National Institute Of Materials Physics - Romania

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5840 articles found

4521. TlSr2CaCu2Oy template for the growth of superconducting Tl(Ba, Sr)(2)Ca2Cu3Oy thin films on CeO2 buffered sapphire

Authors: Sundaresan, A; Crisan, A; Nie, JC; Hirai, M; Asada, H; Kito, H; Tanaka, Y; Ihara, H

Published: JUN 2002, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 15, 963, DOI: 10.1088/0953-2048/15/6/322

Thin films of Tl(Ba, Sr)(2)Ca2Cu3Oy superconductor have been grown on CeO2 buffered r-cut sapphire substrate with the use of a template TlSr2CaCu2Oy by the amorphous phase epitaxy method. By using this template containing Sr, we have overcome the problem of chemical reactions between Ba and Cc present in the superconducting and buffer layers, respectively, at the normal processing temperatures of 840-890 degreesC. Films grown by this method have an excellent c-axis orientation as well as in-plane alignment with the TlSr2CaCu2Oy template, which in turn is aligned with the CeO2 layers. These films have superconducting critical temperature values up to 104 K and critical current density values as high as 1 MA cm(-2) at 77.3 K and 0 T.

4522. Exchange coupling effects in nanocrystalline alloys studied by Monte Carlo simulation

Authors: Crisan, O; Labaye, Y; Berger, L; Coey, JMD; Greneche, JM

Published: MAY 15 2002, JOURNAL OF APPLIED PHYSICS, 91, 8729, DOI: 10.1063/1.1456420

The magnetic behavior of nanocrystalline alloys has been modeled using atomic Monte Carlo simulation. The model consists of a cubic lattice composed of a ferromagnetic nanograin in a ferromagnetic matrix. The magnetizations of nanograin core, surface and interface regions and matrix were studied as a function of the exchange coupling between the nanograin and the matrix, as well as of the nanograin/matrix volume ratio, equivalent to the crystalline fraction in the nanocrystalline alloys. The mechanism of polarization of the matrix by fields penetrating from the nanograin is discussed and correlated with the matrix-nanograin exchange coupling. Competition between interface anisotropy and magnetocrystalline anisotropy produces spin-glass-like magnetic order of the interfacial regions. (C) 2002 American Institute of Physics.

4523. Capacitance in open quantum structures

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

Published: MAY 15 2002, PHYSICAL REVIEW B, 65, DOI: 10.1103/PhysRevB.65.193314

Opposite to the well-known statistical limit, we study a semiconductor quantum system with only one relevant resonance which is in contact with a probe acting as a particle reservoir. We find that the quasibound state that exists in the nearly closed system develops at a transition to the open system into a separate type of resonance. In contrast to the quasibound state (i) it is localized in the space between the probe and the isolated quantum system, (ii) its energy lies in the classically allowed regime, and (iii) its line shape is strongly asymmetric. Excellent quantitative agreement shows that this transition is seen in capacitance experiments on MIS (metal-insulator-semiconductor)-type semiconductor heterostructures in which a field-induced two-dimensional electron gas is formed.

4524. Monte Carlo simulation of the magnetization of a ferromagnet with antiphase boundaries

Authors: Berger, L; Labaye, Y; Crisan, O; Greneche, JM; Coey, JMD

Published: MAY 15 2002, JOURNAL OF APPLIED PHYSICS, 91, 7636, DOI: 10.1063/1.1456404

The magnetic properties of a ferromagnetic cylinder containing an antiphase boundary with antiferromagnetic interactions are studied using atomic Monte Carlo simulation. The approach to saturation shows a reduced magnetization compared with the completely ferromagnetic sample, even in huge applied fields. The magnetization profiles are studied as a function of the number of ferromagnetic planes in the cylinder with 80 atoms per plane and also as function of the ratio of the bulk-to-antiphase exchange coupling. Hysteresis appears after removing the applied field for relatively low antiphase boundary densities. It is shown that the antiferromagnetic coupling across the antiphase boundary leads to a progressive rotation of spins as the two half "domain walls" at the boundary with opposite chirality narrow with increasing applied field. (C) 2002 American Institute of Physics.

4525. Surface anisotropy in ferromagnetic nanoparticles

Authors: Labaye, Y; Crisan, O; Berger, L; Greneche, JM; Coey, JMD

Published: MAY 15 2002, JOURNAL OF APPLIED PHYSICS, 91, 8717, DOI: 10.1063/1.1456419

The effect of surface anisotropy on the magnetic ground state of a ferromagnetic nanoparticle is investigated using atomic Monte Carlo simulation for spheres of radius R=6a and R=15a, where a is the interatomic spacing. It is found that the competition between surface and bulk magnetocrystalline anisotropy imposes a "throttled" spin structure where the spins of outer shells tend to orient normal to the surface while the core spins remain parallel to each other. For large values of surface anisotropy, the spins in sufficiently small particles become radially oriented either inward or outward in a "hedgehog" configuration with no net magnetization. Implications for FePt nanoparticles are discussed. (C) 2002 American Institute of Physics.

