Navegando por Autor "Dumelow, T."
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Artigo Effect of Fe doping in polycrystalline La0.7Sr0.3MnO3 on transport and magnetic properties(Elsevier, 2001-05) Araújo, José Humberto de; Xavier Júnior, M. M.; Cabral, F. A. O.; Feitosa, Carlos Chesman de Araújo; Dumelow, T.; Sasaki, José MarcosThe effect of Fe doping (⩽10%) on polycrystalline samples of La0.7Sr0.3MnO3 has been studied through X-ray diffraction, resistivity, AC susceptibility, magnetization, and magnetoresistance measurements. Fe doping was found to change substantially the magnetic and transport properties of the samples, without modifying the lattice structure. In a magnetic field of 5 T, the magnetoresistance of a sample doped with 10% Fe was 4 times greater than that of an undoped sample, near the resistivity peak. This result was explained in terms of the formation of antiferromagnetic clusters of Fe ions. Magnetoresistance did not depend on Fe doping at 5 K, but there was a large magnetoresistive effect in fields less than 0.5 T in all samples. This behavior was attributed to spin polarized tunneling at grain boundaries of polycrystalline samples.Artigo Giant magnetoimpedance in FeAg granular alloys(Applied Physics Letters, 2002-04-02) Araújo, José Humberto de; Soares, João Maria; Cabral, F. A. O.; Dumelow, T.We have observed giant magnetoimpedance (GMI) in an FeAg granular alloy. Impedance and transverse susceptibility (TS) measurements as a function of the external dc magnetic field were made at room temperature. The variation of the magnetoimpedance ratio with magnetic field has been related to a corresponding TS ratio change as a function of the magnetic field. The peaks in the GMI and TS curves are in the same position and have been associated in both cases to effective anisotropy fields.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.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 Reentrant spin glass behavior in polycrystalline La0.7Sr0.3Mn1-X FeX O 3(Materials Research, 2004) Araújo, José Humberto de; Xavier Jr., M. M.; Cabral, F. A. O.; Dumelow, T.; Coelho, A. A.The magnetic and transport properties of the compound La0.7Sr0.3Mn1-xFexO3(0.1 ≤ x ≤ 0.4) have been studied by means of electrical resistivity, AC magnetic susceptibility, and DC magneti- zation. At low concentrations (x ≤ 0.1), the system displays essentially para-to-ferromagnetic transitions as the temperature is decreased, although a decrease in the magnetic moment has been observed in previous studies at temperatures a little below TC. This ferromagnetism is explained by double exchange theory in terms of the formation of Mn+3/Mn+4 ions pairs in the system. At concentrations in the range 0.1 < x ≤ 0.4 the system is more complex. Increased Fe doping not only weakens the ferromagnetic (FM) order and augments the resistivity of the samples, but also induces the appearance of a reentrant spin glass phase at low temperatures (T < 60 K). Irreversibil- ity of the magnetization measured with zero field cooling and with field cooling has been ob- served. In addition, the AC susceptibility peak position varies with frequency. All these effects are characteristic of spin glass behavior. The results have been interpreted based in an increase of frustration due to increasing competition between FM Mn+3/Mn+4 interactions and antiferromagnetic interactions between ions at the boundaries of Fe clusters.