Publications

The occurrence of a second magnetization peak (SMP) on the dc magnetization curves of superconducting single crystals with random quenched disorder in increasing external magnetic field H is quite common. Many models for this intriguing phenomenon have been proposed so far, but its origin is still under strong debate. Optimally doped and overdoped La2-xSrxCuO4 single crystals (x >= 0.15) exhibit an SMP over a large temperature interval, up to very close to the critical temperature. By decreasing doping, it was found that with H oriented along the c axis the SMP completely disappears in the doping domain of well developed static charge and spin stripes (x similar to 1/8), reappearing for x <= 0.10. This behaviour follows the instability of the Bragg vortex glass generated by the static stripe order (as revealed by small-angle neutron scattering experiments), which is confirmed by our magnetization relaxation measurements. If H is along the (a, b) planes, the SMP is observed for all investigated specimens. The presented analysis supports the scenario in which the SMP is generated by the pinning-induced disordering of the low-field Bragg vortex glass in the dynamic conditions of dc magnetic measurements. (C) 2015 Elsevier B.V. All rights reserved.

Thermo-ionic Vacuum Arc evaporation method was selected for the synthesis of Fe/Cu/Ni/Cu multilayer structures on Si (1 0 0) substrates. The aim of the study was the preparation and characterization of structures featuring a giant magnetoresistance effect. This was accomplished by inducing the formation of nanosized ferromagnetic crystallites in multilayer nonmagnetic solutions via atomic inter-diffusion processes by the tuning of deposition parameters. Layer-by-layer and inter-diffused type structures were prepared and comparatively analyzed by scanning electron microscopy, X-ray microanalysis, atomic force microscopy, X-ray diffraction and high-resolution transmission electron microscopy coupled with selected area electron diffraction. We presented the influence of the microstructure on electric and magnetic properties of the submicron-sized multilayers. The dependence of the electric resistance and the magnetoresistance on the composition, structure, morphology and roughness of the layers was established. We obtained an electric resistance value of 1.22 Omega for the layer-by-layer type structure, and 0.46 Omega for the inter-diffusion designed structure. Using the atomic inter-diffusion we succeeded in achieving an improvement of the magnetoresistive effect, from 0.1% to 2.3%. (C) 2015 Elsevier B.V. All rights reserved.

Pyridoxal isonicotinoyl hydrazone (HL) and its copper(II) complexes: [CuLCI(H2O)(2)] (I), [CuL(OAc)](H2O) (2), [CuL(OAcac)] (3), [CuL(NO3)](H2O) (4), [CuL2](H2O) (5) were synthesized and characterized. The ligand has been obtained by condensation of 3-hydroxy-5-(hydroxymethyl)-2-methylpyridine-4-carbaldehyde (pyridoxal) hydrochloride with isonicotinoyl hydrazide. The characterization of the formed compounds was done by H-1 NMR, C-13 NMR, UV-Vis, IR and EPR spectroscopy as well by thermal studies and elemental analysis. The crystal structure of HL has been determined by single crystal X-ray diffraction studies. The microbiological effect of the ligand and all five complexes against both Gram-positive and Gram-negative strains, Staphylococcus aureus ATTC 6538, Bacillus cereus ATCC 14579, Escherichia call W3110 and Pseudomonas aeruginosa ATCC 9027 was tested. (C) 2015 Elsevier Ltd. All rights reserved.

A two dimensional photonic crystal (2DPhC) with triangular symmetry is investigated using optical reflectivity measurements and numerical calculations. The system has been obtained by direct laser writing, using a pulsed laser (lambda = 775 nm), perforating an In-doped Ge wafer. A lattice of holes with well-defined symmetry has been designed. Analyzing the spectral signature of PBGs recorded experimentaly with finite difference time domain theoretical calculations one was able to prove the relation between the geometric parameters (hole format, lattice constant) of the system and its ability to trap and guide the radiation in specific energy range. It was shown that at low frequency and telecommunication ranges of transvelsal electric modes photonic band gap occur. This structure may have potential aplications in designing photonic devices with applications in energy storage and conversion as potential alternative to Si-based technology.

Within the field of chemical gas sensing, ammonia detection represents one of the nowadays challenges, due to its adverse effects for human health. Real operating investigations are imposed by the high solubility potential of NH3 at room temperature. Thick films of Ba0.75Sr0.25TiO3 material have been investigated by means of electrical resistance and capacitance changes and correlated with the photoacoustic outputs. The experimental findings have been discussed in terms of Grotthuss mechanism, with respect to the ionic/electronic conduction within BST, toward ammonia detection enhanced by the presence of water vapors when operated at room temperature. So an appropriate NH3 sensing mechanism was proposed. (C) 2015 Elsevier B.V. All rights reserved.

