1PO4-30 Magnetoresistivity, Seebeck, Nernst and Hall Effects in the Mixed and Normal State of (Bi_1.6Pb_0.4)Sr₂Ca₂(Cu_1-xNi_x)₃O_y Epitaxial Thin Films

G. Ilonca 1, Aurel V. Pop 1, Isof Gr. Deac 1, Claudiu Lung 1, Teodor Jurcut 2, Nicolae Dulamita 3, and Robert Deltour 4. 1 Babes – Bolyai University, Faculty of Physics, 3400 Cluj-Napoca, Romania. 2 University of Oradea, Romania. 3 University, Faculty of Chemistry, 3400 Cluj-Napoca, Romania. 4 ULB, Physique des Solides, CP 233, 1050 Bruxelles, Belgium.

Presenting Author: G. Ilonca

Measurement of the magnetoresistivity, Seebeck, Nernst and Hall coefficients in the mixed state for Bi-2223 doped with 0 £ x £ 0.05 Ni content in the magnetic fields between 0 and 5 T, and the temperature range 5-300 K are reported. The critical temperatures, the Hall are reported Nernst and Seebeck coefficients depend strongly on the nickel content. In the mixed state and fluctuation regime these behaviors can be explained by Ginsburg - Landau theory. The deviation from linear dependence of m _H^-1 = f(T²) and n_H in normal state can be interpreted in the percolative phase separation theory and S(T), with the asymmetric narrow-band model.

1PO4-31 A study of the growth mode of Re_1+xBa_2-xCu₃O_7-d(Re = Nd, Y, Dy) epitaxial films deposited by physical vapour deposition

Marco Salluzzo 1, Fabio Miletto Granozio 1, Umberto Scotti Di Uccio 1, Ivan Maggio Aprile 2, Oystein Fischer 2, and Carmela Aruta 3. 1 INFM and Dipartimento di Fisica dell’Università Federico II di Napoli, Piazzale Tecchio 80, P.O. Box 80125 Napoli, Italy. 2 Département de Physique de la Matière Condensée, Université de Genève, 24 Quai E.-Ansermet, P.O. Box CH-1211 Genève 4, Switzerland. 3 European Synchrotron Radiation Facility BP 220,6 rue Jules Horowitz, P.O. Box 38043 Grenoble Cedex, France.

Presenting Author: M. Salluzzo

In this contribution we report on the growth modes of Re_1+xBa_2-xCu₃O_7-d (RBCO) compounds deposited by sputtering. Two important phenomena have been studied: the 2D vs.3D growth in c-axis Nd_1+xBa_2xCu₃O₇ films and the crossover between a-axis to c-axis growth in the RBCO family.

We found that c-axis stoichiometric Nd₁Ba₂Cu₃O₇ films exhibit the well known 3D layered island-growth, possibly originated by screw and half loop dislocations. On the contrary, a continuos 2D growth is found in off-stoichiometry films, characterised by Nd excess and Ba depletion [1,2].

The amount of a-axis in Re_1+xBa_2-xCu₃O₇ films deposited at different temperatures has been carefully determined by XRD. As for other compounds, the c-axis percent is an increasing function of the deposition temperature. We have found that in Nd-rich films the content of a-axis grains is lower than in stoichiometric films.

The experimental results are interpreted in the framework of a theoretical model of the nucleation of RBCO films. The crossover from a-axis to c-axis growth is explained in terms of the Gibbs energy of critical nuclei and of the supersaturation at deposition conditions. The model provides a quantitative description and a satisfactory fit of experimental data from our experiment and from the current literature. [3]

[1] M. Salluzzo, I. Maggio-Aprile, and Ø. Fischer, Applied Phys. Letters Vol. 73 (5), 683 (1998).

[2] M. Salluzzo, I. Maggio-Aprile, and Ø. Fischer, IEEE Trans. Applied Superconductivity, Vol. 9, 1856 (1999).

[3] F. Miletto, M. Salluzzo, U. Scotti di Uccio, I. Maggio-Aprile, Ø. Fischer, in press on Phys Rev. B, 1 November (1999).

1PO4-32 PVD Growth and Characterization of YBCO Thin Films on Polycrystalline Alumina Substrates: Phase Separation

Macit A. Ozenbas and Tufan Gungoren, Dept. of Met. and Materials Eng., Middle East Technical University, 06531 Ankara, Turkey

Presenting Author: M.A. Ozenbas

In this study, high T_c superconducting thin films of Y-Ba-Cu-O system have been prepared onto polycrystalline Al₂O₃ substrates by resistive evaporation of YF₃, BaF₂, and Cu powders and a subsequent multi-stage annealing. In the deposition process, two different methods were used: the mixed-powder method, evaporation of a pulverized mixture of powders, and the sequential-deposition method, a layer-by-layer deposition of each constituent powder. The films were characterized by XRD, SEM/EDS and a computerized electrical characterization unit. In the course of the study, effects of deposition method, film thickness, barrier layer, and post-growth annealing parameters on the superconducting properties were searched. Sequential deposition method seems to reduce film-substrate reactions on Al₂O₃ substrates, probably due in part to the prevention of phase separation.

