Navegando por Autor "Ginani, Marconi Floripe"
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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 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 Mössbauer and magnetic study of nanocrystalline strontium hexaferrite prepared by an ionic coordination reaction technique(Elsevier, 2013-10) Araújo, José Humberto de; Soares, João Maria; Ginani, Marconi Floripe; Machado, Fernando L. A.; Cunha, João Batista MarimonHard-magnetic nanocrystalline strontium hexaferrites SrFe12O19 were synthesized using an ionic coordination reaction technique. In this sample preparation technique the biopolymer chitosan was used as a nanoreactor. The obtained precursor powders were calcined at temperatures in the range 600–900 °C. The samples were analyzed by X-ray diffraction, transmission electron microscopy, Mössbauer spectroscopy and vibrating sample magnetometry. A complementary study of X-ray Rietveld refinement and Mössbauer spectroscopy shown that the hexaferrite phase formation is accompanied by formation of maghemite and hematite as intermediate phases. It was found that hexaferrite is present in the samples calcined at and above 600 °C but it is fully developed at 900 °C. For this sample the average particle size was found to be 41.6 nm. Magnetization measurements yielded squared hysteresis loops with a magnetization ratio () of 0.58 and a coercive field of 6.48 kOe. The overall results indicated that the particles in these samples are in the single domain regime and that the magnetization reversal in these particles is mainly due to coherent rotationArtigo Simulation and measurement of inset-fed microstrip patch antennas on BiNbO 4 substrates(Wiley, 2010-03-05) Araújo, José Humberto de; Carneiro Filho, Ranilson; Ginani, Marconi Floripe; D'Assunção Júnior, Adaildo Gomes; Martins, R. A.; Mendonça, L. M.; D'Assunção, Adaildo GomesThis work presents an experimental investigation of the properties of microstrip patch antennas on bismuth niobate ceramic substrates. The substrate fabrication process and characterization are described. Several prototypes were built and measured for wireless communication systems. A good agreement was observed between measured and simulated results. The very high electrical permittivity of the ceramic substrate provided a reduction of the antenna dimensionsArtigo Synthesis and magnetic properties of the SiO2 BaFe12O19 glass ceramic composites(Elsevier, 2006-09-15) Araújo, José Humberto de; Cabral, F. A. O.; Ginani, Marconi Floripe; Soares, João Maria; Machado, Fernando. L. A.Samples of the magnetic glass–ceramics SiO2–BaFe12O19 were obtained from mixtures of Fe2O3–BaSO4–SiO2–Na2CO3–CaCO3 following a two-step route. The samples were characterized using XRD analysis and scanning electron microscopy, and their room temperature magnetization hysteresis loops were measured. It was found that the magnetic phase was completely developed in the precursor BaFe12O19 material when it was calcinated above 1000 °C. The size of the particles of the BaFe12O19 phase depends on the thermal treatment conditions and on the SiO2 concentration. The magnetic data showed that the high value of the coercive magnetic field is preserved despite the fact that both the remanent and the saturation magnetizations diminish substantially