Navegando por Autor "Soares, J. M."
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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 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 Influence of reaction temperature, proportions of iron, cobalt and KOH on the CoFe2O4 synthesis by hydrothermal method assisted by microwave heating(Springer, 2017-06-19) Paskocimas, Carlos Alberto; Conceição, Ozivam Lopes de Aquino; Garcia, Laurênia Martins Pereira; Lovisa, Laura Ximena; Torres, M. M.; Soares, J. M.; Delmonte, Maurício Roberto Bomio; Motta, Fabiana Villela daIn this study, several factors such as reaction temperature, proportions of iron, cobalt and KOH were considered in the synthesis of cobalt ferrite nanoparticles by hydrothermal method assisted by microwave radiation. These parameters were studied using the full factorial experiment design. The effect of these factors on crystallite size was investigated using surface response methodology. The structure, crystallite size, magnetic properties, degree of inversion, as well as the chemical occupation of Fe ions were investigated in this study. The results indicated that all factors were statistically significant by analysis of variance (F-test), with a confidence level of 95%. The factors that influenced the growth of the crystallite were: KOH and iron concentration, precursors of cobalt salts and the effect of the interaction between the concentration of KOH and the concentration of precursors. The analysis of variance at a significance level of 95% indicates that the model adjusted to the experimental points is significantArtigo 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 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.Artigo 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.