排序方式: 共有14条查询结果,搜索用时 15 毫秒
1.
J. A. H. Coaquira H. R. Rechenberg C. Marquina M. R. Ibarra A. M. Benito W. Maser E. Muñoz M. T. Martínez 《Hyperfine Interactions》2001,134(1):103-108
Iron particles encaged in carbon nanocapsules have been produced by the Krätschmer–Huffmann carbon-arc discharge method. Soot, collarette and cathode samples have been characterized by Mössbauer spectroscopy and magnetic measurements in the temperature range 4.2–300 K. Different iron phases and iron-carbon solid solutions have been detected in our samples. The Einstein model has been used to evaluate the coupling constant between the particles and their environment, yielding values of the order 1–10 N/m. Irreversibilities observed at ZFC and FC curves for soot samples would suggest the presence of superparamagnetism only if the particles presented a blocking temperature above 300 K. 相似文献
2.
R. F. C. Marques A. F. R. Rodriguez J. A. H. Coaquira J. G. Santos V. K. Garg M. Jafelicci Jr. S. J. L. Ribeiro M. Verelst J. Dexpert-Ghys P. C. Morais 《Hyperfine Interactions》2009,189(1-3):159-166
Mössbauer spectroscopy was used in this study to investigate magnetite nanoparticles, obtained by spray pyrolysis and thermal treatment under H2 reduction atmosphere. Room temperature XRD data indicate the formation of magnetite phase and a second phase (metallic iron) which amount increases as the time of reduction under H2 is increased. While room temperature Mössbauer data confirm the formation of the cubic phase of magnetite and the occurrence of metallic iron phase, the more complex features of 77 K-Mössbauer spectra suggest the occurrence of electronic localization favored by the different crystalline phase of magnetite at low temperatures which transition to the lower symmetry structure should occur at T ~120 K (Verwey transition). 相似文献
3.
Coaquira Julio C. Cardoso Daniel C. T. Gonçalves Paulo B. Orlando Diego 《Nonlinear dynamics》2021,103(4):3557-3580
Nonlinear Dynamics - The range of applications of fiber-reinforced polymer structures has become increasingly diverse in recent years. However, little is known on their performance under dynamic... 相似文献
4.
Magnetic characterization of maghemite nanoparticles dispersed in surface-treated polymeric template
J. A. H. Coaquira A. F. R. Rodriguez J. G. Santos L. B. Silveira A. C. Oliveira V. K. Garg F. Q. Soares D. Rabelo P. C. Morais 《Hyperfine Interactions》2007,176(1-3):113-117
Magnetic characterization of maghemite nanoparticles dispersed in a polymeric template and treated under different chemical processes is reported in this work. Particle size estimated from magnetic measurements, D M?≈?10 nm, for the free-surfactant sample, is consistent with values determined from XRD analysis and TEM images. The magnetic collapse of sextets towards a quadrupole doublet as the temperature is increased reveals the thermal relaxation of smaller $\upgamma $ -Fe2O3 nanoparticles. Magnetic measurements show a strong irreversibility between ZFC and FC curves suggesting the occurrence of particle–particle interaction. 相似文献
5.
J. André-Filho L. León-Félix J. A. H. Coaquira V. K. Garg A. C. Oliveira 《Hyperfine Interactions》2014,224(1-3):189-196
In this work we present the study of hematite (α-Fe2O3) nanostructures synthesized by the ball milling technique. The structural characterization and the crystallite size estimation have been carried out using the X-ray diffraction (XRD) technique. Data analyses indicate that the hematite phase (space group, R-3C) is preserved after the milling process. As the milling time is increased, a second phase (α-Fe) appears. The mean crystallite size shows a decreasing tendency as the milling time is increased. High-resolution transmission electron microscopy (HRTEM) images show the formation of grains composed of crystallites with irregular shapes. Mössbauer spectra of milled powders carried out at 297 and 77 K are well modeled with a histogram distribution of hyperfine fields. The presence of one additional sextet which corresponds to the ∝-Fe phase is also determined in agreement with XRD data analysis. Magnetic measurements suggest the suppression of the Morin transition in the milled samples and the absence of thermal relaxation effects in agreement with the Mössbauer spectroscopy results. 相似文献
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7.