1PO4-33 Characterization of c-axis Oriented Y-Ba-Cu-O Thin Films Prepared by PVD on LaAlO₃ (100) and SrTiO₃ (100) Substrates

Macit A. Ozenbas and Tufan Gungoren, Dept. of Met. and Materials Eng., Middle East Technical University, 06531 Ankara, Turkey

Presenting Author: M.A. Ozenbas

In this study, high T_c superconducting thin films of Y-Ba-Cu-O system have been prepared onto single crystals of LaAlO₃ (100), and SrTiO₃ (100) by resistive evaporation of YF₃, BaF₂, and Cu powders and a subsequent multi-stage annealing. In the deposition process, two different methods were used: the mixed-powder method, evaporation of a pulverized mixture of powders, and the sequential-deposition method, a layer-by-layer deposition of each constituent powder. The films were characterized by XRD, SEM/EDS and a computerized electrical characterization unit. In the course of the study, effects of deposition method, film thickness, type of substrate and post-growth annealing parameters on the superconducting properties were searched. Characterization studies by SEM/EDS of the films on LaAlO₃ and SrTiO₃ substrates suggested preferential orientation. XRD examinations of these films showed c-axis orientation of the superconducting phase.

1PO4-34 Preparation of YBCO Thin Films by Electron Beam Deposition with Double Sources and its Properties

T. Kawae, S. Kambe, and O. Ishii, Graduate School of Sience and Engineering, Yamagata-Univ. Jounan 4-3-16, Yonezawa, Yamagata 992-8510, Japan

Presenting Author: T. Kawae

One of the merits of using an electron beam (EB) deposition method is its rapid deposition rate, which is suitable for mass production. Previously, YBCO thin films have been fabricated by using the EB deposition with triple hearths, in which Y₂O₃, BaO, and CuO sources are contained. In order to improve productivity and to reduce the material cost, we attended to fabricate YBCO thin films by using the EB deposition apparatus with double hearths, in which Y₂O₃ and Ba₂Cu₃O_x are contained.

To fabricate high quality YBCO thin films, we examined preparation conditions for each sources. The EB currents were fixed at 50mA and 60mA for the Y₂O₃ and Ba₂Cu₃O_x sources, respectively. By changing deposition time for Y₂O₃, the ratio of Y₂O₃:Ba₂Cu₃O_x was controlled. The prepared films were sintered at 950ºC for 6 hours in air and annealed at 450ºC for 5 hours in oxygen. The phase of the prepared films without heating substrate was analyzed by the XRD method. It was found that when the Y₂O₃ deposition time ranged from 4 to 12 minutes, the cation ratio, Y:Ba:Cu was 1:2:3, and T_c was 83K. When the Y₂O₃ deposition time exceeded more than 16 minutes, the amount of the Y211 phase was increased with increasing the Y₂O₃ deposition time.

Substrate heating was effective to improve crystallinity, when the Y₂O₃ deposition time was fixed at 12 minutes. It was proved that the optimum heating temperature was 690ºC, at which the highest c-axis orientation, the highest transition temperature(T_cON=90K) and the narrowest transition width (D T~20K) were achieved.

1PO4-35 Superconducting YBa₂Cu₃O_7-x films on SrTiO₃ via electrodeposition process

A.J.S. Machado 1, S. Moehlecke 2, Y. Kopelevich 2, Alain Robin 1 and C.A.M. dos Santos 1. 1 Departamento de Engenharia de Materiais – FAENQUIL, 12600-000 – Lorena – SP, Brazil. 2 Instituto de Física "Gleb Wataghin", Universidade Estadual de Campinas – UNICAMP, 13083-970 – Campinas – SP, Brazil.

Presenting Author: A.J.S. Machado

An ac electrodeposition method to obtain good quality YBa₂Cu₃O_7-x high-temperature superconductor films was developed. These films were deposited on the (110) surface of SrTiO₃ single crystal substrates previously coated with a thin silver layer (800 Å) that was deposited by chemical reduction. A small amount of Ag (0.5 –0.8 % in weight) added also to the electrolyte bath has shown to improve the quality of the deposited films. YBa₂Cu₃O_7-x films, with thickness between 0.5 and m m, after an appropriate heat treatment present a single phase material with an onset T_c ~ 89 K, D T_c ~ 4 K, homogeneous grain morphology and a strong biaxial texture. X-ray (q -2q , rocking curves and pole figures), SEM(EDX) and ICP characterizations reveal the good quality of the samples, like omega scans with full width half maximum (FWHM) of only 0.9° . Transport properties measurements show a metallic behavior in the normal state and self-field critical current densities ~ 10⁵ A/cm² at 4.2 K and ~ 10⁴ A/cm² at 77K.

