National Institute Of Materials Physics - Romania

Nano-composite magnets with L1(0) structure derived from binary FePt alloys and prepared as melt-spun ribbons are of current interest due to their higher operating temperature and the ability to be cast as a two-phase magnet with exchange spring magnetic properties, as both soft and hard magnetic phase may emerge from the same metastable precursor, i.e. the disordered cubic A1 phase. The present paper studies the effect of Mn addition on the thermal stability and phase structure, on the abundance of the hard magnetic phase and relative proportion of the soft ones, on the microstructure of the alloy as a function of temperature and on the overall magnetic properties. The interplay of the various magnetic sublattices in the ordering of the L1(0) phases as a consequence of introducing antiferromagnetically coupled Mn atoms in the alloy composition is discussed and interpreted in terms of microstructural changes induced by this addition as revealed by high resolution transmission electron microscopy and Xray diffraction. The temperature evolution of the phase composition and structural parameters is monitored using synchrotron radiation powder diffraction, while the compositional aspects are investigated using proton-induced X-ray emission and energy dispersive X-ray spectroscopy. Magnetic measurements reveal the magnetic parameters of interest (coercivity, remanence, Curie temperature, saturation magnetization), as well as the exchange-coupled two-phase nature of these magnets and provide information that hints at possible spin reorientation transitions in the Mn-containing planes of the L1(0) superlattice. (C) 2015 Elsevier Ltd. All rights reserved.

The non-absorption of photons with energies below the bandgap (E-g) and the thermalization of photons with energies higher than E-g are the dominant loss processes of single-junction solar cells. Rare earth doped glasses give the opportunity to convert the incident photons wavelength and hence to increase or decrease their energies. The conversion of photons energies by "up or down conversion" leads to the possibility to increase the efficiencies of all classes of single-junction solar cells. Depending on the nature of doping materials, two low energy photons can be converted into one high-energy photon (up-conversion), or one high energy photon, can be converted into two low energy photons (down-conversion). In this paper, Pr3+-Yb3+ down-conversion co-doped ZBLA glasses were tested as encapsulation materials for silicon solar cells. The J-V characterizations were done under solar simulator irradiation. The influence of Yb3+ concentration on the solar cells performances was investigated, showing that an optimum value between 0.5 and 2 mol% conducts to an increase of the device efficiency comparing to mono-doped ZBLA material. (C) 2015 Elsevier B.V. All rights reserved.

Enhancement at low temperatures of the Raman scattering excited by laser light situated near the edge of the fundamental absorption band is often encountered when studying nanoscale structures. Theory devoted to this phenomenon has established that it originates in an exciton-phonon interaction process, known as the Frohlich interaction, Such a phenomenon was observed in PbI2. The experimental data conclude that the enhancement of the Raman emission results from: (i) an optical excitation near edge of fundamental absorption band; (ii) it is conditioned by the existence of excitonic PL; (iii) its occurrence is different over stokes and anti-stokes Raman branches as result of the different overlapping of the Raman spectral range and the excitonic PL band profile; and (iv) it appears more intense in micrometric powders or in bulk crystalline material. These data are interpreted as stimulated Raman effect resulting from the mixing of the pump laser light and the excitonic light. (C) 2015 Elsevier Ltd. All rights reserved.

Dense samples (relative density > 93%) of bulk MgB2 with GeO2 additions were obtained by Spark Plasma Sintering. The critical current density J(c) of the added samples is improved at high magnetic fields when compared to the pristine sample. The optimum composition is for MgB2(GeO2)(0.005). For this sample, a J(c)(20 K) = 10(2) A/cm(2) is obtained at 5.1 T versus 3.9 T for the pristine sample. Ge substitution in the crystal lattice of MgB2 can be considered negligible, and T-c,T-onset and T-c,T-midpoint from magnetization measurements scatter within 0.2 and 0.6 K, respectively. TEM investigations show some specific details at nano scale: the tendency to form secondary phases (25-100 nm) with sphere-like or irregular shapes is observed and discussed. Samples are composites and the residual strain of MgB2 is constant for pristine and GeO2-added samples. Therefore, pinning enhancement leading to improvement of Jc for the GeO2 added samples is purely a 'microstructure' effect due to the presence of secondary phases. The point pinning is determined to be the predominant mechanism. Addition of a higher amount of GeO2 is shifting the pinning mechanism toward a grain boundary pinning. (C) 2015 Elsevier Masson SAS. All rights reserved.

