National Institute Of Materials Physics - Romania

Publications

5781 articles found

4291. Monte Carlo simulation study of magnetic behaviour of core-shell bimetallic nanoparticles

Authors: Crisan, O; Tornau, EE; Petrauskas, V; Angelakeris, M

Published: 2004, SECOND SEEHEIM CONFERENCE ON MAGNETISM, PROCEEDINGS, 3763, DOI: 10.1002/pssc.200405549

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

4292. Radiation hardness of silicon - a challenge for defect engineering

Authors: Stahl, J; Fretwurst, E; Lindstroem, G; Pintilie, I

Published: DEC 31 2003, PHYSICA B-CONDENSED MATTER, 340, 709, DOI: 10.1016/j.physb.2003.09.238

Thermally stimulated current measurements of silicon particle detectors have been performed for defect characterization after high levels of gamma- and proton-irradiation. Two defects closely correlated with the detector performance were monitored: a deep acceptor (1) and a bistable donor (BD). In oxygen rich silicon the deep acceptor is largely suppressed while the BD generation is strongly enhanced. The influence of the starting material (standard float zone (FZ), oxygen enriched FZ, Cz, epitaxial silicon) on the formation of these two defects is discussed. Identification of the I-defect with the V2O complex, and of BD with the thermal double donor TDD2 is suggested. (C) 2003 Elsevier B.V. All rights reserved.

4293. Second-order generation of point defects in highly irradiated float zone silicon - annealing studies

Authors: Pintilie, I; Fretwurst, E; Kramberger, G; Lindstroem, G; Li, Z; Stahl, J

Published: DEC 31 2003, PHYSICA B-CONDENSED MATTER, 340, 582, DOI: 10.1016/j.physb.2003.09.131

Isothermal and isochronal annealing experiments have shown that the I defect (the main cause for the changes in silicon diodes characteristics after high levels of gamma-irradiation) is stable up to 325-350degreesC. Possible reactions, which can explain the behavior of different defects formed during the annealing experiments, are discussed. All the experimental results concerning the I center (second-order irradiation induced defect, formed most likely via VO center, suppressed in oxygen rich material, high thermal stability) make it the favorite for being associated with the V2O2 complex. (C) 2003 Elsevier B.V. All rights reserved.

4294. Electronic transport in nanosystems

Authors: Wulf, U; Racec, ER; Racec, PN; Aldea, A

Published: DEC 15 2003, MATERIALS SCIENCE & ENGINEERING C-BIOMIMETIC AND SUPRAMOLECULAR SYSTEMS, 23, 681, DOI: 10.1016/j.msec.2003.09.080

We demonstrate the application of the Landauer-Buttiker formalism (LBF) to MIS (metal-insulator-semiconductor)-type quantum nanostructures and analyze their static and dynamical response properties. For the time-dependent problem, we employ the random phase approximation (RPA) and calculate the irreducible polarization from the one-particle scattering wave functions in the Hartree approximation. For their calculation, we employ the R-matrix approach which we have developed over the past years. We find in the low frequency limit a reasonable agreement with the static Hartree calculations and experimental capacitance data. (C) 2003 Published by Elsevier B.V.

4295. Fabrication of novel magnetic nanostructures by colloidal bimetallic nanocrystals and multilayers

Authors: Angelakeris, M; Crisan, O; Papaioannou, E; Vouroutzis, N; Tsiaoussis, I; Pavlidou, E; Crisan, AD; Kostic, I; Sobal, N; Giersig, M; Flevaris, NK

Published: DEC 15 2003, MATERIALS SCIENCE & ENGINEERING C-BIOMIMETIC AND SUPRAMOLECULAR SYSTEMS, 23, 878, DOI: 10.1016/j.msec.2003.09.147

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.

