Investigation of some dielectric properties of phosphate glasses doped with iron oxides, by a microwave technique

The effects of iron ions on dielectric properties of lithium sodium phosphate glasses were studied by non-usual, fast and non-destructive microwave techniques. The dielectric constant (epsilon`). insertion loss (L) and microwave absorption spectra (microwave response) of the selected glass system xFe(2)O(3)center dot(1 - x)(50P(2)O5 center dot 25Li(2)O center dot 25Na(2)O), being x = 0, 3, 6, ....,15 expressed in mol.%, were investigated. The dielectric constant of the samples was investigated at 9.00 GHz using the shorted-line method (SLM) giving the minimum value of epsilon` = 2.10 +/- 0.02 at room temperature, and increasing further with x, following a given law. It was observed a gradual increasing slope Of E in the temperature range of 25 <= t <= 330 degrees C, at the frequency of 9.00 GHz. Insertion loss (measured at 9.00 GHz) and measurements of microwave energy attenuation, at frequencies ranging from 8.00 to 12.00 GHz were also studied as a function of iron content in the glass samples. (C) 2009 Elsevier Ltd. All rights reserved.

Dielectric properties and physical features of phosphate glasses containing iron oxide

Microwave techniques were applied to the study of dielectric properties of phosphate glasses on the basis of contributions from permanent and induced dipolar polarization of local structural units interacting with the electrical component of the electromagnetic radiation. The dielectric constant of the selected glass system (100-x)(50P(2)O(5)center dot 25Li(2)O center dot 25Na(2)O)center dot xFe(2)O(3), where 0 <= x <= 21 is in mol%, was measured using a microwave setup assembled to measure the phase shift of the standing wave pattern produced by the insertion of the sample. It is shown that the Fe2+ ions contribute effectively to the dielectric constant, as expected from the interactions of the dipoles of the local charge compensation pairs with the microwave radiation. However, there is the possibility of occurrence of some ions Fe3+, in general, at low iron content, which reinforces the glass structure and, therefore, decreases the dielectric constant. There is a gradual conversion from Fe3+ to Fe2+ as the iron ions increases. This is possibly the reason of the anomaly in the dielectric constant values observed in the results. These assumptions can be checked by results of electronic paramagnetic resonance (EPR) and optical absorption (OA). The dielectric constant of the glasses studied in this work was found to increase with the temperature in the range of 25-330 degrees C. (C) 2007 Elsevier B.V. All rights reserved.

Ac and dc magnetotransport properties of the phase-separated La(0.6)Y(0.1)Ca(0.3)MnO(3) manganite

The physical properties of the La(0.6)Y(0.1)Ca(0.3)MnO(3) compound have been investigated, focusing on the magnetoresistance phenomenon studied by both dc and ac electrical transport measurements. X-ray diffraction and scanning electron microscopy analysis of ceramic samples prepared by the sol-gel method revealed that specimens are single phase and have average grain size of similar to 0.5 mu m. Magnetization and 4-probe dc electrical resistivity rho(T,H) experiments showed that a ferromagnetic transition at T(C) similar to 170 K is closely related to a metal-insulator (MI) transition occurring at essentially the same temperature T(MI). The magnetoresistance effect was found to be more pronounced at low applied fields (H <= 2.5 T) and temperatures close to the MI transition. The ac electrical transport was investigated by impedance spectroscopy Z(f,T,H) under applied magnetic field H up to 1 T. The Z(f,T,H) data exhibited two well-defined relaxation processes that exhibit different behaviors depending on the temperature and applied magnetic field. Pronounced effects were observed close to T (C) and were associated with the coexistence of clusters with different electronic and magnetic properties. In addition, the appreciable decrease of the electrical permittivity epsilon`(T,H) is consistent with changes in the concentration of e(g) mobile holes, a feature much more pronounced close to T (C).

Study of the spectroscopic properties and first hyperpolarizabilities of disperse azo dyes derived from 2-amino-5-nitrothiazole

The solvatochromism and other spectroscopic and photophysical characteristics of four azo disperse dyes, derived from 2-amino-5-nitrothiazole, were evaluated and interpreted with the aid of experimental data and quantum mechanical calculations. For the non-substituted compound two conformers, E and Z, were proposed for the isolated molecules, being the second one considerably less stable. The optimization of these structures in combination with a SCRF methodology (IEFPCM, Simulating the molecules in a continuum dielectric with characteristics of methanol), suggests that the Z form is not stable in solution. This same behaviour is expected for the substituted compounds, which is corroborated by experimental data presented in previous investigations [A.E.H. Machado, L.M. Rodrigues, S. Gupta, A.M.F. Oliveira-Campos, A.M.S. Silva, J. Mol. Struct. 738 (2005) 239-245]. For the substituted compounds, two forms derived from E conformer (A and R) are possible. Quantum mechanical data suggest for the isolated molecules, that the low energy absorption hand of the E conformers involve at least two close electronic states. having the low-lying excited state a (1)(n,pi*) nature, and being the S-2 state attributed to a (1)(pi,pi*) transition. The data also suggest a small energy gap between the absorption peaks of A and B, related to the easy conversion between these forms. For the structures optimized in combination with the applied SCRF methodology, an states inversion is observed for the Substituted compounds, with a considerable diminish of the energy gap between A and B absorption peaks. The electronic spectra of these compounds are quite sensitive to changes in the solvent polarity. The positive solvatochromism is more evident in aprotic solvents, probably due to the polarization induced by the solute. These compounds do not fluoresce at 298 K, but present a small but perceptible fluorescence at 77 K, which seems to be favoured by the nature of the group in the 2 `-position of the phenyl ring. Moreover, such compounds present expressive values for first hyperpolarizability, which implies in good non-linear optics (NLO) responses and photoswitching capability. (C) 2008 Elsevier B.V. All rights reserved.

Spin-polarization effects in homogeneous and non-homogeneous diluted magnetic semiconductor heterostructures

Spin polarization is a key characteristic in developing spintronic devices. Diluted magnetic heterostructures (DMH), where subsequent layers of conventional and diluted magnetic semiconductors (DMS) are alternate, are one of the possible ways to obtain it. Si being the basis of modern electronics, Si or other group-IV DMH can be used to build spintronic devices directly integrated with conventional ones. In this work we study the physical properties and the spin-polarization effects of p-type DMH based in group-IV semiconductors (Si, Ge, SiGe, and SiC), by performing self-consistent (k) over right arrow . (p) over right arrow calculations in the local spin density approximation. We show that high spin polarization can be maintained in these structures below certain values of the carrier concentrations. Full spin polarization is attained in the low carrier concentration regime for carrier concentrations in the DMS layer up to similar to 2.0 x 10(19) cm(-3) for Si and up to similar to 6.0 x 10(19) cm(-3) for SiC. Partial, but still important spin polarization can be achieved for all studied group-IV DMH, with the exception of Ge for carrier concentrations up to 6.0 x 10(19) cm(-3). The role played by the effective masses and the energy splitting of the spin-orbit split-off hole bands is also discussed throughout the paper.