The study was focused on the influence of small amounts of rare earth (RE=La, Ce, Sm, Gd, Dy, Ho, Er, Yb) addition on the microstructure, phase content and magnetic properties of cobalt ferrite bulk materials. The X-Ray diffraction measurements confirmed the formation of the spinel structure but also the presence of secondary phases of RE oxides or orthoferrite in small percentages (up to 3%). Density measurements obtained by Archimedes method revealed a similar to 1 g cm(-3) decrease for the RE doped cobalt ferrite samples compared with stoichiometric one. Both the Mossbauer and Fourier Transform Infrared Spectrocopy analysis results confirmed the formation of the spinet phase. The saturation magnetization and coercive field values of the doped samples obtained by Vibrating Sample Magnetometry were close to those of the pure cobalt ferrite. For magnetostrictive property studies the samples were analyzed using the strain gauge method. Higher maximum magnetostriction coefficients were found for the Ho, Ce, Sm and Yb doped cobalt ferrite bulk materials as related to the stoichiometric CoFe2O4 sample. Moreover, improved strain derivative was observed for these samples but at higher magnetic fields due to the low increase of the coercive field values for doped samples. (C) 2015 Elsevier B.V. All rights reserved.

L-cysteine-thiolate monolayers spontaneously self-assembled on p- and n-GaAs(100) electrodes have been investigated by electrochemical impedance spectroscopy in H2SO4 solutions. On p-doped samples a potential-induced reversible proton transfer occurs within the L-cysteine-thiolate layer during both forward and backward potential scans; in contrast, on n-doped samples it is observed only in the reverse scan. The XPS data and the fractal analysis of the AFM images point to the field - dipole interactions operating distinctively in the L-cysteine-thiolate layer formed at p-and n-doped semiconducting substrates as the origin of the observed difference. The interaction of this small but highly polar molecule with the electrostatic field driven by the diffuse distribution of the excess charge in the semiconductor subsurface region both in equilibrium and under polarization conditions turned out to play a key role in determining the optimal orientation of the two polar groups. The latter one seems to be a prerequisite for the potential-induced internal proton transfer. (C) 2015 Elsevier Ltd. All rights reserved.

Mn doped ZnO films and wires, having different manganese concentrations were synthesized by thermal oxidation of the corresponding ZnMn alloy films and wires electrodeposited on a gold substrate. Structural and optical properties were addressed with scanning electron microscopy, X-ray diffraction (XRD), Raman scattering and photoluminescence (PL). To estimate the manganese concentration in Mn doped ZnO films, X-ray photoelectron spectroscopy was used. XRD patterns indicate that the incorporation of Mn2+ ions into the Zn2+ site of ZnO lattice takes place. Quenching of the ZnO PL appears due to Mn2+ ions in the ZnO lattice. Moreover, a significant decrease in the green emission of ZnO is reported in the case of the Mn doped ZnO wire array with a Mn concentration of 1.45%. The wurtzite ZnO has a total of 12 phonon modes, namely, one longitudinal acoustic (LA), two transverse acoustic (TA), three longitudinal optical (LO), and six transverse optical branches. Compared to the undoped ZnO, a gradual up-shift of the Raman lines assigned to the 2LA and A(1) (LO) vibrational modes, from 482 and 567 cm(-1) to 532 and 580 cm(-1), respectively, takes place for the Mn doped ZnO films having a Mn concentration between 2 and 15%. Additionally, in the case of the Mn doped ZnO films with 7 and 15% Mn concentration, Raman spectra show the appearance and increase in the relative intensity of the ZnO Raman line assigned to the TA + LO vibrational mode in the 600-750 cm(-1) spectral range. For the Mn-doped ZnO wires, the presence of the Raman line peaking at 527 cm(-1) confirms the insertion of Mn2+ ions in ZnO lattice. (C) 2015 Elsevier B.V. All rights reserved.

The new 1 wt % Au/TiO2-UVM-7 catalyst was prepared and fully characterized. This heterogeneous catalyst proved to be active, selective and recyclable for the unprecedented gold-catalyzed 1,4-addition of various functionalized arylboronic acids to 2-cyclohexen-1-one and other selected enones using toluene as a solvent. The gold-based catalyst was recycled two times and played an active role in this reaction, and the nature of the solvent determined a remarkable change in the products' selectivities.