Navegando por Autor "Xavier Jr, M. M."
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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 Magnetic and transport properties of polycrystalline La0.7Sr0.3Mn1-xFexO3(American Physical Society, 2000-12-12) Araújo, José Humberto de; Feitosa, Carlos Chesman de Araújo; Dumelow, T.; Xavier Jr, M. M.; Cabral, F. A. O.The effect of Fe doping (<10%) on polycrystalline samples of La0.7Sr0.3MnO3 has been studied by means of x-ray diffraction, resistivity, ac susceptibility, magnetization, and magnetoresistance measurements. Fe doping does not affect the lattice structure, but it weakens the ferromagnetism and substantially augments the resistivity of the samples. The magnetoresistance of a sample with 10% Fe doping, in a 5 T magnetic field, was 4 times greater than that of the undoped sample near the peak in the resistivity. The results were explained in terms of the formation of antiferromagnetic clusters of Fe ions. At 5 K the magnetoresistance showed no significant dependence on Fe doping, but it displayed large variations in fields below 0.5 T for all samples. This behavior was attributed to spin-polarized tunneling at the grain boundaries of the polycrystalline samples. A variable-range hopping behavior in the resistivity was also encountered in all the samples at low temperatures.