Papers by Author: Pedro B. Tavares

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.

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.

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.