National Institute Of Materials Physics - Romania

Critical current density J(c) of Na0.35CoO2.1.3H(2)O superconductor (T-c=4.4 K) has been estimated from magnetization measurements: J(c)(1.9 K,0 T)=5x10(4) A/cm(2), J(c) (1.9 K,0.4 T)=4x10(2) A/cm(2) and J(c) (3.4 K,0 T)=10(3) A/cm(2). J(c) versus applied magnetic field H cannot be fitted, as for high-T-c superconductors (HTS), by a power law for any temperature. The temperature dependence for a given H is not of the linear type as for MgB2, but is close to quasiexponential (associated with the thermally activated flux creep) as for HTS. Irreversibility field data points H-irr versus (1-T/T-c) obey a power law with power exponent n=3.4. The irreversibility line is located below, but in vicinity of, the line for Bi-2223. Details of morphology are also presented. (C) 2004 American Institute of Physics.

Precursor powders of Ba/Zr = 1:1 chloride have been prepared by freeze drying (FD) and conventional mixing. A simple, laboratory-made apparatus for sublimation of the frozen water from the cryo-particles is presented. Both powders have the same phase formation behavior. Up to 500-600 degreesC, Zr chloride transforms into ZrO2. At high temperatures, BaCl2 starts melting and decomposing and two subsequent processes take place: (a) chlorination of the Zr atoms from the oxide and (b) formation of BaZrO3 phase, These processes depend on treatment atmosphere and reactivity of the starting powders. Freeze-dried powders with higher reactivity and/or O-2 atmosphere are suitable to obtain high content of BaZrO3 phase. In the freeze-dried powder, decomposed in O-2 atmosphere at 1190 degreesC for 20 h, 90% of BaZrO3 phase content was attained. No new phases were observed during decomposition of the freeze-dried powder. In the as-synthesized freeze-dried powder, some XRD lines remained unidentified. (C) 2003 Elsevier B.V. All rights reserved.

The Fe spin configurations in both the soft- and hard- magnetic phases of nanocomposite Fe/RE2Fe14B exchange-spring ribbons have been analysed by Mossbauer spectroscopy. New criteria for evaluating the inter-phase exchange-coupling strength are proposed. The coupling strength is discussed in relation to the annealing conditions and the relative content of the soft magnetic phase.

The phase composition, local structure and magnetic interactions were investigated in FexOx-SiO2 nanocomposites obtained by modern sol-gel methods. The very sensitive method of Mossbauer spectroscopy has evidenced the strong influence of the silica matrix precursor on the iron phase composition in the initial composites as well as on the size distribution of the hematite nanoparticles obtained by suitable thermal annealing treatments.

The paper presents experimental results concerning the influence of additives from a solution of the PVA on the optical properties of the polymer. The refractive index of the poly (vinyl-alcohol) was modified through the addition of inorganic materials and showed values between 1.5 and 2 (depending on the compositions and the curing conditions of the polymer. The transmittance indicated values above 80%. The additives we used were metal oxides/inorganic salts. The composition and the deposition processes were optimized to allow a better control of the optical parameters (refractive index, transmittance), to reduce the processing temperatures and to improve photosensitivity. PVA with these new light-sensitive compositions can be easily spin coated onto variety of semiconductor substrate and directly patterned. A process was developed for channel waveguides fabrication.

The magnetic properties of core-shell-type nanoparticles with non-magnetic core and ferromagnetic shell are modeled using Monte Carlo simulation. The influence of the surface spin disorder on the magnetization and Curie temperature of magnetic states is studied. Due to competition between exchange coupling and surface anisotropy, rotation of the magnetization axis is observed, as the surface anisotropy of the ferromagnetic shell is increased. It is shown that the exchange coupling at the core-shell interface can influence the shape of the temperature dependence of total magnetization. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Recent developments of lithographic techniques as well as improved chemical synthesis methods allow researchers to engineer novel nanostructured materials consisting of arrays of self-organized nanocrystals and multilayers grown as patterns on different substrates. In our case, the magnetic nanostructures consist either of multilayers directly deposited on pre-patterned substrates to form regular arrays of stripes and grooves or colloidal solutions of self-organized bimetallic Ag/Co nanoparticles on patterned and nonpatterned substrates. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed in order to study the surface morphology of the 2D patterning arrays and the 3D nanostructures. The development of periodic arrays of magnetic patterns of micrometer size is strongly dependent on technological parameters such as: film, thickness, distances, size and shape of the patterns. Moreover, it is shown that the substrate morphology significantly affects the colloidal crystallization of magnetic nanoparticles and leads to different growth modes. This will ultimately affect the overall magnetic behavior of the nanostructures. Consequently, the combination of self-assembly and patterning allows for the controlled fabrication of the novel magnetic nanostructures at a macroscopic level and the study of fundamental aspects in magnetism such as quantum tunneling magnetization and magneto-transport properties along well-defined nanosized patterns. (C) 2003 Elsevier B.V. All rights reserved.

Films of (119) Bi-2223 high-T-c superconductor, potentially useful for future sandwich-stacked structures exhibiting Josephson effect have been prepared by MOCVD on (100) NdGaO3 and (110) SrTiO3 flat and vicinal substrates with different off-angles. It is shown that off-angle is a key parameter in growth control mechanism of the films. In-plane aligned films, with regular morphology and low roughness as well as having maximum zero-resistivity critical temperature: of T-c0=67.2K and T-c0=74K when "single" and "two"-temperature growth routes are used, respectively, have been obtained for high of-angles (20 degrees). Growth mechanism is changing from a two-dimensional type for the flat substrate to a step-flow one for vicinal substrates.

Ag2O-doped (1.2% wt.) nitrate freeze-dried powders (Bi : Pb : Sr : Ca : Cu = 1.7 : 0.3 : 2 : 2.5 : 3.5) were processed under an external electrical field and 17.5 MPa pressure at 800 degreesC, for 4 min in vacuum. Final heat treatments (HT) were applied at 835 - 850 degreesC for 70 h. in air (Bi, Pb)(2)Sr2CaCu2Ox (2212-phase) was formed by electrical field processing in just 4 min. Electrical field application enhanced (Bi, Pb)(2)Sr2Ca2Cu3Oy (2223-phase) formation during final HT. Ag2O additions to field sintered BSCCO ceramics increased the amount of 2223-phase and the zero resistance critical temperature (T-e(R=0)) by similar to 4 k.

The paper presents a review of our data and the current status on synthesis and processing of Bi-2223 superconducting ceramic, by spray frozen, freeze drying method (SF-FD). By this method powders of 200-300 nm were obtained and processed by several different routes to the final superconducting product. Optimization process and comparative analysis with the literature data revealed intrinsic problems associated to the specific nature (i.e high degree of mixing and large surface area relative to the volume) of the SF-FD material. The paper focuses on the effects induced by the specific features of the SF-FD material, sometimes opposite to those observed in the conventional materials produced by solid-state method. It resulted that in order to take full advantage of the specific features for a nano SF-FD material, it is necessary to consider a special, non-conventional, processing approach and some examples in this regard will be presented.