National Institute Of Materials Physics - Romania

Publications

903 articles found

741. 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.

742. BPSCCO superconductor with addition of Ag2O synthesized under electrical field

Authors: Aldica, G; Groza, JR; Badica, P; Bunescu, MC

Published: DEC 2003, JOURNAL OF SUPERCONDUCTIVITY, 16, 1005, DOI: 10.1023/A:1026295408106

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.

743. Bi-2223 freeze-dried ceramic: Specific features, related problems and search for new solutions

Authors: Badica, P; Aldica, G

Published: DEC 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 1039, DOI:

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.

744. Magnetism and anisotropy in core-shell nanoparticles

Authors: Crisan, O; Angelakeris, M; Flevaris, NK; Filoti, G

Published: DEC 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 962, DOI:

The subject of magnetic anisotropy in nanostructured materials is of considerable interest from both experimental and theoretical points of view. Systems that consist of either isolated or interacting nanoparticles are shown to exhibit different magnetic properties of the nanoparticle surface than those of the bulk. This altering of magnetic behavior is mainly related to surface spins structure cot-related with finite size effects, providing that the nanoparticles are sufficiently small so that the surface-to-volume atomic ratio would be high enough. This work reports on both the simulation and experimental approach on the magnetic behavior of ferromagnetic nanoparticles. We investigate the competition between surface and bulk magnetocrystalline anisotropy in small magnetic particles. The experimentally obtained magnetic data for ferromagnetic core-shell-type AgCo nanoparticles are interpreted consistently with the results of Monte Carlo simulation of the magnetic behavior of single domain nanoparticles.

745. Anomalous suppression of superconductivity in LiCl-doped Bi-2223

Authors: Mihalache, V; Aldica, G; Badica, P

Published: OCT 2003, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 392, 188, DOI: 10.1016/S0921-4534(03)01102-X

From ac susceptibility measurements of the [Bi(Pb)](2)Ca2Sr2Cu3Oz (Bi-2223) samples doped with LiCl, we have found and report for the first time anomalously suppressed superconductivity (T, is anomalously decreasing) in the slightly under-doped region. For our BPSCCO/(LiCl)(y) samples this region is situated around y = 0.07. Hole concentration (p) per number of Cu-O planes of the Bi-2223 unit cell, determined from room temperature thermopower (S-300 K) measurements, is approximately 0.15 when y is within anomalous region. Literature data shows that for the La-based cuprates, similar suppression of superconductivity was observed at p similar to 0.12 (so-called "1/8 problem"). This discrepancy would appear because charge ordering phenomenon (probably induced by Li known as a pair breaker as e.g. Zn) may be accompanied by changes in oxygen content or oxygen ordering effects. (C) 2003 Elsevier B.V. All rights reserved.

746. Monte Carlo simulation of magnetic properties in nanocrystalline-like systems

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

Published: SEP 24 2003, JOURNAL OF PHYSICS-CONDENSED MATTER, 15, 6344, DOI: 10.1088/0953-8984/15/37/003

Those nanocrystalline materials which consist of ferromagnetic nanograins embedded in a ferromagnetic matrix were modelled as a cubic lattice composed of a central sphere with strongly coupled spins surrounded by weakly coupled spins. The magnetic behaviour was studied by Monte Carlo simulation, especially the low temperature spin ordering and features exhibited in the temperature range between the Curie temperatures of the two phases. The magnetization and magnetic susceptibility are calculated as a function of temperature for different values of the interfacial exchange interactions. It is shown that the exchange coupling between matrix and nanograin influences the magnetic properties of the nanograin, the matrix and the interfacial regions differently. The magnetic behaviour of different regions has been explained in terms of a polarization mechanism acting on the surface and in the core, leading to magnetic correlation between the spins. Such features are quite consistent with the experimental results obtained on nanocrystalline alloys.

