Publications

In this work, we present a new, fast and high yield synthesis method for obtaining microcrystalline yttrium orthoferrite YFeO3, based on hydrothermal technique. The advantage of the method consists in the direct crystallization of the material without the necessity of the calcination. The quality of the so-obtained compound is checked by X-ray diffraction and XPS measurements. Unusual behavior of the zero field cooled (ZFC) - field cooled (FC) magnetization curves (ZFC > FC) was also observed in YFeO3 and a possible explanation could be related to the effect of the synthesis methods on the magnetic properties of yttrium orthoferrite. (C) 2014 Elsevier B.V. All rights reserved.

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Atomic intermixing processes in relation to structural aspects and phase formation in Be based thin films subjected to different annealing treatments simulating the case of re-deposited layered structures on plasma facing components in nuclear fusion devices are reported. Accordingly, bilayers of Be/W and Be/C have been deposited on Si(001) substrates with Fe buffer layers. The Fe films have been prepared by radiofrequency sputtering and further processed by annealing in hydrogen atmosphere at 300 degrees C, for 90 min, at a pressure of 10 bars of H-2. After the Be/W and Be/C bilayer deposition by means of thermionic vacuum arc method, annealing in vacuum at 600 degrees C, for 10 min has been applied to the complex structures. The influence of annealing on the phase composition and atomic intermixing processes in the complex structures has been studied by means of X-ray photoelectron spectroscopy (XPS) and conversion electron Mossbauer spectroscopy (CEMS). The layered structures present an oxidation gradient with oxide phases in the uppermost layers and non-oxidized phases in the lower layers, as observed from the XPS data. The CEMS results revealed that the as-deposited structures contain a main metallic Fe phase and secondary superparamagnetic Fe oxide phases at the Fe/Be interface, while annealed samples present a large contribution of Fe-Be and Fe-C mixtures. The annealing treatment induces considerable atomic interdiffusion, strongly dependent on the nature of the upper layer. In the case of Be/W system, the annealing provides a much rougher Be/W interface, while in case of the Be/C structure, the annealing treatment only homogenize the structure over the whole depth. (C) 2014 Elsevier B.V. All rights reserved.

Frequency-dependent AC susceptibility measurements have been used to estimate the critical current densities and pinning potential of two 1.1-mu m-thick YBCO films with nanoengineered pinning centres. The films were grown by pulsed laser deposition using a target with 4 % BaZrO3 nanoinclusions. Data were analysed in the framework of the Anderson-Kim model, collective pinning model, and a model due to Zeldov that implies a logarithmic dependence of pinning potential on the current density. It was found that the latter describes better our experimental data, and that the resulting pinning potential has reasonable high values in high magnetic field up to 9 T, indicating a high current-carrying capability in high magnetic fields.

Cobalt chromite (CoCr2O4) was synthesized through the precursor method. The precursors: (NH4)(3)[CoCr2(C4O6H4)(4)(OH)(3)]center dot 4H(2)O, (NH4)(3)[CoCr2(C6O7H10)(4)(C6O7H9)]center dot 5H(2)O were characterized by elemental chemical analysis, infrared (IR) and ultraviolet-visible (UV-vis) spectroscopy, and thermal analysis. The final oxides were characterized by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM/TEM), UV-vis, IR, Raman spectroscopy (RS), magnetic measurements, N-2 adsorption-desorption analyses and X-ray photoelectron spectroscopy (XPS). XRD confirmed the cubic CoCr2O4 phase only and determined average crystallite sizes between 14 and 21 nm. Electron microscopy revealed morphology corresponding to the complete crystallization into cubic CoCr2O4. All the samples presented ferrimagnetic ordering below the Currie temperature (T-c), and a phase transition at T-s similar to 26 K attributed to the onset of long-range spiral magnetic order. The CoCr2O4 nanoparticles generated through the gluconate route following calcination at 700 degrees C for 1 h were found to have the best catalytic activity in the total oxidation of methane. (C) 2014 Elsevier Ltd. All rights reserved.

Spark plasma sintering (SPS) was applied for fabrication of dense (relative density > 97 %) bulk MgB2 samples added with Ge. Mixtures of MgB2 and Ge powders with starting compositions (MgB2) Ge (x) , x=0.005, 0.01, and 0.03 were used. Added samples show enhancement of the critical current density at high magnetic fields when compared to a pristine reference sample. The optimum composition is for x=0.005 and for this sample J (c)(20 K) =10(2)A/cm (2) is obtained at 5 T, while for the reference sample is obtained at 3.9 T. Ge does not substitute in the crystal lattice of MgB2 and T (c,onset) or T (c,midpoint) from magnetization measurements scatter within 0.15 or 0.3 K, respectively.

AC susceptibility and DC magnetization relaxation measurements have been performed for YBa2Cu3O7 films containing BaZrO3 nanorods preferentially oriented along the c axis, with the external magnetic field perpendicular to the film surface. It was found that the effective vortex activation energy U (e) supplied by the standard analysis of the frequency dependent AC magnetic response remains very high even in the vicinity of the irreversibility line indicated by the DC relaxation. We show that the large U (e) values appearing in the AC signal are related to the pinning enhanced viscous drag and significant vortex velocities.

The Mossbauer spectra of a FeSe0.3Te0.7 single crystal grown by the Bridgman method were analysed across the superconducting transition by considering the interplay between the structure and electron configuration of the transition metal. The magnetically determined superconducting critical temperature is T-C similar to 14 K. The Fe-57 Mossbauer spectra collected in the temperature range from 5 to 200K mainly have an asymmetric doublet pattern, which was conveniently fitted by the full Hamiltonian method. No effective magnetic moment ascribed to the superconducting phase was observed down to 5K. The unusual behaviour observed below similar to 17K for the chemical isomer shift and quadrupole splitting may be associated with an electron reconfiguration process intimately related to an unusual lattice distortion accompanying the superconducting transition. The decreasing trend of the total absorption spectral area and second-order Doppler shift during cooling the sample below the critical temperature, point to enhanced phonon activation in the superconducting state.

In this paper we report conformational changes recorded on a protein molecule (alpha-amylase) under green light irradiation. In order to explain the experimental results we advanced the hypothesis that green light induces electric dipoles in the protein, which interact with each other, generating conformational modifications toward a more compact design, with different physical properties. The experiments were carried out with un-polarized light (lambda = 520 nm) from a light-emitting-diode (1000 lm, 20 W, 105 mW on the target). In view of the character of our hypothesis, and corroborated with all our experimental results, we suggest that this phenomenon may be more extended and general, specific for a larger class of proteins, occurring on the protein macromolecules under the green light. The effects of a-amylase protein irradiation were revealed by circular dichroism, fluorescence, Raman and FTIR-spectroscopies, zeta potential, cyclic voltammetry, electric impedance spectroscopy and atomic force microscopy. Tentatively, we term the novel conformations as P* (polarized) proteins.

Quasi-static polarization induced by dc electric field was studied in a broad range of temperatures in epitaxial films of relaxor ferroelectric PbMg1/3Nb2/3O3 and PbSc1/2Nb1/2O3. The electric field was applied and the response was measured along the out-of-plane crystal direction of the epitaxial perovskite-structure (001) oriented films. The films remained in the relaxor state in zero-field cooling. A new polar state can be induced by electric field at a critical temperature below 100 K. The critical field and the induced polarization increased on cooling and reached the bulk-like values at 20 K. The orientational anisotropy of thin-film dipolar systems is discussed as a possible reason for the observed stable relaxor state.