Publications

Intelligent machines using multi-level memory chips are at the ends of our fingers. The first prototypes of smart memories are under construction. The science behind the devices is growing up very rapidly. New materials with multi-level storage are discovered and tested. The applications of the smart memories are extensive and in the same time rewarding for the welfare of the human kind.

Recent experiments evidenced room temperature ferromagnetism in ZnO:Co diluted magnetic semiconductors, however the diluted semiconductor nature and the ferromagnetism strongly depends on the preparation method employed. We have investigated the ability of the sol-gel route for the synthesis of cobalt doped ZnO films grown on Si (100) and glass substrates. A homogeneous and stable Zn (1-x) CoxO sol was prepared by dissolving zinc nitrate hexahydrate and cobalt acetate tetrahydrate in a PVP (polyvinylpyrrolidone) solution, followed by annealing at 800 degrees C. Local structure studies of the Zn1-xCoxO thin films by XANES (X-ray absorption near-edge structure) and EXAFS (extended X-ray absorption fine structure) proved the co-existence of a few amount of small metal cobalt aggregates with Co in non-stoichiometric ConOm aggregates, with n > m, and m similar to 4. Low temperature (1.5 K) Kerr magnetometry give a direct proof of a superparamagnetic behavior of the magnetic aggregates and allowed an estimate of the number of magnetically active atoms in Co aggregates, which is close to the number of Co excess atoms inferred from XANES and EXAFS considerations: n - m similar to 3 atoms per aggregate. To this main superparamagnetic phase, a weak ferromagnetic phase with coercitive field of similar to 200 Oe is superimposed, most probably due to metal cobalt nanoclusters.

This paper presents new concepts for pumping very thin active laser media at pump level of hundreds of watts, output performances obtained from Yb-based microchip laser. Continuous-wave (cw) laser operation at 1.03 mu m with slope efficiency of 0.40 in Yb:YAG and low thermal lens was realized using an edge-pump configuration and a laser diode emitting at 0.94 Pm. Over 90 W cw output power was demonstrated from Yb:YAG volume of 2 x 2 x 0.4 mm(3). The optical phase distortion analysis gives focus shift bellow to 0.05 m and proves the absence of astigmatic effects. Two new architectures of thin disc concept were analysed numerically and experimentally. These new design allow very thin disk to be operated at high power level.

Bioglass films were prepared by reactive magnetron sputtering from a 45S5 target and than annealed in air tip to 550 T in order to relax the mechanical stresses. The mixtures (0, 7 and 15%) of oxygen and argon were used in order to modify the films stoichiometry. Morphological and compositional studies were performed by SEM coupled with an EDAX system. FTIR and micro Raman measurements were performed in order to investigate the molecular vibrations of the BG structure. A modified composition respect to the target one Was found for all the films, with lower silicon dioxide concentrations. The films displayed rough surfaces with spherical particles of 10-20 mu m diameter embedded in the surfaces. The Raman results evidenced important carbonate lines besides the silica-phosphate bands.

The displacement devices using piezoceramic sensors play an important role in many fields. Soft type piezoelectric materials used for such sensors arc more appealing because of their lower coercive field, relatively low modulus of elasticity, high electromechanical coupling factors and piezoelectric strain coefficients. We designed a PZT type material doped with niobium and nickel to increase the strain coefficient and still retaining a high coupling coefficient. The chemical formula was Pb0.9Ba0.08Nb0.03Ni0.06Zr0.46Ti0.40O3. The main piezoelectric parameters were: d(33)=680 pm/V. d(31)=-235 pm/V, k(p)=0.64 and k(31)=0.42. A unimorph transducer was made from a thin sheet of such a piezoceramic material, tightly bonded on a thin optically polished glass plate and mounted as a cantilever. The performance of this transducer was evaluated by means of a new optical method which makes use of the Moire deflectometry and provides high precision. The method is rather simply and consists mainly in the following: the Moire fringes rotate when the bimorph bends under the influence of the electric field and by measuring the rotating angle of the fringes one can easily determine the bimorph curvature and the free end displacement.