747. Nanocrystallization of soft magnetic Finemet-type amorphous ribbons

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

Published: SEP 15 2003, SENSORS AND ACTUATORS A-PHYSICAL, 106, 250, DOI: 10.1016/S0924-4247(03)00177-8

The nanocrystallization of two soft magnetic amorphous Finemet-type ribbons of the nominal composition Fe73.5Cu1Nb3Si13.5B9 and Fe73.5Cu1Gd1Nb2Si13.5B9 synthesized by a melt spinning technique is reported. The crystallization process was monitored by differential scanning calorimetry (DSC) that allows identification of the primary crystallization temperature as well as the other exothermic events occurring during crystallization process. The resulting phase structure of the properly annealed samples is investigated by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM) and is shown to consist of size-distributed body-centred cubic (bcc) alpha-Fe(Si) (rather than Fe3Si, DO3-type), nanograins, embedded into the amorphous Nb-rich residual matrix. The degree of dispersion of the nanograins in the amorphous matrix is estimated from the nanocrystalline volume fraction and the mean size of the nanograins obtained both from XRD data and TEM images. (C) 2003 Elsevier B.V. All rights reserved.

748. Anisotropies in ferromagnetic nanoparticles: simulation and experimental approach

Authors: Crisan, O; Angelakeris, M; Flevaris, NK; Sobal, N; Giersig, M

Published: SEP 15 2003, SENSORS AND ACTUATORS A-PHYSICAL, 106, 133, DOI: 10.1016/S0924-4247(03)00150-X

Magnetic behavior of the isolated ferromagnetic bimetallic nanoparticles is investigated in relation with the competition between exchange interaction and surface and bulk anisotropies. Experimental results on AgCo bimetallic nanoparticles are presented and correlated with Monte Carlo simulation of low temperature spin ordering. The nature of spin disorder at the surface, responsible for the reduction of the overall saturation magnetization is briefly discussed. (C) 2003 Elsevier B.V. All rights reserved.

749. Preparation and structural investigation of epitaxially grown antiferromagnetic FeSn2(001) thin films on InSb(001)

Authors: Kuncser, V; Doi, M; Sahoo, B; Stromberg, F; Keune, W

Published: SEP 1 2003, JOURNAL OF APPLIED PHYSICS, 94, 3581, DOI: 10.1063/1.1595144

Antiferromagnetic FeSn2(001) thin films with different thicknesses and relatively low Neel temperatures were grown on InSb(001)(4x2) surfaces by molecular-beam epitaxy. The Neel temperature could be increased to above room temperature by subsequent thermal annealing. In situ structural characterization was performed by high- and low-energy electron diffraction. The degree of the structural (001) texture as a function of the preparation and annealing conditions was investigated by x-ray diffractometry. The local magnetic properties and the spin structure were studied using Fe-57 conversion electron Mossbauer spectroscopy (CEMS) at different temperatures. The epitaxial FeSn2(001) thin films exhibit in-plane Fe spin orientation and appear to be suitable antiferromagnets for studying the interfacial spin structure in exchange-biased bilayers by CEMS. (C) 2003 American Institute of Physics.

750. Metastable phases and crystallization products in nanocrystalline FINEMET-type alloys with RE addition

Authors: Crisan, AD; LeBreton, JM; Crisan, O; Filoti, G

Published: SEP 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 711, DOI:

The melt spun nanocrystalline alloys such as Fe73.5Cu1Nb3Si13.5B9 (FINEMET) have attracted great deal of interest in recent years due to their excellent soft magnetic properties mostly related to the exchange coupling between nanograins, through the amorphous matrix. The microstructural evolution of both nanocrystalline and amorphous residual phases during annealing of the ribbons give rise to crystallization products that determine the expected magnetic properties for specific applications. Upon addition of the rare earth (RE), the evolution of the crystalline phases which emerge from metastable precursors during annealing is investigated. It is expected that the RE presence should strongly modify both the phase structure via new ternary metastable precursors and the magnetic properties of the ribbons by inducing enhanced exchange correlation due to novel ternary crystalline phases.