CuOx nanorods grown by calcination on a Cu plate and TiO2 successive layers were assembled by chemical means to obtain a broad band light captor. The synthesis of composite with complex architecture aimed at maximization of light absorption and at improvement of electron-holes separation of via the multiple n-p heterojunctions created at TiO2-CuOx boundaries, formed mostly by filling the pore network composite material with CuOx. Au nanoparticles were embedded into TiO2 matrix to enhance the light harvesting ability in visible domain via surface plasmon resonance (SPR) phenomenon and to help the photo-generated charge separation through the formation of Schottky barriers. Dual p-n semiconducting character of the composite was clearly evidenced by photoelectrochemical measurements. The composite material exhibited photoresponse even at wavelengths higher than 420 nm (in visible domain), both in cathodic and anodic regions. The enhanced light adsorption characteristics, the advancement in synthesis procedure of CuOx-Au_TiO2 composite material provide rationale for further development of novel light captor composites for photoenergy conversion. Other applications such electronics and gas sensors are envisaged. (C) 2015 Elsevier B.V. All rights reserved.

A numerical extension of the simple Stoner-Wohlfarth model to the case of bi-dimensional angular distributions of easy axis is provided. The results are particularized in case of step-like, Gaussian-like and user defined distributions. In spite of its simplicity, the model can be applied to magnetically textured thin films and multilayers with in-plane magnetic anisotropy, independently on the texture source. Exemplifications are provided for a simple ferromagnetic textured FeCo film as well as for a FeMn/FeCoi CufFeCo spin valve structure. (C) 2015 Elsevier B.V. All rights reserved.

xRuO(2)-(1-x)alpha-Fe2O3 (x=0.1, 0.3,0.5 and 0.7) nanoparticle systems were successfully synthesized by mechanochemical activation of RuO2 and alpha-Fe2O3 mixtures for 0-12 h of ball milling. Mossbauer spectroscopy, X-ray diffraction (XRD), magnetic measurements, simultaneous differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) were employed to investigate the phase evolution of xRuO(2)- (1-x)alpha-Fe2O3 nanoparticle systems under the mechanochemical activation process. The Mossbauer studies showed that the spectrum of the mechanochemically activated composites evolved from a sextet for alpha-Fe2O3 (hematite) to four sextets and a doublet upon duration of the milling process with ruthenium oxide. Rietveld refinement of the XRD patterns yielded the values of lattice parameters as function of composition and milling times. The presence of Ru-substituted hematite and Fe-doped ruthenium oxide was evidenced and correlated with differences between ionic radii of Fe3+ and Ru4+. Magnetic measurements recorded at 5 and 300 K in applied magnetic fields of 50,000 and 100,000 Oe showed that the estimated saturation magnetization of the milled samples increased with ball milling time while preserving a multidomain magnetic structure. The Morin transformation was evidenced by zero-field cooling-field cooling (ZFC-FC) measurements in 200 Oe and 1 T, for samples milled for 0 and 8 h of mechanochemical activation. These results correlate well with the DSC-TGA measurements, which support the formation of mixedoxide solid solutions in the system under investigation. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Single molecular switches are basic device elements in organic electronics. The pentacene analogue anthradithiophene (ADT) shows a fully reversible binary switching between different adsorption conformations on a metallic surface accompanied by a charge transfer. These transitions are activated locally in single molecules in a low-temperature scanning tunneling microscope. The switching induces changes between bistable orbital structures and energy level alignment at the interface. The most stable geometry, the "off" state, which all molecules adopt upon evaporation, corresponds to a short adsorption distance at which the electronic interactions of the acene rings bend the central part of the molecule toward the surface accompanied by a significant charge transfer from the metallic surface to the ADT molecules. This leads to a shift of the lowest unoccupied molecular orbital down to the Fermi level (E-F). In the "on" state the molecule has a flat geometry at a larger distance from the surface; consequently the interaction is weaker, resulting in a negligible charge transfer with an orbital structure resembling the highest occupied molecular orbital when imaged close to E-F. The potential barrier between these two states can be overcome reversibly by injecting charge carriers locally into individual molecules. Voltage-controlled current traces show a hysteresis characteristic of a bipolar switching behavior. The interpretation is supported by first-principles calculations.

Chromium substituted cobalt ferrites (CoFe2-xCrxO4, 0 <= x <= 2) were synthesized through solution combustion method using glycine as fuel, named glycine-nitrates process (GNP). As evidenced by X-ray diffraction data (XRD), single cubic spinel phase was formed for all CoFe2-xCrxO4 (0 <= x <= 2) series. The cubic lattice parameter (a) decreases with increasing chromium content. Room temperature Fe-57 Mossbauer spectra revealed the Fe3+ and Cr3+ site occupancy, the local hyperfine magnetic fields and the substitution of Fe3+ by Cr3+ in the lattice. Scanning electron microscopy (SEM) showed a refinement of particle size with the increase of Cr3+ content. Magnetic measurements from 5 K to 120 K have shown a dropping in the saturation magnetization as the chromium content increases. This behaviour has been explained in terms of substitution of Fe3+ by Cr3+ in the cubic lattice of cobalt ferrite.