Navegando por Autor "Melo, Daniel Macedo A."
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Artigo Evaluation of (BH)max and magnetic anisotropy of cobalt ferrite nanoparticles synthesized in gelatin(Elsevier, 2015-11) Araújo, José Humberto de; Lima, Aldenir Cavalcanti; Torres, Marco Morales; Soares, João Maria; Melo, Daniel Macedo A.; Carriço, Artur da SilvaCoFe2O4 nanoparticles were synthesized using gelatin as a polymerizing agent. Structural, morphological and magnetic properties of samples treated at different temperatures were investigated by X-ray diffraction, scanning electron microscopy, Mössbauer spectroscopy and magnetization measurements. Our results revealed that the samples annealed at 623 K and temperatures above 973 K have a cation distributions given by (Co0.19Fe0.81)[Co0.81Fe1.19]O4 and (Co0.06Fe0.94)[Co0.94Fe1.06]O4, respectively. The particle sizes varied from 73 to 296 nm and the magnetocrystalline anisotropy, K1, has values ranging from 2.60×106 to 2.71×106 J/m3, as determined from the law of approach to saturation applied to the M×H data at high field. At 5 K, the saturation magnetization, coercive field and (BH)max varied from 76 to 95 Am2/kg, 479.9 to 278.5 kA/m and 9.7 to 20.9 kJ/m3, respectively. The reported values are in good agreement with near-stoichiometric cobalt ferrite samplesArtigo Nanoferrites of nickel doped with cobalt: influence of Co2+ on the structural and magnetic properties(Elsevier, 2015-01-15) Araújo, José Humberto de; Rodrigues, A. P.G.; Gomes, Danielle Karinne Souza; Melo, Daniel Macedo A.; Oliveira, N.A.S.; Braga, Renata M.Nanoferrites of nickel substituted with cobalt of composition Ni1−xCoxFe2O4 (0≤x≤0.75), were synthesized by combustion reaction assisted in microwaves. The influence of the substitution of Ni2+ by Co2+ content and the concentration of Co2+ in the structural and magnetic properties was investigated. The powders were prepared by combustion according to the concept of chemical propellants and heated in a microwave oven with a power of 7000 kW. The synthesized powders were characterized by absorption spectroscopy in the infrared region (FTIR), X-ray diffraction (XRD) together with Rietveld refinement, surface area (BET) method, scanning electron microscopy (MEV) and magnetic measurements (MAV). The results indicated that it was possible to obtain nickel ferrite doped with cobalt in all compositions and that an increase of cobalt concentration caused an increase in particle size (9.78–21.63 nm), a reduction in surface area, and reduction in magnetic concentrations greater than 50%Artigo The effect of Sr2+ on the structure and magnetic properties of nanocrystalline cobalt ferrite(Elsevier, 2015-04-15) Araújo, José Humberto de; Lima, Aldenir Cavalcanti; Peres, Ana Paula Da Silva; Torres, Marco Antônio Morales; Medeiros, Susana Nóbrega de; Soares, João Maria; Melo, Daniel Macedo A.; Carriço, Artur da SilvaWe report the effect of Sr2+ on the structure, morphology and magnetic properties of CoFe2O4 synthesized by microwave-assisted combustion method. Results from X-ray diffraction and Mössbauer spectroscopy at 298 K suggest the formation of a mixed spinel Sr2+ substituted CoFe2O4. However, secondary phases like CoO, SrCO3 and SrFeO2.96 were also observed. A decrease in the lattice parameter and particle size were observed with the addition of Sr2+. The crystallite size of ferrite phase ranged from 53 to 62 nm. Magnetic studies showed a gradual decrease in the saturation magnetization and remanence with the increase of Sr2+ content, and an increasing value of the coercive field