National Institute Of Materials Physics - Romania

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5781 articles found

4351. Transport and magnetic properties of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiCl)(y) superconductors

Authors: Mihalache, V; Aldica, G; Popa, S; Miu, D

Published: JUN 2003, JOURNAL OF SUPERCONDUCTIVITY, 16, 580, DOI: 10.1023/A:1023837508196

We investigated the superconducting critical temperature, the intra- and intergranular critical current density, and the thermopower properties of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiCl)(y) samples. All these properties have been compared with those of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiF)(y) specimens. It was found that the critical temperature determined from resistive and AC complex susceptibility measurements show a maximum and the transition width shows a minimum for the intermediate values of y. Powder X-ray diffraction studies and the AC complex susceptibility measurements reveal that in our samples the amount of Bi2Sr2Ca2Cu3O10+delta high-temperature superconducting phase is maximum for y approximate to 0: 02: The amount of LiCl in Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiCl)(y) changes the superconducting properties of the grains as well as of the intergrain matrix. The splitting of the peak in the temperature dependence of the imaginary part of the complex susceptibility, corresponding to the dissipation inside the grains, was also observed.

4352. Growth by MOMBE of c-axis superconducting YBCO thin films on different substrates: In situ RHEED monitoring of the growth

Authors: Endo, K; Badica, P; Moriyasu, Y; Abe, K

Published: JUN 2003, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, 13, 2794, DOI: 10.1109/TASC.2003.812014

Superconducting YBCO c-axis oriented thin films have been grown by MOMBE (metalorganic molecular-beam-epitaxy). Growth of the films on different substrates (100) (Y, Nd)AlO3, (100) SrTiO3 and (100) MgO has been investigated by, in situ monitoring the reflection high energy electron diffraction (RHEED). The paper discusses our results suggesting significant differences in films growth on the three presented substrates. The highest quality (high uniformity, epitaxy and low roughness) has been attained for the films prepared on SrTiO3. All films, regardless substrates, have shown values between 81 and 84 K for zero resistance critical temperature T-c0.

4353. (119) Oriented Bi-2223 thin films grown on (100) NdGaO3 by MOCVD

Authors: Endo, K; Badica, P; Abe, K

Published: JUN 2003, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, 13, 2712, DOI: 10.1109/TASC.2003.811963

Orientation control of high T-c superconductors is essential for superior device performance, because the coherence length is longer along the non c-axis directions than along c-axis direction. In this study, we report on the first successful preparation of (119) oriented Bi-2223 films by MOCVD using (100) NdGaO3 Substrates. Atomic force microscopy observations of the (119) Bi-2223 films are also reported. Our films have an in-plane aligned mountain-range-shaped surface morphology. This morphology is resulting from the epitaxial relationship between the (119) Bi-2223 film and (100) NdGaO3 substrate. Films exhibited a large in-plane anisotropy as revealed by, resistivity measurements and surface morphology observations.

4354. Nanodots-induced pinning Centers in thin films: Effects on critical current density, activation energy and flux jump rate

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

Published: JUN 2003, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, 13, 3729, DOI: 10.1109/TASC.2003.812527

By very short time rf sputtering in certain deposition conditions we have grown three-dimensional Ag nanodots on the substrate prior to the growth of TI-based superconducting films. These nanodots create pinning centers, leading to an increase in the critical current density about 10 times. From the frequency dependence of the critical current density we estimated also the, actiation energy for the flux jumps, which resulted to. be several times higher. The rate of the thermally-activated flux jumps decreases several orders of magnitude. We suggest that our method can be used for the reduction of thermal noise in high-T-c dc SQUID's.

4355. Hall probe imaging of local hysteresis inversion and negative remanent fields near the edge of a YBCO thin film

Authors: Crisan, A; Pross, A; Humphreys, RG; Bending, S

Published: JUN 2003, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 16, 698, DOI: 10.1088/0953-2048/16/6/307

We report vortex imaging and local magnetometry studies near the edge of a YBCO thin film using scanning Hall probe microscopy (SHPM). Our data show some unusual features, namely local hysteresis inversion and negative remanent fields, which are semi-quantitatively explained in terms of a theoretical model of flux penetration in an infinite-long thin superconducting strip.

