National Institute Of Materials Physics - Romania

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903 articles found

801. The effect of pinning centers in Zn-doped CuBa2Ca3Cu4O12-y high-temperature superconductors

Authors: Crisan, A; Agarwal, SK; Koganezawa, T; Kuroda, R; Tokiwa, K; Watanabe, T; Iyo, A; Tanaka, Y; Ihara, H

Published: JUN-AUG 2002, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 63, 1076, DOI: 10.1016/S0022-3697(02)00030-6

CuBa2Ca3Cu4O12-y (Cu:1234) high-temperature superconductors (HTS) doped with up to 2% Zn were grown using the high-pressure synthesis technique. Magnetization loops of the samples were measured at various temperatures between 5 and 77.3 K and magnetic fields up to 14 T. Critical current densities J(c) of the samples were estimated using the critical state model. The results show that Zn-induced pinning centers increase J(c) of Cu:1234 several times, depending on field and temperature. From the experimentally determined field-temperature region in which a higher Zn concentration lead to a higher J(c), we suggest the existence of a cross-over from quite efficient, extended (in the c-axes direction) pinning centers to point-like (inefficient) pinning centers at a certain temperature, depending on field. This effect can be attributed to the fact that, unlike other HTS, in Cu: 1234 there is a second critical temperature T-c2 of about 70-80 K (in zero field, and 50-60 K in 15 T), related to the over-doping of pyramidal basal plane (outer CuO2 plane). (C) 2002 Elsevier Science Ltd. All rights reserved.

802. Change of the nuclear charge radius for the Au-197 Mossbauer resonance

Authors: Palade, P; Wagner, FE; Filoti, G

Published: 2002, HYPERFINE INTERACTIONS (C), VOL 5, PROCEEDINGS, 188, DOI:

The electron density at the gold nuclei in metallic gold and in the cubic intermetallic compounds CsAu, RbAu, AuMg, AuZn, AuCd, AuSc, AuAl2, AuGa2, AuIn2, AuSn2 and AuSb2 have been calculated using the Wien97 computer code. The electron densities thus obtained and the experimental values of the isomer shifts define a linear correlation spanning nearly the whole range of isomer shifts observed for the 77 keV transition of Au-197. From the slope of this correlation one obtains Delta = (4.6 +/- 0.2).10(-3) fm(2) for the change of the mean square nuclear charge radius accompanying 77 keV transition in Au-197.

803. Sputtered nanodots: A costless method for inducing effective pinning centers in superconducting thin films

Authors: Crisan, A; Fujiwara, S; Nie, JC; Sundaresan, A; Ihara, H

Published: DEC 31 2001, APPLIED PHYSICS LETTERS, 79, 4549, DOI: 10.1063/1.1428632

A straightforward and cheap method for creating extended defects, strong pinning centers, in superconducting thin films is proposed. Clearly, by very short time (3-5 s) rf sputtering at suitable substrate temperatures, we deposited Ag nanodots on SrTiO3 substrates prior to the growth of superconducting thin films. The nanodots were studied by atomic force microscopy. Due to the lattice mismatch and/or chemical poisoning, on top of the nanodots the superconducting phase does not form, creating in this way extended and effective pinning centers which increase the critical current density of the film. The method was applied to (Cu, Tl)BaSrCa2Cu3Oy films grown by amorphous phase epitaxy. Thin films grown in similar conditions, with and without nanodots, were characterized by x-ray diffraction and ac susceptibility. The results show that the nanodots increased the critical current density more than one order of magnitude. (C) 2001 American Institute of Physics.

804. Influence of LiF addition on the superconducting properties of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox high-temperature superconducting oxide

Authors: Mihalache, V; Aldica, G; Giusca, C; Miu, L

Published: OCT 2001, JOURNAL OF SUPERCONDUCTIVITY, 14, 579, DOI: 10.1023/A:1012983831008

We investigated the superconducting critical temperature, transport critical-current density and the thermo-power properties of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiF)(y) samples. It was found that the midpoint critical temperature increases with increasing y, attaining approximate to 119.5 K for y = 0.15. The transition width shows a minimum and the critical temperature (zero electrical resistance) exhibits a maximum as a function of y. The observed behavior is correlated with the SEM analysis. Powder X-ray diffraction studies reveal that in our samples the amount of Bi2Sr2Ca2Cu3O10+delta high-temperature superconducting phase is maximal (70-78%) for y = 0.05-0.07.

805. Mechanically alloyed Cu-Fe studied by Mossbauer spectroscopy

Authors: Principi, G; Spataru, T; Gupta, R; Enzo, S; Kuncser, V; Filoti, G

Published: AUG 9 2001, JOURNAL OF ALLOYS AND COMPOUNDS, 326, 192, DOI: 10.1016/S0925-8388(01)01301-9

Recent studies of mechanically alloyed Fe-Cu powder mixtures have suggested differences in the local magnetic environment of iron atoms. For a more accurate definition of this point, ball-milled Cu70Fe30 and Cu50Fe50 alloys were investigated by Mossbauer spectroscopy in the, temperature range 4.2-300 K. The low temperature Mossbauer spectra exhibit a broad magnetic pattern. typical of a defect structural configuration. The magnetic splitting strongly decreases with increasing temperature, especially in the case of Cu70Fe30 alloy. But even for this composition there is, at room temperature, an unresolved magnetic pattern. Applying a magnetic field of 3 T, parallel to gamma rays, at 4.2 K a rotation of all magnetic moments along the external field is observed. The samples behave as an alloy with continuously distributed local fields. (C) 2001 Elsevier Science BY. All rights reserved.

