Publications

The UV exposure of Fe:PVA thin films induces modifications of both the refractive index and the absorption coefficient. M-line spectroscopy showed the variation of the refractive index while Mossbauer measurements present the Fe2+/Fe3+ ratio versus the exposure energy. The diffraction efficiency experimental data were satisfactorily explained in correlation with the Fe2+/Fe3+ ratio.

Samples of Fe40Ni38Mo4B18, Fe78B13Si9 and Fe66Co18B15Si1 metallic glasses, selected with respect to the different values of their magnetostriction constants, were irradiated with alpha particle beams (W = 2.8 MeV) using radiation doses of 10(16) and 10(17)/cm(2). The fundamental effects underlying the interaction of alpha particle beams with amorphous magnets were studied by transmission and conversion electron Mossbauer spectroscopy (CEMS), hysteresis loops and a.c. susceptibility measurements, and scanning electron microscopy (SEM). The evolution of phases and microstructure during the radiation-induced amorphous-to-crystalline transformation was found to depend on the particle flux and sample composition. Differences between bulk and surface behaviors of the magnetic texture and constituent phases were evidenced. Radiation-driven changes in the saturation magnetic moment, coercive held, real and imaginary components of the a.c. response were observed for all samples studied and found to depend on the radiation dose employed. The surface morphology of the radiation-induced crystalline precipitates was shown to vary as a function of composition, such that both spherical and dendritical modes of growth were observed. By stimulating unconventional pathways for the crystallization process, the interaction of alpha particle beams with glassy ferromagnets offers unique opportunities to understand the fundamentals of nucleation and growth in these systems. (C) 1998 Acta Metallurgica inc. Published by Elsevier Science Ltd. All rights reserved.

The electronic structure of different (001) (AlAs)(m)(InAs)(1)(GaAs)(m) multilayer structures is studied. We have thus seen the influence of the relative thicknesses of the constituent materials and of the positions of the InAs principal layers in the multilayer structures on the energy eigenstates and on their spatial localization. The calculations are based on an sp(3)s* empirical tight-binding model and on the surface Green function matching method. (C) 1998 Elsevier Science B.V. All rights reserved.

The synthesis of nanosized (1-100 nm) particles is an active research field in chemical processing technologies. The process may be of particular relevance in the case of the widely used iron-based materials. In this paper, we report on the preparation and characterization of ultrafine powders produced by pulsed infrared laser pyrolysis of iron pentacarbonyl, Fe(CO)(5), in sensitized mixtures. The synthesis is performed in a flow reactor, at rather high pressure (400 mbar) and high laser fluence (similar to 5 J cm(-2)). The average diameter of the particles is 30 nm. Different characterization methods of the fine powder aged in atmosphere indicate the presence of a substantial oxidic portion, identified as the beta-Fe2O3 . H2O form. Some experimental observations suggest the possibility of iron-oxides formation during synthesis in the flow reactor. The relevant mechanisms for this reaction route are discussed. (C) 1998 Elsevier Science S.A. All rights reserved.

Surface enhanced Raman scattering (SERS) is being increasingly used for the study of the structural properties of conducting polymer thin films. It is generally accepted that the enhancement process has an electromagnetic origin, arising from the excitation of surface plasmons in the metal support on which the polymer film is deposited. However, the electromagnetic enhancement is also accompanied by a chemical process, for which available experimental data are scarce. The chemical process originates from the increase in the polarizability of the molecules at the metal surface under the action of the incident radiation, which leads to the formation of new chemical bonds with the atoms of the metal support. The present work was devoted to the study of the SERS spectra of poly(3-hexylthiophene) deposited on rough Ag and Au supports by evaporating the solvent from a solution of known concentration. The experiments revealed the existence of a chemical surface effect. The results obtained show that the SERS spectra depend on the oxidizing properties of the metal surface and on the nature of the solvent. This dependence is explained by the existence of some interfacial reactions that lead to the formation of interface compounds of the type MeX (Me = Ag or Au, X = Cl or O). The SERS measurements reported here reveal an increase in the intensities of the Raman lines, accompanied by a modification of the corresponding intensity ratios, when the degree of doping is increased. It was observed for the first time by SERS spectroscopy that the doping of 3-PHT with FeCl3 leads to the appearance of a state of disorder in the structure of the macromolecular chain, as a result of steric hindrance effects. (C) 1998 John Whey & Sons, Ltd.

