National Institute Of Materials Physics - Romania

Publications

892 articles found

641. Angle-resolved photoemission study of the cobalt oxide superconductor NaxCoO2 center dot yH(2)O: Observation of the Fermi surface

Authors: Shimojima, T; Ishizaka, K; Tsuda, S; Kiss, T; Yokoya, T; Chainani, A; Shin, S; Badica, P; Yamada, K; Togano, K

Published: DEC 31 2006, PHYSICAL REVIEW LETTERS, 97, DOI: 10.1103/PhysRevLett.97.267003

The cobalt oxide superconductor NaxCoO2 center dot yH(2)O is studied by angle-resolved photoemission spectroscopy. We report the Fermi surface (FS) topology and electronic structure near the Fermi level (E-F) in the normal state of NaxCoO2 center dot yH(2)O. Our result indicates the presence of the hexagonal FS centered at the Gamma point, while the small pocket FSs along Gamma-K direction are absent, similar to NaxCoO2. The top of the e(g)(') band, which is expected in band calculations to form the small pocket FSs, extends to within similar to 30 meV below E-F, closer to E-F than in NaxCoO2. We discuss its possible role in superconductivity, comparing with other experimental and theoretical results.

642. Magnetic structure and interlayer exchange coupling in spring magnets - studied via nuclear resonant scattering

Authors: Klein, T; Rohlsberger, R; Crisan, O; Schlage, K; Burkel, E

Published: DEC 5 2006, THIN SOLID FILMS, 515, 2534, DOI: 10.1016/j.tsf.2006.03.035

Magnetic properties of FePt/Ag/Fe and FePt/Pd/Fe layer systems, prepared by magnetron sputtering, were investigated using the nuclear resonant forward scattering of synchrotron radiation. This technique allows the accurate determination of magnetic hyperfine field orientations by using an extremely thin 57 Fe probe layer suitably embedded within the soft magnetic layer. From an evaluation of these measurements within a one-dimensional micromagnetic model, the interlayer exchange coupling constants between the magnetically hard (FePt) and soft (Fe) layers were determined as function of the Ag and I'd interlayer thickness. The interlayer thickness dependence of the bilinear coupling constants provides evidence for the superposition of Ruderman-Kittel-Kasuya-Yoshida coupling and a magnetostatic interaction between the magnetic layers. (c) 2006 Elsevier B.V. All rights reserved.

643. Structure effects on the magnetism of AgCo nanoparticles

Authors: Crisan, O; Angelakeris, M; Simeonidis, K; Kehagias, T; Komninou, P; Giersig, M; Flevaris, NK

Published: NOV 2006, ACTA MATERIALIA, 54, 5260, DOI: 10.1016/j.actamat.2006.06.053

Fabrication by colloidal chemistry and an extended structural and magnetic investigation are reported for AgCo bimetallic colloidal nanoparticles using X-ray diffraction, transmission electron microscopy and superconducting quantum interference device magnetometry. The AgCo nanoparticles exhibit a core-shell structure, with a face-centred cubic Ag core and a hexagonal close-packed Co (either complete or partial) shell. A bimodal size distribution of superparamagnetic (SPNI) nanoparticles together with a small fraction of nanoparticles ferrornagnetic at room temperature has been determined and their influence on magnetic properties is discussed. The arrays of self-assembled nanoparticles exhibit a lack of saturation of magnetisation and typical SPM behaviour. The annealing of the arrays greatly enhances the ferromagnetic character of the samples. Finally, the observed magnetic behaviour of the AgCo nanoparticles is correlated with surface spin disorder induced by the particular structural features of the sample, high fraction of magnetic atoms in surface states and finite-size effects. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