4296. Superior radiation tolerance of thin epitaxial silicon detectors

Authors: Kramberger, G; Contarato, D; Fretwurst, E; Honniger, F; Lindstrom, G; Pintilie, I; Roder, R; Schramm, A; Stahl, J

Published: DEC 11 2003, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT, 515, 670, DOI: 10.1016/j.nima.2003.07.021

For the LHC upgrade (fluences up to 10(16) p/cm(2)) epi-Si devices are shown to be a viable solution. No type inversion was measured up to 1.3 x 10(15) 24 GeV/c protons/cm(2) and the charge collection efficiency (CCE) remained close to 100%. For reactor neutrons CCE was measured to be 60% at 8 x 10(15) n/cm(2). Annealing measurements have shown that only moderate cooling during beam off periods would be necessary. As a tentative explanation for the superior quality of these devices, we assume that radiation-induced donor generation leads to compensation effects of deep acceptors. In the future, we will extend the experiments to fluences up to 10(16) p/cm(2) and use also different variants of the epi-Si material and device geometry. (C) 2003 Elsevier B.V. All rights reserved.

4297. Influence of the reaction conditions on the activity properties of vanadium-aluminium oxynitride propane ammoxidation catalyst

Authors: Florea, M; Silvy, RP; Grange, P

Published: DEC 8 2003, , DOI: 10.1016/S0926-860X(03)00591-X

4298. A study of the SnO2 and CuSb2O6 based solid solutions: Electrical and magnetic properties

Authors: Scarlat, O; Payne, AC; Mihaiu, S; Aldica, G; Zaharescu, M

Published: DEC 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 1002, DOI:

Two types of solid solutions (i.e, Sn1-xCux/3Sb2x/3O2 and Cu1-xSb2(1-x)Sn3xO6 mixed crystals), have been experimentally evidenced over a large concentration range, in the SnO2 - CuSb2O6 binary system, For these two types of solid solutions, the measured values of the unit cell parameters obey Vegard's rule. The electric and magnetic properties of the Sn0.50Cu0.167Sb0.33O2 and Cu0.923Sb1.836Sn0.231O6 mixed crystals, with compositions placed at the maximum solid solubility limit, on either side, were evaluated over 77 to 1123 K and 2 to sample exhibited semiconducting behaviour, with 300 K, respectively. Sn0.50Cu0.167Sb0.33O2 sample exhibited semiconducting behaviour, with activation energy E-d (eV) and Seebeck coefficient (muV/grd) values close to the values reported for metals, while the Cu0.923Sb1.836Sn0.231O6 sample can be treated as insulators. Short - range magnetic ordering has been evidenced at about 60 K for the CuSb2O6 binary compound and Cu0.923Sb1.836Sn0.231O6 mixed crystal, wiib calculated magnetic moments in good agreement with literature data. In contrast, the Sn0.50Cu0.167Sb0.33O2 mixed crystal presented only a paramagnetic behaviour.

4299. Control of the growth mechanism of (119) Bi-2223 superconducting thin films. Two-dimensional nucleation growth and step-flow growth

Authors: Endo, K; Badica, P; Aldica, G

Published: DEC 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 1028, DOI:

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.

4300. Field effect assisted thermally stimulated currents in CdS thin films deposited on SiO2/Si substrates

Authors: Lisca, M; Pentia, E; Saran, G; Pintilie, L; Pintilie, I; Botila, T

Published: DEC 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 852, DOI:

Field effect assisted Thermally Stimulated Currents (TSC) measurements were used to differentiate between electron and hole traps in CdS thin films deposited on SiO2/Si substrate. The electrode configuration was designed as for a pseudo-MOS structure, with drain and source contacts on the CdS film surface and using the Si substrate as gate electrode. Electron or hole traps will be favored to fill-up depending on the polarity of the gate voltage applied during illumination at low temperatures. Collection of the thermally released electrons or holes in the drain circuit will depend also on the polarity of the gate voltage applied during heating at constant rate. Comparing the results with those obtained in case of US films deposited on glass, it is possible to differentiate between SiO2/CdS interface traps and bulk CdS ones.