4356. Preparation of Tl-2212 and Tl-1223 superconductor thin films and their microwave surface resistance

Authors: Sundaresan, A; Asada, H; Crisan, A; Nie, JC; Kito, H; Iyo, A; Tanaka, Y; Kusunoki, M; Ohshima, S

Published: JUN 2003, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, 13, 2916, DOI: 10.1109/TASC.2003.812045

Epitaxial Tl2Ba2CaCu2Oy and Tl(Ba,Sr)(2)Ca-2 Cu3Oy thin films have been prepared on CeO2 buffered sapphire substrate by an ex situ process with high reproducibility. Microstructure analysis of the surface of the TI-2212 films showed well connected and smaller sized grains of 1 mum or less. In the case of TI-1223 film, plate-like crystals with pits and pin holes could be observed. The superconducting critical temperatures of both films are around 96 K. Critical current density measured by ac susceptibility technique is as high as 1 MA/cm(2) at 77 K. The microwave surface resistance of these films was measured over a wide range of temperatures by a HTS-sapphire-HTS resonator method at 38 GHz on films with area 10 mm x 10 mm. The temperature dependence of surface resistance of TI-2212 film follows very closely to that of the best YBCO films on MgO substrate, whereas the TI-1223 exhibits slightly higher values resulting from an inferior surface morphology.

4357. Mossbauer acquisition system for Rayleigh scattering experiments

Authors: Kluger, A; Enescu, SE; Bibicu, I; Ciortea, C; Enulescu, A

Published: JUN 2003, REVIEW OF SCIENTIFIC INSTRUMENTS, 74, 3038, DOI: 10.1063/1.1575921

A data acquisition system is reported with a number of improvements over similar systems currently used in Rayleigh scattering of Mossbauer radiation (RSMR) experiments. It is based on a data acquisition board including a Mossbauer function generator with adjustable frequency reference signal delivery, an internal single channel analyzer, and an internal 32 kB random access memory for data storage. The system performances were tested in RSMR measurements on pyrolytic graphite C (002) single crystal by using an iron enriched Fe-57 ( 30 at.%) in rhodium matrix absorber. We want to demonstrate that the reported data acquisition system is suitable for the experimental separation of the elastic and inelastic components of the scattered radiation. The system can be used also in the ordinary Mossbauer measurements as a compact and rapid data acquisition system. In this case it presents the advantage of including also the function generator. (C) 2003 American Institute of Physics.

4358. Surface particularities in pulsed laser ablation/deposition of the ferromagnetic alloy NiMnSb

Authors: Grigorescu, CEA; Manea, SA; Mitrea, M; Monnereau, O; Notonier, R; Tortet, L; Keschawarz, R; Giapintzakis, J; Klini, A; Zorba, V; Androulakis, J; Fotakis, C

Published: MAY 15 2003, APPLIED SURFACE SCIENCE, 212, 84, DOI: 10.1016/S0169-4332(03)00404-5

In this work we investigate the surface morphology and stoichiometry of the ablated targets employed with pulsed laser deposition (PLD) of thin films of the ferromagnetic half-Heusler alloy NiMnSb. The NiMnSb targets were: a pressed and sintered pellet and, respectively, polycrystalline discs grown by RF melting. The films were deposited using a KrF excimer laser (lambda = 248 nm, tau = 30 ns, 600 mJ/pulse maximum) delivering pulses at a repetition rate of 10 Hz. SEM observations show different morphologies of the ablated surfaces in the two cases, which are further correlated with EDX analyses to explore surface segregation phenomena. The corresponding films are also considered. Apparently, the crystalline targets keep better the initial stoichiometry than the pressed ones, which in principle would indicate higher film stoichiometry and hence enhanced quality. (C) 2003 Elsevier Science B.V. All rights reserved.

4359. Intensification of synthesis and examination of technological parameters influence on the properties and structure of Bi-(Pb)-Sr-Cu-Ca-O ceramics (vol 384, pg 111, 2003)

Authors: Yurchenko, IA; Alekseev, AF; Yurchenko, DO; Badica, P; Gridasova, TY; Morozov, VV; Nemirovsky, AV; Peklun, VF

Published: MAY 15 2003, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 387, 420, DOI: 10.1016/S0921-4534(03)00703-2

4360. Intra- and inter-grain critical current density in (Cu,C): 1234 superconductors

Authors: Crisan, A; Badica, P; Hirai, M; Kito, H; Iyo, A; Tanaka, Y

Published: MAY 2003, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 388, 422, DOI: 10.1016/S0921-4534(02)02550-9

From DC magnetization studies in fields up to 14 T and temperatures between 20 and 100 K, we estimated the intra-and inter-grain critical current density J(c) of (Cu,C): 1234 high-T-c superconductors, in the frame of critical state models. The inter-grain J(c) was determined by comparing the magnetization loops of as-grown sample and of the ground sample. Finally, short comments on J(c) resulted from AC susceptibility measurements and of the impressive increase of intra-grain J(c) due to heavy-ion and neutron irradiation are presented. (C) 2003 Elsevier Science B.V. All rights reserved.