A theoretical study of the radiation effects, from the point of view of the non-ionising energy loss and of the concentration of primary defects, in some A(3)B(5) semiconductors, has been performed. These effects have been analysed for charged pions, in the energy range 50 MeV-50 GeV. The investigated materials have been GaAs, InP, GaP, InAs and InSb, and the results have been compared with silicon, as a reference material, keeping into account the peculiarities of the pion-nucleus interaction, and the recoil energy redistribution in the lattice. The results of the calculations have put in evidence the higher radiation resistance of Si, GaP and GaAs in the whole energy range investigated, with respect to the other analysed materials: InP, InAs and InSb, that proved to be of interest from this point of view only in the intermediate energy region. (C) 1998 Elsevier Science B.V. All rights reserved.

The Mossbauer spectra of the melt-spun and subsequently intensively ball-milled Fe65Rh35, Fe50Rh50 and Fe26Rh74 alloys with fee structure are characterized at 4.2 K by broad hyperfine field distributions. The magnetic critical temperatures are 60(5) for the first one and, respectively, 85(5) K for the second and the latter composition, in good agreement with recent magnetic susceptibility studies. The distributions arise as result of the chemical disorder, local strains and atomic displacements, owing to the drastic ball-milling process. The values of the Fe magnetic moments at the peaks of the hyperfine field distributions reached 1.1, 1.6 and 1.9 mu(B) with decreasing iron concentration in semi-quantitative agreement with earlier experimental and recent theoretical evaluations of Fe moments in gamma-Fe and fee Fe50Rh50. Above 400 K an irreversible transformation to the bcc structure occurs for the two Fe-rich alloys with three types of Fe sites in the Fe richest alloy and two different Fe sites in the partially disordered Fe50Rh50. Our analysis allowed us to describe the nn atomic arrangements around these sites. (C) 1998 Elsevier Science S.A. All rights reserved.

The persistent extinction of fluorescence emission of Eu(3+) in glasses activated with europium and cerium is reported for the first time to the authors' knowledge. The glass samples containing EU(3+) and Ce(3+) were initially colorless and transparent and exhibited intense emission peaks at 592 ana 612 nm assigned to the (5)D0-(7)F(1,2) transitions of EU3+. Th, complete extinction of the Eu(3+)-ion emission was obtained as an effect of multipulse excimer-UV-laser (lambda = 248 nm, tau(FWHM) greater than or equal to 20 ns) irradiation of the glass samples. Fluorescence microscopy, Mossbauer spectrometry, and electron spin resonance were applied for investigation of the modifications induced by the laser treatment. As a decisive proof of the extinction of fluorescence me succeeded in recording three-dimensional fluorescent photographic patterns within the activated samples. (C) 1998 Optical Society of America.

The crystal structure of (Bi1.82Pb0.18) (ST1.84Pb0.16) CuO6 was determined from single-crystal intensity data measured on a four-circle diffractometer. The structure is orthorhombic, space group Cccm (No. 66); the lattice parameters: a=5.392(3) Angstrom, b=24.603(5) Angstrom, c=5.300(3) Angstrom, V=703.2 (5) Angstrom(3) (the space group can be denoted as Amaa by a cyclic transformation of the cell constants). The R value was 0.048 for 418 unique reflections with I>3.00 sigma(I). One of the O atoms bonded to Bi ions is disordered at two sites with a half occupancy and B-eq=3.6(6) Angstrom(2); although the thermal parameter is not so large as that reported by Torardi et al. [Phys. Rev. B 38, 225 (1988)], the disorder suggests that the crystal has a lower symmetry. The valence state of the Cu ion in the present compound was evaluated to be trivalent based on the Cu-O distances [Asbrink and Norrby, Acta Crystallogr. B 26, 8 (1970)] and the valence of Bi was estimated in comparison with that in other compounds. The O(3) sublattice dynamics in these types of superconductors is discussed based on the structural investigations.

The improvement of intragranular critical current density, j(c), and flux pinning obtained by thermal neutron irradiation (fluences between 10(16) - 10(18) neutrons/cm(2)) in YBa2Cu3O7,(-x) doped with 8 mol% lithium fluoride (LiF) (YBCO-LIF) is explained by the occurrence of various irradiation defects (point defect clusters, dislocation loops, stacking faults) observed by transmission electron microscopy (TEM), A twin fading process is also revealed accompanied by a lowering of the orthorhombicity index. At higher irradiation fluences (similar to 5 x 10(17) 10(18) neutrons/cm(2)), a new mechanism related to the dispersion of fine Cu2O precipitates in the superconducting YBCO-LiF matrix could explain the enhancement of up to eight times obtained for j(c) values. A good agreement between the dependence of the measured j(c) values on the applied magnetic field and the calculated curves, j(c) = f(B), derived from a theoretical model, previously proposed for the pinning contribution of fine particles dispersed in YBCO matrix, is obtained. The role of thermal neutron irradiation for the flux pinning improvement by introducing of strong pinning centers (irradiation defects and Cu2O microdispersoids) is also discussed. (C) 1998 Elsevier Science B.V. All rights reserved.