Departamento de Física
URI permanente desta comunidadehttps://repositorio.fei.edu.br/handle/FEI/785
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8 resultados
Resultados da Pesquisa
- Metal-insulator transition in Nd1-xEuxNiO3: Entropy change and electronic delocalization(2015) Jardim R.F.; Barbeta V.B.; Andrade S.; Escote M.T.; Cordero F.; Torikachvili M.S.© 2015 AIP Publishing LLC.The metal-insulator (MI) phase transition in Nd1-xEuxNiO3, 0 ≤ x ≤ 0.35, has been investigated through the pressure dependence of the electrical resistivity ρ(P, T) and measurements of specific heat CP(T). The MI transition temperature (TMI) increases with increasing Eu substitution and decreases with increasing pressure. Two distinct regions for the Eu dependence of dTMI/dP were found: (i) for x ≤ 0.15, dTMI/dP is nearly constant and ∼-4.3 K/kbar; (ii) for x ≤ 0.15, dTMI/dP increases with x and it seems to reach a saturation value ∼-6.2 K/kbar for the x = 0.35 sample. This change is accompanied with a strong decrease in the thermal hysteresis in ρ(P, T) between the cooling and warming cycles, observed in the vicinity of TMI. The entropy change (ΔS) at TMI for the sample x = 0, estimated by using the dTMI/dP data and the Clausius-Clapeyron equation, resulted in ΔS ∼ 1.2 J/mol K, a value in line with specific heat measurements. When the Eu concentration is increased, the antiferromagnetic (AF) and the MI transitions are separated in temperature, permitting that an estimate of the entropy change due to the AF/Paramagnetic transition be carried out, yielding ΔSM ∼ 200 mJ/mol K. This value is much smaller than that expected for a s = 1/2 spin system. The analysis of ρ(P, T) and CP(T) data indicates that the entropy change at TMI is mainly due to the electronic delocalization and not related to the AF transition.
- 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.
- Magnetothermopower in Nd1-xEuxNiO3 compounds(2007) Barbeta V.B.; Jardim R.F.; Escote M.T.; Dilley N.R.We have measured magnetization M (T,H), thermal conductivity κ (T,H), and thermopower S (T,H) of polycrystalline samples of Nd1-x Eux Ni O3, 0≤x≤0.35, as a function of temperature and external magnetic field. The data indicate a metal-insulator (MI) transition in a wide range of temperature (200< TMI <325 K). The magnetic susceptibility (T) data, after the subtraction of the rare-earth contribution, exhibit a Curie-Weiss-like behavior at temperatures above TMI. Although a clear antiferromagnetic AF transition of the Ni sublattice is observed at TN ≤ TMI, (T) still increases down to 5 K, suggesting a heterogeneous ground state. The thermal conductivity of the NdNi O3 compound is not affected by an external magnetic field of 90 kOe in a wide range of temperature, and its temperature dependence below 15 K is approximately quadratic, strongly suggesting the presence of disorder. S (T) is negative above TMI and varies linearly with temperature. Below TMI, there is a minimum close to 120 K, and S (T) changes its sign at T∼30 K, indicating a competition between two types of charge carriers. A pronounced peak in S (T) at TS ∼20 K is observed and the peak remains unaltered under magnetic fields up to 90 kOe. However, its magnitude is enhanced by ∼25% with applied magnetic field, exhibiting a clear magnetothermopower effect. The combined results indicate a coexistence of ordered and disordered phases below TN and that an applied magnetic field is suitable for enhancing the thermoelectric properties close to TS. © 2007 American Institute of Physics.
- Spin-wave fluctuations in ferrimagnetic Mgx Fe3-x O4 nanoparticles(2010) Franco A.; Zapf V.S.; Barbeta V.B.; Jardim R.F.We have performed a systematic study of the magnetic properties of a series of ferrimagnetic nanoparticles of Mgx Fe3-x O4 (0.8times;1.5) prepared by the combustion reaction method. The magnetization data can be well fitted by Bloch's law with T3/2. Bloch's constant B determined from the fitting procedure was found to increase with Mg content x from ∼3.09× 10-5 K-3/2 for x=0.8 to 6.27× 10-5 K-3/2 for x=1.5. The exchange integral JAB and the spin-wave stiffness constant D of Mgx Fe3-x O4 nanoparticles were also determined as ∼0.842 and 0.574 meV and 296 and 202 meV Å2 for specimens with x=0.8 and 1.5, respectively. These results are discussed in terms of cation redistribution among A and B sites on these nanostructured spinel ferrites. © 2010 American Institute of Physics.