The potential of X-ray micro-tomography in characterizing the relevant material issues as identified during manufacturing processes was investigated. For this purpose, the 3D X-ray micro-tomography (X mu RT) analysis was applied to the characterization of structural integrity of Nb3Sn superconducting wires of differing topology and the evolution of the density distribution of ceramic samples manufactured by Field Assisted Sintering Techniques (FAST). The latter technique was used to consolidate the ceramic, metal (Ni) and composite powders (MgB2). The usual scanning electron microscope (SEM) technique was enhanced by the high resolution X mu RT in order to describe the volumetric density distribution before sintering and at different moments of the thermal cycle. Two types of samples sintered Ni and high density MgB2 superconductors with typical volume of 0.8 divided by 1.5-10-9 m(3) were sampled at space resolution around 5 mu m. For the sintered Ni, the 3D reconstructed volumes revealed the grain connectivity, necks formation and particle rearrangement in the densification process. The X mu RT analysis was essential in explaining the differences in superconducting properties of MgB2 samples in terms of different volumetric structures. For the Nb3Sn multifilamentary wire which is at basis of many practical superconductors, the 3D tomography enabled the determination of the number of inter-filament contacts as well as the twist-pitch parameter. Advantages of our method versus the invasive etching techniques for the determination of the twist-pitch parameter were outlined.

The efficiency of different surfactants (anionic, cationic, neutral) as chemical agents favoring in the unbinding of nanotube bundles in isolated entities is investigated by Raman and UV-VIS-NIR spectroscopy. ne neutral surfactants Tween 20 - polyoxyethylene sorbitan monolaurate (T-20-PSM) and Tween 80 - polyoxyethylene (80) sorbitan monooleate (T-80-PSM) have been found the most efficient. The isolated single walled carbon nanotubes (SWNTs) nanotubes are apprised at lambda(exc) = 1064 nm by two simultaneous modifications of the Raman spectra: i) narrowing of the band associated to the transversal vibration modes, i.e., G band (similar to 1592 cm(-1)); ii) change of shape of the complex band associated to the radial vibration modes (RBM) consequent on the decrease of the intensity of the components associated with the bundled nanotubes. The individualization of nanotubes is evidenced also in the UV-VIS-NIR absorption spectra by the appearance of a fine structure in the wide absorption band (similar to 1400-1750 nm) that is associated with the transitions E-S(11) of semiconducting SWNTs. The fine structure grows gradually with the separation degree in isolated nanotubes entities.

Hg-based multi- and super-multi-layered cuprates HgBa2Can-1CunOy (Hg:12(n-1)(n)) (n >= 4) are composed of the charge reservoir layer (CRL) HgBa2Ox, and of CuO2 planes: outer planes (OP) with pyramidal oxygen coordination and inner planes (IP) with square oxygen coordination, with different charge distribution. This fact gives rise to very interesting phenomena, like coexistence of superconductivity (SC) in OPs with antiferromagnetism (AF) in IPs in Hg:1245. The dependence of T-c on the number n of CuO2 planes is also interesting: for n increasing from 4 to 5 and to 6, T-c decrease from 125 to 108 and then to about 103 K; further increase in n keeps T-c constant at about 103 K. In Hg:1245 we show that the AF ordering at T-N of 60 K has no influence on vortex dynamics, and there is a quite robust c-axis supercurrent. Vortex melting lines of various samples showed that Hg-based multi- layered cuprates (n = 4, 5) behaves in the usual way (increasing n resulted in decrease of Josephson coupling (JC) and a less sharp melting line) and are well described by the usual anisotropic GL-based theory. Samples with n >= 6 have all the same melting line, suggesting that, in this case, the short-range JC becomes much less important than the long-range magnetic coupling (MC). However, since the JC between two pancakes in OPs of adjacent unit cells (pancakes separated by the thin CRL) is large, we suggest that the vortex matter in Hg-based super-multilayered cuprates is composed of magnetically-coupled pancake-vortex-molecules.

The spectroscopic investigation of Tm3+ or (Nd, Yb, Tm)-doped Sc2O3 transparent ceramics, as laser-active media for visible and IR emission, was performed. The spectra are dominated by Tm3+ ions in sites of C-2 symmetry and an energy level scheme and other spectral parameters were determined. The strong concentration quenching of H-3(4) level emission in Tm:Sc2O3 at 300 K is discussed by considering the resonant cross-relaxation (H-3(4)-> F-3(4))-(H-3(6)-> F-3(4)) process and multipolar interactions of various orders. The main energy-transfer processes leading to the blue upconversion emission from the (1)G(4) Tm3+ level in (Tm, Nd, Yb):Sc2O3 ceramic, under pulsed pumping at 808.3 nm were evidenced. (c) 2008 Elsevier B.V. All rights reserved.