Navegando por Autor "Costa, J. A. P. da"
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Artigo Evidence of magnetic polaronic states in La_{0.70}Sr_{0.30}Mn_{1-x}Fe_{x}O_{3} manganites(American Physical Society, 2003-02-19) Araújo, José Humberto de; Souza Filho, A. G.; Faria, J. L. B.; Guedes, I.; Sasaki, José Marcos; Freire, P. T. C.; Mendes Filho, J.; Xavier Jr, M. M.; Cabral, Francisco de Assis Olímpio; Costa, J. A. P. da; Freire, V. N.We have performed x-ray, electrical resistivity, and Raman scattering measurements in La0.70Sr0.30Mn12xFexO3 manganites, with x varying from 0 to 0.40. The appearance of new peaks in the Raman spectra as the Fe content increases is attributed to either otherwise forbidden Raman modes that become active due to symmetry breaking effect or to the phonon density of states features, which arise due to a disorder-induced phenomenon. The softening of some phonon modes suggests the evidence of magnetic polaronic states in the insulating paramagnetic phase of these manganites. Our observations are discussed in connection with the x-ray diffraction and electrical resistivity data.Artigo Nitriding in cathodic cage of stainless steel AISI 316: influence of sample position(Vacuum, 2009) Sousa, R. R. M. de; Araujo, F. O. de; Costa, J. A. P. da; Dumelow, T.; Oliveira, R. S. de; Alves Jr, C.Artigo Preparation and Characterization of FeAg Granular Alloys(Materials Research, 2004) Araújo, José Humberto de; Soares, J. M.; Cabral, F. A. O.; Costa, J. A. P. da; Sasaki, M.Fe10Ag90 granular alloys have been fabricated, using a sol-gel method, for a range of nitric acid concentrations in the start solution. The samples have been characterized by X-ray diffraction and magnetization measurements. The average Fe particle sizes derived from X-ray diffraction are in the range 24-29 nm, indicating a large variation. The coercivity obtained from the hysteresis curve is two orders of magnitude larger that of pure iron in all the samples. Moreover, the hysteresis curves do not saturate, even in fields of up to 1 T. These observations indicate that the samples contain both superparamagnetic and blocked particles. A comparison between the coercive field and the average particle diameter, determined by the Sherrer's formula, is displayed in all acid concentrations.