644. Anomalous vortex melting line in the two-component superconductor (Cu,C)Ba2Ca3Cu4O10+delta

Authors: Crisan, A; Tanaka, Y; Iyo, A; Cosereanu, L; Tokiwa, K; Watanabe, T

Published: NOV 2006, PHYSICAL REVIEW B, 74, DOI: 10.1103/PhysRevB.74.184517

Using the on-set of third-harmonic susceptibility we have measured vortex melting lines of high-pressure synthesized (Cu0.6C0.4)Ba2Ca3Cu4Oy [(Cu,C):1234] and (Cu0.5C0.5)Ba2Ca2Cu3Oy [(Cu,C):1223] with preferentially oriented crystallites and having various carrier concentrations. Vortex melting lines of all (Cu,C):1223 samples and of optimum-doped (Cu,C):1234 were very well described by the commonly accepted theory of melting lines within the two-fluid model. Overdoped (Cu,C):1234 proved to have an anomalous melting line, that was phenomenologically explained by the rather special gaps evolution, that is the significant opening of a second superconducting gap due to CuO2 outer planes at a temperature lower than the critical one, where the first superconducting gap due to inner planes opens. The experimental melting lines of overdoped (Cu,C):1234 were modelled theoretically by assuming an empirical formula for the temperature dependence of anisotropy factor.

645. Novel approach to growth of precipitate-free, high-quality oxide thin films suitable for device applications

Authors: Endo, K; Badica, P; Sato, H; Akoh, H

Published: OCT 25 2006, THIN SOLID FILMS, 515, 495, DOI: 10.1016/j.tsf.2005.12.278

To eliminate precipitates-segregates that can easily occur on the thin film surfaces of the multicomponent materials for electronics, a new approach is proposed, consisting of the following aspects: first, on the substrates, artificial steps of predefined height and width are produced, and second, films are grown on such substrates. The width of the step is taken equal to the 'double of the migration length' of the atomic species depositing on the substrate. In these conditions, precipitates migrate and gather at the step edges where the free energy is lowest and the resulting totally precipitate-free surface of the film on the step is suitable for device applications or integration purposes. The method has several other important advantages and they are discussed in the text. Using this new approach we present successful fabrication of a mesa structure showing intrinsic Josephson effect. We have used thin films of Bi-2212/Bi-2223 superstructure grown by MOCVD on (001) SrTiO3 single crystal substrates with artificial steps of about 20 mu m width. (c) 2006 Elsevier B.V. All rights reserved.

646. Magnetic interactions in NdFeB bulk permanent magnets with additions

Authors: Chiriac, H; Lupu, N; Chiriac, C; Valeanu, M; Kuncser, V

Published: OCT 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 1769, DOI:

NdFeB-based bulk nanocomposite permanent magnets (with Dy and Co partially substituting Nd and Fe, respectively) with additions of Early Transition Metals (ETM = Mo, Ti, Zr, Cu, Nb, V, W), in form of rods with diameters ranging from 0.5 to 0.8 mm, have been prepared by devitrification annealing of amorphous and partly-amorphous precursors produced by injection die casting. A fully amorphous structure was obtained for rods with the diameter as large as 0.6 mm. The as-cast rod samples are magnetically soft and the magnetic hardness develops as the samples are devitrified to the optimum nanostructure. The best-achieved hard magnetic properties have been obtained for the optimum devitrified (15 min at 630 degrees C) Nd(3)Dy(1)F(3)e(66)Co(10)B(20) samples of 0.6 mm in diameter: H-i(c) = 296 kA/m, mu M-0(r) = 0.86 T, Mr/M-max = 0.65 and (BH)(max), = 74 kJ/m. The Mossbauer spectra indicate the coexistence of 4 magnetic phases: 2:14:1, alpha-Fe, Fe3B and a short-range ordered intermetallic Fe-B-type phase.

647. Review on Bi-Sr-Ca-Cu-O whiskers

Authors: Badica, P; Togano, K; Awaji, S; Watanabe, K; Kumakura, H

Published: OCT 2006, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 19, R99, DOI: 10.1088/0953-2048/19/10/R01

We review the current status of the growth, characterization and applications of Bi-Sr-Ca-Cu-O (BSCCO) whiskers from the materials science point of view. Our analysis leads to the conclusion that reconsideration of the importance of the whisker studies is necessary; whiskers can play a major role in studies of the growth mechanism and defects, properties control and their understanding, and new devices and applications. Arguments favourable to the idea that BSCCO whiskers can possibly have a significant impact on future synthesis of nano-objects in this system or other high-T-c superconducting systems, as well as in investigation and understanding of the growth processes under elevated magnetic fields, are discussed. Emphasis is made on superconducting whiskers, but non-superconducting ones are also briefly addressed.