L. León-Félix J. Chaker M. Parise J. A. H. Coaquira L. De Los Santos Valladares A. Bustamante V. K. Garg A. C. Oliveira P. C. Morais 《Hyperfine Interactions》2014,224(1-3):179-188
We report on the synthesis and characterization of uncoated and gold coated magnetite nanoparticles. Structural characterizations, carried out using X-ray diffraction, confirm the formation of magnetite phase with a mean size of ~7 and ~8 nm for the uncoated and gold covered magnetite nanoparticles, respectively. The value of the gold coated Fe3O4 nanoparticles is consistent with the mean physical size determined from transmission electron microscopy images. Mössbauer spectra at room temperature are consistent with the thermal relaxation of magnetic moments mediated by particle-particle interactions. The 77 K Mössbauer spectra are modeled with four sextets. Those sextets are assigned to the signal of iron ions occupying the tetrahedral and octahedral sites in the core and shell parts of the particle. The room-temperature saturation magnetization value determined for the uncoated Fe3O4 nanoparticles is roughly ~60 emu/g and suggests the occurrence of surface effects such as magnetic disorder or the partial surface oxidation. These surface effects are reduced in the gold-coated Fe3O4 nanoparticles. Zero-field–cooled and field-cooled curves of both samples show irreversibilities which are consistent with a superparamagnetic behavior of interacting nanoparticles. 相似文献
8.
F.H. Aragón P.E.N. de Souza J.A.H. Coaquira P. Hidalgo D. Gouvêa 《Physica B: Condensed Matter》2012,407(13):2601-2605
Nickel oxide nanoparticles successfully synthesized by a polymer precursor method are studied in this work. The analysis of X-ray powder diffraction data provides a mean crystallite size of 22±2 nm which is in a good agreement with the mean size estimated from transmission electron microscopy images. Whereas the magnetization (M) vs. magnetic field (H) curve obtained at 5 K is consistent with a ferromagnetic component which coexists with an antiferromagnetic component, the presence of two peaks in the zero-field-cooled trace suggests the occurrence of two blocking process. The broad maximum at high temperature was associated with the thermal relaxation of uncompensated spins at the particle core and the low temperature peak was assigned to the freeze of surface spins clusters. Static and dynamic magnetic results suggest that the correlations of surface spins clusters show a spin-glass-like behavior below Tg=7.3±0.1 K with critical exponents zν=9.7±0.5 and β=0.7±0.1, which are consistent with typical values reported for spin-glass systems. 相似文献
9.
F. H. Arag??n R. Cohen J. A. H. Coaquira G. V. Barros P. Hidalgo L. C. C. M. Nagamine D. Gouv??a 《Hyperfine Interactions》2011,202(1-3):73-79
In this work, we report on the study of SnO 2 nanoparticles prepared by a polymer precursor method. X-ray diffraction (XRD) data analysis evidenced the formation of only the tetragonal rutile-type phase for the as-grown and thermally annealed samples. A mean grain size of about 11 nm for the as-prepared sample has been determined. This mean size increases after the thermal annealing and with the annealing temperature. The room temperature M?ssbauer spectra (MS) were well fitted using a quadrupole splitting (QS) distribution. The isomer shift (IS) tends to increase when the grain size decreases. That increase has been associated to the extra s-electron density generated by the oxygen vacancies. 相似文献
10.
Zr(Fe0.5Cr0.5)2 Laves phase samples have been made to absorb different hydrogen amounts up to 3.3 H/f.u. XRD showed all ZrFeCrH y samples to have the C14 structure and, for 0 < y < 2.6, to be composed of a H-poor α-phase and a H-rich β-phase having different c/a ratios. Both phases could be distinguished in the Mössbauer spectra. The β-phase fraction was determined both from X-ray and Mössbauer analysis, with excellent agreement. β-phase lattice constants and hyperfine parameters vs. y exhibited a small jump at y ≈ 1.75, suggesting a phase transition. Mössbauer spectra of ZrFeCrH3.3 in the 78–350 K range revealed that, for Fe at the 6h site, quadrupole splitting and linewidth increased for decreasing temperature. These effects were attributed to H atomic jump diffusion, and an activation energy of ≈70 meV was estimated. 相似文献