4526. Strong reduction of thermally activated flux jump rate in superconducting thin films by nanodot-induced pinning centers

Authors: Crisan, A; Badica, P; Fujiwara, S; Nie, JC; Sundaresan, A; Tanaka, Y; Ihara, H

Published: MAY 13 2002, APPLIED PHYSICS LETTERS, 80, 3568, DOI: 10.1063/1.1478782

From frequency-dependent ac susceptibility studies of (Cu,T1)BaSrCa2Cu3Oy superconducting thin films, with and without nanodot-induced artificial pinning centers, we estimated the activation energy of flux jumps. The result was that, in the film with nanodots, the pinning potential is several times higher, leading to a probability of thermally activated flux jumps several orders of magnitude lower than in the film without artificial pinning centers. We suggest that our no cost straightforward method for creating extended defects can be successfully employed for the reduction of thermal noise in superconducting electronic devices. (C) 2002 American Institute of Physics.

4527. Field-effect-assisted photoconductivity in PbS films deposited on silicon dioxide

Authors: Pintilie, L; Pentia, E; Matei, I; Pintilie, I; Ozbay, E

Published: MAY 1 2002, JOURNAL OF APPLIED PHYSICS, 91, 5786, DOI: 10.1063/1.1468277

Lead sulfide (PbS) thin films were deposited from a chemical bath onto SiO2/Si (n-type) substrates. Pseudo-metal-oxide-semiconductor devices were obtained by evaporating source and drain gold electrodes on a PbS surface and aluminum gate electrode on a Si substrate. Field-effect-assisted photoconductivity in the PbS layer was investigated at room temperature, in the 800-2700-nm-wavelength domain for different values and polarities of the drain and gate voltages. The best results were obtained for a positive gate, when both semiconductors are in depletion. An enhancement of about 25% of the photoconductive signal is obtained compared with the case when the gate electrode is absent or is not used. A simple model is proposed that explains the behavior of the dark current and photoconductive signal in PbS film with changing the gate voltage. (C) 2002 American Institute of Physics.

4528. A familial translocation t(3;5) identified by chance in a case of essential thrombocythemia (ET)

Authors: Arghir, AA; Berbec, NM; Rodewald, A; Serghei, L; Lungeanu, AG

Published: MAY 2002, EUROPEAN JOURNAL OF HUMAN GENETICS, 10, 155, DOI:

4529. A Mossbauer study of the Verwey transition in cobalt-doped magnetite

Authors: Sorescu, M; Grabias, A; Brand, RA; Voss, J; Tarabasanu-Mihaila, D; Diamandescu, L

Published: MAY 2002, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 246, 403, DOI: 10.1016/S0304-8853(02)00112-9

The low-temperature behavior of the Co-doped magnetite is studied. The Fe3-xCoxO4 powders (x = 0.01 - 1) have been prepared by the hydrothermal method. In order to investigate the cobalt influence on the structure of magnetite, transmission Mossbauer spectroscopy measurements were performed at temperatures close and below the Verwey transition temperature characteristic for pure magnetite (122 and 77 K, respectively). All spectra were fitted with five components and related to nonequivalent Fe2+ and Fe3+ sites. The hyperfine parameters of these sites as well as their relative fractions in the powder samples were determined. (C) 2002 Elsevier Science B.V. All rights reserved.

4530. Stud of Ag-- and Ag-0-type centers in X- and gamma-ray irradiated NaCl : Ag+, KCl : Ag+ and NaCl : Ag+ + Ca2+ crystals

Authors: Topa, V; Tsuboi, T; Dragusin, M

Published: MAY 2002, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 191, 300, DOI: 10.1016/S0168-583X(02)00579-7

Creation of Ag and Ag-0 centers by irradiation with ionizing radiations has been investigated using NaCl and KCl crystals containing various concentrations of Ag+ ions and using Ca2+-codoped NaCl:Ag+ and KCI:Ag+ crystals. From the absorption spectra, it is observed that Ag- center perturbed by Ca2+ ion is also created besides the Ag- and Ag-0 centers. It is confirmed that Ag- and Ag-0 centers increase with increasing the concentration of doped Ag- ions. It is suggested that there are two kinds of Ag(c)(0)v(a) centers (i.e. Ag-0 accompanied by vacancy at the nearest neighbor) with different site symmetries depending on the location of vacancy. (C) 2002 Published by Elsevier Science B.V.