806. A magnetic study of magnetoresistive Cu-(SmCo5)-Fe heterogranular alloys

Authors: Crisan, O; Le Breton, JM; Jianu, A; Maignan, A; Nogues, M; Teillet, J; Filoti, G

Published: AUG 2001, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 234, 99, DOI: 10.1016/S0304-8853(01)00279-7

The magnetic interactions between the nanograins in heterogeneous granular Cu-(SmCo5)-Fe ribbons were evaluated with a classical mean field model to obtain the exchange coupling as well as the magnetic moment per nanoparticle. The samples were subjected to various thermal treatments and a significant magnetoresistive effect was obtained when the optimal annealing conditions had been achieved. (C) 2001 Elsevier Science B.V. All rights reserved.

807. Some superconducting properties of the inter-domain border of melt-textured YBa2Cu3O7-delta

Authors: Crisan, A; Gordeev, SN; de Groot, PAJ; Beduz, C

Published: JUN 15 2001, PHYSICA C, 355, 237, DOI: 10.1016/S0921-4534(01)00023-5

The resistive transitions R(T) and the current-voltage (I-V) in fields up to 5 T were measured in a melt-textured YBa2Cu7-delta (YBCO) sample consisting of two single-grain domains separated by an inter-domain border, and in a single-grain sample cut from the same pellet and oxygenated in such conditions that ensure the optimum oxygenation of the grains. From comparing the two sets of results, we were able to suggest that the inter-domain border consists of oxygen-deficient YBCO with thickness smaller than 6 mum, having a slightly lower critical temperature, a higher anisotropy factor and without strong pinning centers. From applications point of view, our work shows that, for improving the superconducting properties of large multiple-domain melt-textured pellets. an oxygenation process much longer than that required for optimum grain properties is needed. (C) 2001 Published by Elsevier Science B.V.

808. Fluctuation magnetoconductivity of BSCCO-2212 films in parallel magnetic field

Authors: Balestrino, G; Crisan, A; Livanov, DV; Manokhin, SI; Milani, E

Published: JUN 1 2001, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 355, 139, DOI: 10.1016/S0921-4534(00)01771-8

The in-plane (delta sigma (ab)(T,H)) and out-of-plane (delta sigma (c)(T,H)) components of the magnetoconductivity tenser of Bi2Sr2CaCu2O8+x films have been simultaneously measured in magnetic fields parallel to the film conducting planes (B less than or equal to 1 T). A negative c-axis magnetoresistance is measured for the first time in this geometry. Both delta sigma (ab) and delta sigma (c) are well described by the fluctuation theory in the temperature range T-c < T < T-c + 20 K, using a single set of microscopical parameters for both directions. The negative c-axis magnetoresistance in the field parallel to conducting layers is shown to be induced by the density of states Zeeman fluctuation contribution. (C) 2001 Elsevier Science B.V. All rights reserved.

809. The influence of BaZrO3 on the magnetic response of (Bi,Pb): 2223 high-temperature superconductors

Authors: Mihalache, V; Aldica, G; Popa, S; Nita, P; Crisan, A

Published: JUN 2001, JOURNAL OF SUPERCONDUCTIVITY, 14, 386, DOI: 10.1023/A:1011174321955

DC magnetization and AC complex susceptibility measurements on (Bi,Pb) : 2223 high-temperature superconductors impurified with various amounts of BaZrO3 are presented. The results are discussed in the frame of the critical state model. and the values of the inter- and intragranular critical current density as well as of the field for full penetration are estimated. The values of the intergranular critical current density are consistent with those obtained from transport measurements. The intragranular critical current density and the field for full penetration have similar values from both DC magnetization and AC susceptibility measurements. It was shown that, in the (Bi,Pb) : 2223 system. BaZrO3 impurification changes only the properties of the intergrain matrix, while the superconducting properties of the grains are not modified.

810. Improvement of the superconducting transport properties of YBa2Cu3O7-delta by BaZrO3 doping

Authors: Aldica, G; Bradea, I; Jaklovszky, J; Crisan, A

Published: JUN 2001, JOURNAL OF SUPERCONDUCTIVITY, 14, 409, DOI: 10.1023/A:1011186724681

We have doped the YBa2Cu3O7-delta,S superconducting ceramics with BaZrO3 up to 75 wt.% and studied the changes of some physical properties. The most important finding is the enhancement of the critical current density, which has a maximum at around 5 wt.% doping level. Compared with the undoped samples, the critical current density is four times higher if the doping compound is introduced prior to the calcination treatment, or two times higher if BaZrO3 is introduced prior to the sintering treatment, The dependence of the critical current density on the doping level is consistent with the dependence of the Seebeck coefficient. We also observed a very small decrease of the critical temperature. The real density of the samples decreases with increasing doping level as expected from the ratio between the theoretical densities of YBa2Cu3O7-delta and BaZrO3, the effect being important for doping levels higher than 25 wt.%.