1PO4-36 Superconducting YBCO Thin Film on Multicrystalline Ag Film Evaporated on MgO Substrate

Jacob Azoulay, Armen Verdyan, and Igor Lapsker, Center for Technological Education Holon affiliated with Tel-Aviv University, Dept. of Science, Holon, Israel

Presenting Author: J. Azoulay

Superconducting YBa₂Cu₃O_7-d films were grown by resistive evaporation on multicrystalline silver film which was evaporated on MgO substrate. A simple inexpensive vacuum system equipped with resistively heated boat was used for the whole process. Silver film was first evaporated on MgO substrate kept at 400°C during the evaporation after which with no further annealing a precursor mixture of yttrium small grains and Cu and BaF₂ in powder form weighed in the atomic proportion to yield stoichiometric YBa₂Cu₃O₇ was evaporated.

The films thus obtained were annealed at 740°C under low oxygen partial pressure of about 1Pa for 30 minutes to form the superconducting phase. X-ray diffraction and scanning electron microscopy techniques were used for texture and surface analysis. Electrical properties were determined using a standard dc four-probe for electrical measurements.

The physical and electrical properties of the YBCO films are discussed in light of the fact that X-ray diffraction measurements done on the silver film have revealed a multicrystalline structure.

1PO4-37 Growth of a-axis Oriented YBa₂Cu₃O_7-d Films on Ion-Beam Cleaned LaSrGaO₄ Substrates

T. Goto, S. Takekawa, M. Moriya, K. Usami, and T. Kobayashi, The University of Electro-Communications, Chofu-Shi, Tokyo, 182-8585, Japan

Presenting Author: T. Goto

In-plain aligned a-axis YBa₂Cu₃O_7-d (YBCO) films are known to grow on LaSrGaO₄ substrates. However, usually limited substrate temperature range is required for a-axis orientation.

In this work the LaSrGaO₄ substrates were cleaned by ion-beam and YBCO films were deposited by temperature gradient method. The best result was obtained at 45 degree as for the angle of incident ion. It was found that high quality a-axis oriented films were formed at the initial substrate temperature in the range of 520 ° C~600 ° C and final temperature of 680 ° C. Zero voltage superconductive transition temperature of an a-axis film was 81K. Electrical transport properties of c-axis direction have been also studied.

1PO4-38 Electrodeposited Films of YBCO on Ag and on SrTiO₃: a comparison

Miguel Á. Alario-Franco, Marisol Martín-González and Emilio Morán, Laboratorio de Química del Estado Sólido, Facultad de Ciencias Químicas, Universidad Complutense, 28040 Madrid, Spain (EU)

Presenting Author: M.A. Alario-Franco

Thick films (~30 m ) of YBa₂Cu₃O₇ (YBCO) have been obtained through cationic electrodeposition on silver and on reduced SrTiO₃, so as to make it conducting and eliminate the need for a conducting buffer layer (Spanish patent P9802655).

The superconducting films morphology is, in both cases, of a columnar type, as shown by SEM, leaving some empty spaces in between the columns. These spaces can be diminished by a multiple sequential electrodeposition. Although the films are then not continuous, critical temperatures are as high as usual (~92 K) and critical currents (as determined by the Bean method) are, in A/cm², respectively 9 500 for YBaCuO/Ag and 3 250 for YBCO/SrTiO₃.

1PO4-39 Dependence of the microstructure of ReBa₂Cu₃O_7-x films on the conditions of high-oxygen pressure dc-sputtering

Mikhail I. Faley, Chunlin L. Jia, Jinsong Wu, Ulrich Poppe, and Knut Urban, Institut fuer Festkoerperforschung, FZ-Juelich GmbH, D-52425 Juelich, Germany

Presenting Author: M.I. Faley

A significant dependence of the bulk- and surface-microstructure of ReBa₂Cu₃O_7-x films on the conditions of high-oxygen pressure dc-sputtering was observed. A variation of the oxygen pressure, substrate-target distance and a substrate temperature lead in a reproducible way to changes of the surface morphology of the films. The films can be perfectly flat, or contain holes, precipitates, and inclusions of secondary phases. In the optimized films TEM studies demonstrate the presence of the secondary phases epitaxially covered by c-axis oriented top layers. A control of the surface by Atomic Force Microscope reveals the step-flow and spiral growths of the films prepared by sputtering at different conditions. Some possible explanations of the observed effects will be presented. The optimized films were used for preparation of the high quality multilayer HTS structures like quasiplanar Josephson junctions, dc-SQUIDs and multi-turn flux transformers. For these applications the films should be flat, but contain inclusions of secondary phases serving as a magnetic flux pinning centers.