National Institute Of Materials Physics - Romania

Double perovskite Sr2FeMoO6 type compounds show special properties, such as magnetoresistance response at relatively small-applied fields and at rather high temperatures (ca. 400 K). Perovskite precursors were obtained unconventionally, in liquid phase, using oxalic acid as complexing agent, in the presence of ethylene glycol as polymerization agent. Ordering control concerning the magnetic properties is predictable to be strongly influenced by the synthesis route. The thermal stability of the precursors and the phase transitions of the oxide samples were analyzed by thermal analysis method, IR spectroscopy, Mossbauer spectroscopy, X-ray diffraction method and magnetic measurements. The experimental data for the perovskite compounds prepared by using oxalic acid as complexing agent revealed a double perovskite structure of analyzed samples, convenient magnetorezistive response (3.51 mu(B)/f.u), in good agreement with the Mossbauer spectra. Moreover, Mossbauer spectreoscopy allows the distinction between the ordered and the disordered phases of the perovskite structure and put in evidence the strong dependence of the structural and magnetic characteristics of the double perovskites on the synthesis route.

Angle-resolved X-ray photoelectron spectroscopy (ARXPS) analysis has been performed on GaAs (10 0) surfaces in different conditions as naturally oxidized, Ar+ ion sputtering (E = 1-5 keV) and chemical etching in H2SO4/H2O2/H2O(3: 1: 1). The most sensitive angle to the surface compositional changes was the take-off angle (TOA): 25 degrees. Native oxide phases on GaAs consist of a mixture of Ga2O3, As2O3 and As2O5. Ar+ ion sputtering procedure modifies the surface composition, in the altered layer where the concentration ratio C-Ga/C-As tends to 1.5-1.6. Wet chemical etching removes the oxide layer and the As-rich region from the surface. In the experiment combining chemical etching with Ar+ ion sputtering for cleaning purpose, the native oxides are removed from the surface and C-Ga/C-As tends to stoichiometry. The experiment on native oxide reconstruction after storage in high-vacuum conditions (P similar to 10(-8) Torr) provides evidence of the high reactivity of GaAs (10 0) surfaces. We have observed the presence of an As oxide (BE = 43 eV) within a concentration range of 2-3%. (C) 2007 Elsevier B.V. All rights reserved.

In the last years the ternary stabilized zirconia ceramics have attracted a considerable attention as a source of materials for structural ceramics. In present paper, the simultaneous influence of Eu3+ and Y3+ cations on zirconia powders (obtained by co-precipitation of aqueous salts solutions) on stabilization process in conjunction with the structural characteristics was investigated. A conventional sintering method (in electric furnace) at 1350 degrees C temperature range was applied. SEM+EDAX and XPS (X-ray Photoelectron Spectroscopic) analysis were performed. The Eu3+ Mossbauer parameters: isomer shift and quadrupole splitting have been obtained for 21.6 keV radiation of Eu-151 are in good agreement with the XPS and XRD-obtained crystallographic data of the studied compositions.

Nanosized magnetite particles embedded in polypyrrole matrix have been studied by Mossbauer and electron magnetic resonance spectroscopy. Comparison with as grown magnetite is made. Hyperfine fields distribution is determined and line shape of resonance curves are discussed in terms of composite structure, sizes and treatments.

Fe-Pt alloys have gained much interest for the occurrence of permanent magnetic features resulting from the L1(0) ordered face-centred-tetragonal FePt phase with very high magnetic crystalline anisotropy (7x10(6) J/m(3)). An amorphous melt spun ribbons of the composition Fe51Pt27Nb2B20 has been synthesized by the rapid solidification technique and its microstructure and magnetic properties were studied. After appropriate annealing, an ordered face-centred-tetragonal (f.c.t.) L1(0) phase is formed. X-ray analysis revealed a structural phase transformation from the body-centered-cubic Al to f.c.t. L1(0) phase and this produce magnetic hardening of the alloy, upon appropriate annealing conditions. Extremely performant magnetic properties, typical for exchange spring magnets, are obtained.

Fluorescence measurements have been made on pure and doped bulk, mechanically polished wafers of crystalline m-DNB and benzil obtained by cutting ingots grown by the Bridgman-Stockbarger method modified for organic compounds crystallization. By comparison with pure matrices, we have investigated the effect of an inorganic dopant (iodine, silver, sodium) and of an organic dopant (m-DNB, naphthalene) on the emission characteristics (position and shape) of these molecular crystals. A slight shift of the emission peaks through high energy and an intense emission peak situated around 2.35 eV correlated with the local trapping level attributed to structural defects, which are involved in radiative processes, have been evidenced in iodine-doped m-DNB. The emission peak of m-DNB-doped benzil situated in the high-energy range (2.97 eV) is associated with direct emission activity of m-DNB, suggesting that this is an active impurity in benzil molecular matrix. We have not observed in benzil any evidence of indirect action of the impurity molecules (atoms) associated with the traps represented by the structural defects that generate changes in the energy levels of the neighbouring molecules and are correlated with different growth conditions. We have not remarked any involvement of the studied inorganic metallic impurities and of some organic impurities, such as naphthalene, in the radiative recombination processes in benzil matrix. (c) 2007 Elsevier B.V. All rights reserved.

Leakage currents determine bumps in the capacitance-voltage characteristics of metal-oxide-semiconductor-type heterostructures with capacitance values larger than the insulator capacitance. A nearly parallel shift with frequency is also observed. These aspects are connected with the presence of a high density of interface states. We illustrate these effects for the case of nano-PbS/SiO2/Si heterostructures. The phenomena were simulated and we propose a method to quickly estimate the interface state density level of a heterostructure when leakage channels are present. (C) 2008 American Institute of Physics.

Magnetite nanoparticles were synthesized from Fe3+/Fe2+ salts in the presence of glycine (2:1:8-2:1:3 ratio metal ions/glycine) using NH4OH 25% as precipitating agent. The parameters which influence the synthesis are discussed. The magnetite powders were characterized by XRD, IR spectra, magnetic measurements and Mossbauer spectra. The average diameter of crystallites is in the range 10 - 13 nm. The magnetization value is similar to 43 emu/g.

The thermoluminescence of undoped and Mn-doped ZnF2 Was investigated at the dopant concentration of 1.5 % mole Mn after gamma - ray irradiation at room temperature. The glow curve from undoped ZnF2 presents two peaks centered at 86 degrees C and 273 degrees C, while the ZnF2:Mn presents two peaks located at higher temperatures (325 degrees C and 365 degrees C), whose height ratio is modified with dose. The dose responses and fading process were also examined. The radiative thermostimulated recombination was found to be of the second kinetic order based on the geometric factor calculation. The trap parameters were obtained as activation energy and frequency factor.

The optical transmission of a liquid crystal cell with in-plane switching (IPS) mode is studied as function of the thickness for different rubbing angles and anchoring energy. It was found that the light transmission has a maximum at the value of similar to 0.66 for the ratio of the optical path difference (introduced by the undisturbed cell) to X and that this value does not depend on lambda, in the case of a very strong anchoring energy and of a rubbing angle of 13 degrees. These experimental results agree very well with simulations using a simplified model. Moreover, the simulations have shown that the thickness corresponding to such a maximum optical path difference depends non-monotonously on the anchoring strength for five values of the rubbing angles (in the range 0 to 40 degrees). For a given anchoring energy this thickness decreases with increasing rubbing angle.