Departamento de Física
URI permanente desta comunidadehttps://repositorio.fei.edu.br/handle/FEI/785
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Resultados da Pesquisa
- Magnetic properties of Fe3 O4 nanoparticles coated with oleic and dodecanoic acids(2010) Barbeta V.B.; Jardim R.F.; Kiyohara P.K.; Effenberger F.B.; Rossi L.M.Magnetic nanoparticles (NP) of magnetite (Fe3 O4) coated with oleic acid (OA) and dodecanoic acid (DA) were synthesized and investigated through transmission electron microscopy (TEM), magnetization M, and ac magnetic susceptibility measurements. The OA coated samples were produced with different magnetic concentrations (78%, 76%, and 65%) and the DA sample with 63% of Fe3 O4. Images from TEM indicate that the NP have a nearly spherical geometry and mean diameter ∼5.5 nm. Magnetization measurements, performed in zero-field cooled (ZFC) and field cooled processes under different external magnetic fields H, exhibited a maximum at a given temperature TB in the ZFC curves, which depends on the NP coating (OA or DA), magnetite concentration, and H. The temperature TB decreases monotonically with increasing H and, for a given H, the increase in the magnetite concentration results in an increase in TB. The observed behavior is related to the dipolar interaction between NP, which seems to be an important mechanism in all samples studied. This is supported by the results of the ac magnetic susceptibility ac measurements, where the temperature in which ′ peaks for different frequencies follows the Vogel-Fulcher model, a feature commonly found in systems with dipolar interactions. Curves of H versus TB / TB (H=0) for samples with different coatings and magnetite concentrations collapse into a universal curve, indicating that the qualitative magnetic behavior of the samples may be described by the NP themselves, instead of the coating or the strength of the dipolar interaction. Below TB, M versus H curves show a coercive field (HC) that increases monotonically with decreasing temperature. The saturation magnetization (M S) follows the Bloch's law and values of MS at room temperature as high as 78 emu/g were estimated, a result corresponding to ∼80% of the bulk value. The overlap of M/ MS versus H/T curves for a given sample and the low HC at high temperatures suggest superparamagnetic behavior in all samples studied. The overlap of M/ M S versus H curves at constant temperature for different samples indicates that the NP magnetization behavior is preserved, independently of the coating and magnetite concentration. © 2010 American Institute of Physics.
- Metal-insulator transition in Nd1-x Eux NiO 3 probed by specific heat and anelastic measurements(2011) Barbeta V.B.; Jardim R.F.; Torikachvili M.S.; Escote M.T.; Cordero F.; Pontes F.M.; Trequattrini F.Oxides RNiO3 (R=rare-earth, R≠La) exhibit a metal-insulator (MI) transition at a temperature TMI and an antiferromagnetic (AF) transition at TN. Specific heat (CP) and anelastic spectroscopy measurements were performed in samples of Nd1-xEu xNiO3, 0≤x≤ 0.35. For x≥0, a peak in C P is observed upon cooling and warming at essentially the same temperature TMI TN ∼ 195 K, although the cooling peak is much smaller. For x 0.25, differences between the cooling and warming curves are negligible, and two well defined peaks are clearly observed: one at lower temperatures that define TN, and the other one at TMI. An external magnetic field of 9 T had no significant effect on these results. The elastic compliance (s) and the reciprocal of the mechanical quality factor (Q-1) of NdNiO3, measured upon warming, showed a very sharp peak at essentially the same temperature obtained from CP, and no peak is observed upon cooling. The elastic modulus hardens below T MI much more sharply upon warming, while the cooling and warming curves are reproducible above TMI. Conversely, for the sample with x 0.35, s and Q-1 curves are very similar upon warming and cooling. The results presented here give credence to the proposition that the MI phase transition changes from first to second order with increasing Eu doping. © 2011 American Institute of Physics.