Navegando por Autor "Machado, F. L. A."
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Artigo Anisotropy field and transverse susceptibility in nanocrystalline hexaferrites(Physica B: Condensed Matter, 2006-10-01) Araújo, José Humberto de; Soares, J. M.; Machado, F. L. A.; Cabral, F. A. O.; Rodrigues, H. A. B.; Ginani, Marconi FloripeNanocrystalline strontium and barium hexaferrites were produced by an ionic coordination reaction method. The average particle size obtained using the Rietveld X-ray refinement technique and by scanning electron microscopy was quite uniform and close to 50 nm. Transverse susceptibility measurements yielded both the coercive and the anisotropy magnetic fields. The results were analysed using a theoretical model proposed by Aharoni et al. [Bull. Res. Counc. Isr. A 6 (1957) 215]. This overall procedure seems to be quite useful in determining the distribution of the anisotropy magnetic fields in granular materialsArtigo Anisotropy field and transverse susceptibility in nanocrystalline hexaferrites(Elsevier, 2006-10-01) Araujo, Jose Humberto; Soares, J. M.; Machado, F. L. A.; Cabral, F. A. O.; Rodrigues, H. A. B.; Ginani, Marconi FloripeNanocrystalline strontium and barium hexaferrites were produced by an ionic coordination reaction method. The average particle size obtained using the Rietveld X-ray refinement technique and by scanning electron microscopy was quite uniform and close to 50 nm. Transverse susceptibility measurements yielded both the coercive and the anisotropy magnetic fields. The results were analysed using a theoretical model proposed by Aharoni et al. [Bull. Res. Counc. Isr. A 6 (1957) 215]. This overall procedure seems to be quite useful in determining the distribution of the anisotropy magnetic fields in granular materials.Artigo Exchange-spring behavior in nanopowders of CoFe2O4–CoFe2(AIP Publishing, 2011-02-15) Araújo, José Humberto de; Cabral, F. A. O.; Soares, João Medeiros; Machado, F. L. A.The nature of the magnetic interactions among core-shell nanoparticles of CoFe2O4–CoFe2 having an average particle size of 32 nm were investigated by δm plots. It was found that the interactions are mainly demagnetizing in nature for nanoparticles with core diameters larger than 26 nm. For smaller values of core diameters, the exchange-spring phenomenon takes place due to the exchange-coupling at the core-shell interface. The critical thickness of the shell was estimated to be 7.8 nm. The overall results are in excellent agreement with current theoretical modelsArtigo Fe interparticle interactions in Fe_{x}Ag_{100?x} granular alloys (2(American Physical Society, 2005-11-03) Araújo, José Humberto de; Soares, J. M.; Machado, F. L. A.; Cabral, F. A. O.; Ginani, Marconi FloripeSamples of FexAg100−x, with 2% x50%, were produced using a new preparation route based on the ionic coordination reaction technique. X-ray diffraction and transmission electronic microscopy showed that the samples are granular alloys and that they have a high degree of purity, good homogeneity, small variations in the Fe concentration, and the mean size of the grains was approximately 30 nm for any Fe composition. Magnetization measurements showed that the coercive field of the samples presented a fairly complex behav- ior. It was also observed, by means of time decay of the thermoremanent magnetization, three dynamical regimes. The existence of these regimes are in good agreement with what is expected from the analysis using the Henkel and M plots. In addition, it was observed an onset to a long range magnetic order LRMO in the Fe30Ag70 sample, culminating in a superferromagnetic order in Fe50Ag50Artigo Magnetic properties of BaFe12O19 prepared by an ICR technique(Elsevier, 2007-03) Araújo, José Humberto de; Soares, J. M.; Cabral, F. A. O.; Machado, F. L. A.; Ginani, Marconi FloripeHigh purity BaFe12O19 samples were synthesized by an ionic-coordination-reaction (ICR) technique using chitosan as a complex agent. The mean size Dm of the particles and their magnetic properties were controlled by varying the concentration of chitosan in the starting solution. It was found that Dm, the coercive field Hc, and the Curie temperature Tc decrease when the concentration of chitosan is increased. In addition, the variation of Hc with Dm in these samples can be accounted for using the Stoner–Wolhfarth model. Finally, the magnetic field dependence of the transverse susceptibility were in good agreement with the theoretical predictions of Aharoni et al. [Bull. Res. Counc. Isr. A 6 (1957) 215Artigo Particle size distribution in FeAg granular alloy(Journal of Non-Crystalline Solids, 2008-11) Araújo, José Humberto de; Soares, João Maria; Cabral, F. A. O.; Machado, F. L. A.; Dumelow, Thmas; Xavier Júnior, Milton Moraes; Sasaki, José MarcosA combination of Langevin and hyperbolic tangent functions weighted by a log-normal particle size distributions were used to fit the magnetization curves of Fe10Ag90 granular alloys, produced by a sol–gel process under different chemical conditions. The fitting technique allows one to get both the superparamagnetic and the blocked particle size distributions. The average size of the Fe particles obtained from the fit were in good agreement with those obtained by X-ray diffraction. The correlation between the coercive field and the particle distribution was also investigated. Zero-field cooling dc magnetic susceptibility measurements were fitted using a similar technique to that used for fitting the magnetization curves. The saturation magnetization and mean Fe particle size values obtained from the two fittings were in good agreementArtigo Structural and magnetic properties of nanoparticles of La2/3Sr1/3MnO3(Elsevier, 2006-10-01) Araújo, José Humberto de; Moreira, M. L.; Soares, J. M.; Azevedo, W. M. de; Rodrigues, A. R.; Machado, F. L. A.Nanoparticles of the manganite were synthesized using the Pechini process and their structural properties were characterized by X-ray diffraction (XRD) and by transmission electron microscopy (TEM). The magnetization of compacted powders was measured for temperatures in the range . The estimated average diameter (D) of the particles continuously increased from 20 to 95 nm when the calcination temperature was varied from 873 to 1273 K. From the magnetic data we found that the room temperature saturation magnetization scaled with while the magnetic transition temperature follows a power-law suggesting that the correlation length is mainly determined by the surface properties of the nanoparticles.