Navegando por Autor "Cabral, F. A. O."
Agora exibindo 1 - 15 de 15
- Resultados por página
- Opções de Ordenação
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 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 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 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 Magnetic behavior of nanocrystalline MnCo2O4 spinels(Elsevier, 2006-07) Araújo, José Humberto de; Borges, Filipe Martel de Magalhães; Melo, Dulce Maria de Araújo; Câmara, M. S. A.; Martinelli, Antonio Eduardo; Soares, João Maria; Cabral, F. A. O.A series of powders with different particle sizes of the spinel MnCo2O4 was synthesized by the Pechini method. The resulting precursor was calcined between 400 and 900o and characterized by thermogravimetric analyses (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and magnetization measurements. TGA analysis showed a loss of mass in the region from 200 to 650oC and stability after this. XRD patterns showed an increase of the crystallite size with calcination temperature. Magnetization measurements on the sample calcined at 900oC showed an unusual behavior of the hysteresis curve and irreversibility of the magnetization with field cooled (FC) and zero field cooled (ZFC) procedures. An evolution of this behavior was observed with increasing crystallite sizeArtigo 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 A Mössbauer study of plasma nitrided iron(AIP Publishing, 1998-10-13) Araujo, Jose Humberto; Cabral, F. A. O.; Araujo, R. C.; Gama, S.Iron powder has been nitrided in a glow discharge plasma of a mixture of nitrogen and hydrogen, in order to verify the existence of a metastable iron nitrogen solid solution, previously observed from the Sm2Fe17N3 phase decomposition process. Our results show that, for short nitridation times, we obtain a mixture of Fe4N, a small amount of α-Fe and an iron nitrogen solid solution the amount of which increases after heat-treatment at 610 °C. This iron nitrogen solid solution is characterized by Mössbauer spectroscopy which shows a typical α-Fe spectrum, but with lower hyperfine field and broader linewidth. TMA measurements show that the Curie temperature is similar to that of α-Fe.Artigo 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 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 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.Artigo Synthesis and characterization of poly(ethyl methacrylate-co-methacrylic acid) magnetic particles via miniemulsion polymerization(Elsevier B. V. Ltda, 2006-10-18) Araújo, José Humberto de; Nunes, J. S.; Vasconcelos, C. L. de; Cabral, F. A. O.; Pereira, M. R.; Fonseca, J. L. C.Magnetic particles are very important systems with potential use in drug delivery systems, ferrofluids, and effluent treatment. In many situations, such as in biomedical applications, it is necessary to cover inorganic magnetic particles with an organic material, such as polymers. In this work, latices based on magnetite covered by poly(ethyl methacrylate-co-methacrylic acid) were obtained via miniemulsion polymerization. The resultant latices had particles in the nanometric range and presented a pronounced superparamagnetic behavior.Artigo 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