648. In situ structural changes during toluene complete oxidation on supported EuCoO3 monitored with Eu-151 Mossbauer spectroscopy

Authors: Alifanti, M; Florea, M; Filotti, G; Kuncser, V; Cortes-Corberan, V; Parvulescu, VI

Published: SEP 30 2006, CATALYSIS TODAY, 117, 336, DOI: 10.1016/j.cattod.2006.05.036

Ceria-zirconia supported (10 and 20 wt.% EuCoO3) versus bulk EuCoO3 perovskite were prepared via citrate decomposition at 700 degrees C and tested for toluene complete oxidation. S-BET, XRD, XPS, H-2-TPD and in situ Eu-151 Mossbauer investigations were performed. Electron delocalization processes around the Eu cations are induced by the reaction, as proved via the evolution of the Mossbauer parameters. All characterization data indicate the formation of a well-dispersed EuCoO3 at the surface of Ce0.9Zr0.1O2 for the 10 wt.% loading and larger crystallite formation for 20 wt.% loading. In terms of intrinsic activity for toluene combustion, supported catalysts were more active than bulk EuCoO3. All structural changes during operation time are reversible. (C) 2006 Elsevier B.V. All rights reserved.

649. Study of crystallization processes in Gd-substituted Finemet alloys

Authors: Crisan, O; Le Breton, JM; Crisan, AD; Filoti, G

Published: SEP 28 2006, JOURNAL OF ALLOYS AND COMPOUNDS, 422, 202, DOI: 10.1016/j.jallcom.2005.12.002

The crystallization processes that occur in amorphous melt-spun ribbons of nominal composition Fe73.5Cu1Nb3Si13.5B9 with Gd addition in different concentrations are studied by differential scanning calorimetry (DSC). The aim of Gd addition is to study the changes induced in the calorimetric behavior of Finemet alloys, the crystallization sequences at intermediate stages of annealing as well as the nature of obtained crystallization products. The nature and structural properties of the phases formed at the end of the calorimetric process are investigated using X-ray diffraction (XRD) and Mossbauer spectrometry (MS). The crystallization sequence and also the phase composition at different stages of annealing were investigated for the Fe68.5Gd5Cu1Nb3Si13.5B9 alloy. Structure and magnetic properties of resulting crystallized samples have been thoroughly investigated. It is shown that in the above-mentioned composition, the decomposition at around 690 degrees C of the metastable Gd3Fe62B14 phase formed at incipient stages of crystallization leads to the co-existence of Gd2Fe14B, Fe2B and alpha-Fe(Si) phases. Magnetic measurements of as-cast and annealed samples are in good agreement with the XRD and MS results and with the proposed crystallization sequences and decomposition of intermediate metastable phases. (c) 2005 Elsevier B.V. All rights reserved.

650. Criteria for the occurrence of negative resonant magnetoresistance in magnetic tunnel junctions

Authors: Tolea, M; Aldea, A; Tolea, F

Published: SEP 2006, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 243, R86, DOI: 10.1002/pssb.200642347

The dependence of the driven-current through a tunnel junction on the relative orientation of the magnetization in the electrodes deviates from the Julliere formula when the current is resonant through impurity levels. For asymmetric coupling of the impurities to the electrodes, the magnetoresistance can even be negative on the resonance. Such inversion in sign is due to the fact that not only the density of states in the electrodes is important (for both orientations of spins), but also the density of states at the impurity position, which has a specific behavior. We find the exact conditions under which the negative magnetoresistance occurs, as function of both polarization and coupling asymmetry. The resonant magnetoresistance may have single or double-dip aspect, depending on the mentioned parameters.