremely sensitive and effective to display all the magnetic phase transitions revealed by the previous d.c. transport measurements, plus an additional spectrum of other critical features.
Authors: Nuno Franco, Eduardo Alves, M. Rickart, Anibal Guedes, Paulo Freitas, Nuno P. Barradas
Abstract: Antiferromagnets (AF) such as MnPt and MnNi can be used in spin valves and tunnel junctions due to their high exchange coupling, high blocking temperature, and fair corrosion resistance. They are used as pinning layer in a AF/FM/barrier/FM structure, where FM is a ferromagnet such as CoFe and the barrier is an ultra-thin insulating layer. However, as deposited MnPt and MnNi films with thickness around 20 nm are in the fcc phase, and show no exchange bias in AF/FM bilayers. A transition from fcc to fct is required, and takes place upon annealing. We present an X-ray diffraction study of the phase transition in glass/Ta 7nm/Ru 3 nm/MnPt 20 nm/CoFe 5 nm/Ta 3 nm AF/FM bilayers. We observe the MnPt phase transition from fcc to fct around 250°C. We correlate the phase transition with the change of the bilayers magnetic properties.
Authors: Pablo Hernández-Gómez, Carlos Torres, Carlos de Francisco, José Ignacio Iñiguez, João M. Perdigão
Abstract: The magnetic disaccommodation on polycrystalline Nd doped YIG NdxY3-xFe5O12 (0
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: Cezarina C. Mardare, Pedro B. Tavares, Andréi I. Mardare, Raluca Savu
Abstract: A dense ceramic target of BiFeO3 was synthesized by the urea combustion method. X-ray diffraction indicates that this target is composed of a mixture of phases, the main one is BiFeO3, but Bi46Fe2O72 and Bi2Fe4O9 are also present in small amounts. The BiFeO3 target was used for depositing thin films on Pt/Ti/SiO2/Si substrates by the laser ablation technique. The depositions were made in oxygen atmosphere at pressures in the range between 5x10-3 and 2x10-2mbar, using a KrF laser. The substrate temperatures were 450 or 500°C and the laser energy, the frequency and the distance between the target and the substrate were kept constant at 125mJ, 10Hz and 4cm, respectively. After a deposition time of 30minutes the thickness of the films was approximately 400nm. Some of the films were heat-treated in situ, in 100mbar O2 for 30minutes, at the same temperatures used for deposition. X-ray diffraction results show the BiFeO3 phase, as well as some Bi46Fe2O72 and Bi2Fe4O9. The films were crystallized without any preferential orientation, but the ones made at 2x10-2mbar and 450°C were partially amorphous. For measuring the ferroelectric hysteresis loops, either Al top electrodes were deposited by thermal evaporation or Pt, by sputtering. The distorted shapes of the hysteresis loops obtained indicated that the films exhibit weak ferroelectric properties and high leakage current values.
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: Maria Fátima Carrasco, Jorge Monteiro, Vitor S. Amaral, Rui F. Silva, Joaquim M. Vieira, Florinda M. Costa
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.

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