Departamento de Física
URI permanente desta comunidadehttps://repositorio.fei.edu.br/handle/FEI/785
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Resultados da Pesquisa
- 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.
- 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.