- Anelastic spectroscopy study of the metal-insulator transition of Nd 1-xEuxNiO3(2011) Cordero F.; Trequattrini F.; Barbeta V.B.; Jardim R.F.; Torikachvili M.S.Measurements are presented of the complex dynamic Young's modulus of NdNiO3 and Nd0.65Eu0.35NiO3 through the metal-insulator transition (MIT). Upon cooling, the modulus presents a narrow dip at the MIT followed by an abrupt stiffening of ∼6%. The anomaly is reproducible between cooling and heating in Nd0.65Eu 0.35NiO3 but appears only as a slow stiffening during cooling in undoped NdNiO3, in conformance with the fact that the MIT in RNiO3 changes from strongly first order to second order when the mean R size is decreased. The elastic anomaly seems not to be associated with the antiferromagnetic transition, which is distinct from the MIT in Nd 0.65Eu0.35NiO3. It is concluded that the steplike stiffening is due to the disappearance or freezing of dynamic Jahn-Teller (JT) distortions through the MIT, where the JT active Ni3 + is disproportionated into alternating Ni3+δ and Ni3-δ. The fluctuating octahedral JT distortion necessary to justify the observed jump in the elastic modulus is estimated as ∼3% but does not have a role in determining the MIT, since the otherwise-expected precursor softening is not observed. © 2011 American Physical Society.
- 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.
- Metal-insulator transition in Nd1-xEuxNiO 3 compounds(2006) Escote M.T.; Barbeta V.B.; Jardim R.F.; Campo J.Polycrystalline Nd1-xEuxNiO3 (0≤x≤0.5) compounds were synthesized in order to investigate the character of the metal-insulator (MI) phase transition in this series. Samples were prepared through the sol-gel route and subjected to heat treatments at ∼1000 °C under oxygen pressures as high as 80bar. X-ray diffraction (XRD) and neutron powder diffraction (NPD), electrical resistivity ρ(T), and magnetization M(T) measurements were performed on these compounds. The NPD and XRD results indicated that the samples crystallize in an orthorhombic distorted perovskite structure, space group Pbnm. The analysis of the structural parameters revealed a sudden and small expansion of ∼0.2% of the unit cell volume when electronic localization occurs. This expansion was attributed to a small increase of ∼0.003 of the average Ni-O distance and a simultaneous decrease of ∼-0.5° of the Ni-O-Ni superexchange angle. The ρ(T) measurements revealed a MI transition occurring at temperatures ranging from TMI∼193 to 336K for samples with x ≤ 0 and 0.50, respectively. These measurements also show a large thermal hysteresis in NdNiO3 during heating and cooling processes, suggesting a first-order character of the phase transition at TMI. The width of this thermal hysteresis was found to decrease appreciably for the sample Nd 0.7Eu0.3NiO3. The results indicate that cation disorder associated with increasing substitution of Nd by Eu is responsible for changing the first-order character of the transition in NdNiO3. © 2006 IOP Publishing Ltd.
- Separation technology meets green chemistry: Development of magnetically recoverable catalyst supports containing silica, ceria, and titania(2018) Vono L.L.R.; Damasceno C.C.; Matos J.R.; Jardim R.F.; Landers R.; Masunaga S.H.; Rossi L.M.© 2018 IUPAC & De Gruyter.Magnetic separation can be considered a green technology because it is fast, efficient, consumes low energy, and minimizes the use of solvents and the generation of waste. It has been successfully used in laboratory scale to facilitate supported catalysts' handling, separation, recovery, and recycling. Only few materials are intrisically magnetic, hence the application of magnetic materials as catalyst supports has broaden the use of magnetic separation. Iron oxides, silica-coated iron oxides, and carbon-coated-cobalt are among the most studied catalyst supports; however, other metal oxide coatings, such as ceria and titania, are also very interesting for application in catalysis. Here we report the preparation of magnetically recoverable magnetic supports containing silica, ceria, and titania. We found that the silica shell protects the iron oxide core and allows the crystalization of ceria and titania at high temperature without compromising the magnetic properties of the catalyst supports.