National Institute Of Materials Physics - Romania

Publications

5781 articles found

4081. Experiments on pulsed laser deposition and characterization of epitaxially in-situ grown YBa2Cu3O7-x thin films

Authors: Branescu, M; Teodorescu, VS; Socol, G; Balasz, I; Ducu, C; Jaklovszky, J

Published: APR 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 972, DOI:

Experimental results concerning the pulsed laser deposition parameters of in-situ epitaxially grown high crystalline quality YBa2Cu3O7-x thin films are reported. The structural and morphological properties of the deposited films analyzed by transmission electron microscopy, selected area electron diffraction, X-ray diffraction, and Raman spectroscopy are also presented. The YBa2Cu3O7-x thin films, predominantly c-axis oriented had required the smallest oxygen pressure during the cool-down time reported to date.

4082. Ni nanowires electrodeposited in single ion track templates

Authors: Daub, M; Enculescu, I; Neumann, R; Spohr, R

Published: APR 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 870, DOI:

30-mu m thick polycarbonate foils were irradiated with single swift heavy ions (,e.g. Au 11.4 MeV/nucleon). After etching with solutions containing NaOH and methanol, templates containing single conical pores were obtained. The size and the shape of the nanopores depend on etching conditions such as time of etching, concentration and temperature of the etching solution. By electrochemical deposition of nickel, nanowires were grown in the single pores. The nanowires were contacted by sputtering a gold layer on top of the membrane. The magnetic measurements indicated that the nanowires possess around 1% anisotropic magnetoresistance. The current-voltage characteristic of a Ni nanowire shows a linear behavior for current densities smaller than 10(8) A/cm(2). The maximum current density that a single Ni wire can withstand was found to be 3x 10(8) A/cm(2).

4083. Microspheres, planoconvex microlenses and optical fibres based on glassy As2S3

Authors: Popescu, M; Sava, F; Lorinczi, A; Savastru, D; Miclos, S; Savastru, R

Published: APR 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 1106, DOI:

Microspheres and planoconvex lenses (50 divided by 400 mu m in diameter) based on glassy As2S3, an important infrared material, have been produced by a special flame melting technique. Glassy fibres of 45 mu m in diameter, with the length of 10-20 cm have been drawn from melt. The morphological properties of these optical elements have been investigated.

4084. Preparation and properties of langasite and YAG amorphous films

Authors: Popescu, M; Sava, F; Lorinczi, A; Stegarescu, M; Georgescu, S; Mihailescu, IN; Socol, G; Stanoi, D; Daroczi, L; Kokenyesi, A; Leonovici, M; Wagner, D

Published: APR 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 966, DOI:

Amorphous thin films of langasite and YAG have been prepared from crystals targets by pulsed laser deposition, at room temperature, in vacuum, on silicon wafers. Bulges of micrometer size are formed on the film surface. Larger size bulges (10-50 micrometers in diameter) are characteristic to the annealed langasite films. Annealing at high temperatures leads to the crystallization of the films. The annealed langasite films (850 degrees C) are polycrystalline and partially oriented with the plane (001) parallel to the surface of the silicon wafer. The bulges break easily in the heat treated films. Their empty structure was remarked on the electron microscope images. The YAG films annealed at 1100 degrees C shows a crystalline YAG phase depleted in Y and traces of Y2O3. An interesting feature is the presence of silver particles spread along the macrodefects (fracture lines) of the film.

4085. Piezoresponse force microscopy for imaging of GaN surfaces

Authors: Calarco, R; Meijers, R; Stoica, T; Luth, H

Published: APR 2005, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 202, 789, DOI: 10.1002/pssa.200461298

Surfaces of GaN films have been investigated by Atomic Force Microscopy (AFM) with implemented Piezoelectric Force Microscopy (PFM) technique. The GaN layers were grown by molecular beam epitaxy (MBE) on Si(111) substrates with an AlN buffer layer. Simultaneous imaging of surface morphology in the AIM and PFM response was performed. The PFM imaging is sensitively dependent on measurement conditions. By increasing the ac voltage the contrast of the PFM images is increased. Using the Voltage-Modulated Force Microscopy (VMFM) regime, with the frequency close to the first resonant frequency of the piezoelectric signal, PFM images with high contrast and high resolution on the nanometer scale were obtained. Regions of opposite piezoresponse were observed and explained by the presence of inversion domains. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

4086. YBa2Cu3O7-delta : Li superconductor system investigated by direct microwave absorption method

Authors: Velter-Stefanescu, M; Duliu, OG; Sandu, V

Published: APR 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 977, DOI:

The influence of thermal neutrons irradiation at different fluencies on the superconductivity properties of Li (2 at. %) doped YBa2Cu3O7-delta system has been investigated by direct microwave absorption method. We have noticed changes of the superconducting properties of the 123 phase that depend on the neutron fluency. By increasing fluency, initially, the phase structure supports a disordering, then a re-ordering accompanied by a significant increase of the average critical temperature T-c to 85.7 K and finally we observed a degradation of the 123 the phase accompanied by a diminishing of T-c. The intragranular dephasing magnetic field B-d, i.e. the direct microwave absorption (DMA) parameter associated with the magnetic field dependence of the critical current density exhibited an opposite tendency, initially decreasing and then increasing at a value close to those of unirradiated samples. We also noticed only on the irradiated samples the presence of a flatness of the DMA curves around 0.5 T, consistent with the expression of a peak effect.

4087. Soft ferrite materials for magnetic temperature transducers and applications

Authors: Miclea, C; Tanasoiu, C; Miclea, CF; Gheorghiu, A; Tanasoiu, V

Published: APR 2005, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 290, 1509, DOI: 10.1016/j.jmmm.2004.11.561

Soft ferrites in the CuZnTi system, having the formula Cu1-xZnxTiyFe2-4O4, with 0.50 <= x <= 0.60 and 0.00 <= y <= 0.05 were investigated as a function of composition, sintering temperature and cooling speed, in order to obtain materials with controlled T-C and very high change rate of permeability with temperature around their T-C. The effect of Zn and Ti additions was a drastic change of the Curie temperature with about 10-12 degrees C for each atomic percent of Zn and Ti introduced into the spinel lattice while the cooling speed changed the behaviour of magnetic permeability with temperature around the Curie point. An application using such magnetic temperature sensors for a thermostat is presented. (c) 2004 Elsevier B.V. All rights reserved.

4088. The nature of the hydrogen bond in the LaNiSnH2 and NdNiSnH hydrides

Authors: Spataru, T; Palade, P; Principi, G; Blaha, P; Schwarz, K; Kuncser, V; Lo Russo, S; Dal Toe, S; Yartys, VA

Published: MAR 22 2005, JOURNAL OF CHEMICAL PHYSICS, 122, DOI: 10.1063/1.1867436

The electronic structure of LaNiSn and NdNiSn compounds and their hydrides has been studied by first principles calculations and variable temperature Sn-119 Mossbauer spectroscopy and the nature of the hydrogen-metal bond is discussed. The analysis of the electronic density of states (DOS) in both compounds before and after hydrogenation indicates an hybridization of the Sn, Ni, and H orbitals. The partial Sn-p DOS of LaNiSnH2 gives evidence for a lower symmetry of electron density around tin atoms compared to LaNiSn, according to the larger quadrupole splitting in the corresponding Mossbauer spectrum. Theoretical and experimental Mossbauer parameters agree very well for all samples.

4089. Mossbauer effect study of correlation between structure and exchange-bias effect in ferromagnetic Fe/antiferromagnetic FeSn2 bilayers

Authors: Kuncser, VE; Stromberg, F; Acet, M; Keune, W

Published: MAR 15 2005, JOURNAL OF APPLIED PHYSICS, 97, DOI: 10.1063/1.1858879

Antiferromagnetic (AF) FeSn2(001) epitaxial and polycrystalline layers were grown in ultrahigh vacuum under various conditions on clean InSb(001) substrates and covered by polycrystalline ferromagnetic Fe layers, forming a new system with exchange bias. Isotopically enriched Fe-57- and (FeSn2)-Fe-57-tracer layers were placed on either side of the Fe/FeSn2 interface for a microscopic investigation of the spin structure and atomic interdiffusion phenomena in near-interfacial regions by Fe-57 conversion electron Mossbauer spectroscopy (CEMS) at room temperature and T=10 K. Several spectral components, assigned to pure bcc Fe, Sn-containing bcc Fe, AF-ordered FeSn2, and paramagnetic FeSn2 were resolved in the CEM spectra. Evidence is provided for interdiffusion across the interface. The temperature dependence of the exchange-bias field H-E was measured by magnetometry. The CEMS data provide evidence for a correlation between H-E at low T and chemical disorder (defects) in the FeSn2 films via the intensity of the paramagnetic line. These results support the assumption that exchange bias is related to the presence of AF domains formed via magnetic defects in the antiferromagnet. The T dependence of H-E suggests interfacial exchange-coupling energies higher than the AF wall energy. (C) 2005 American Institute of Physics.

4090. Supported Co-based perovskites as catalysts for total oxidation of methane

Authors: Alifanti, M; Blangenois, N; Florea, M; Delmon, B

Published: MAR 8 2005, , DOI: 10.1016/j.apcata.2004.11.005