4PO6-10 Electrochemical Oxygen Intercalation in La₂CuO₄ Prepared by Nitrates Method: Microstructural Effects

Carlos R. Michel, Centro de Microscopía Electrónica, C.U.C.E.I. Universidad de Guadalajara. Blvd. M. García Barragán 1421 Guadalajara, Jalisco, Mexico. 44430, and Nieves Casañ-Pastor, Institut de Ciència de Materials de Barcelona (CSIC) Campus UAB, 08193 Bellaterra, Barcelona, Spain

Presenting Author: C.R. Michel

In order to determine microstructural effects in the electrochemical oxygen intercalation for La₂CuO₄, and the relative abundance of the two superconducting phases (T_c=33 and 44K), this oxide was prepared using the nitrates method. This method involves the dissolution of La and Cu oxides, in stoichiometric amounts, using concentrated nitric acid. The dissolution was slowly heated up to 600° C, giving a blue-green powder; which was pressed in pellets of 1 gr. each. Chemical reaction was followed using XRPD on the product obtained at different temperatures. Annealing at 950° C gave a well crystallized and pure oxide. SEM was used to observe pellet microstructure. Grain sizes for these compacts were in the range: 0.5-1 mm, and a density higher than 95% rT was obtained.

To evaluate the influence of porosity and grain size on the electrochemical intercalation, some pellets were ground, pressed and annealed at the same temperature, giving porous compacts, with 80% rT and grain sizes around 1-10 mm. Oxidation potentials for the samples were measured using cyclic voltammetry, giving for them similar values (0.7 V vs. Ag/AgCl). Magnetic measurements performed on oxidized materials showed that densifed pellets gave a main superconducting phase with T_c=33K, whereas porous pellets had a T_c = 44K. Oxygen contents were determined by TG in an Ar/5%H flow. Phase separation using highly diluted hydrochloric acid solutions was proposed.

4PO6-11 Transmission Electron Microscopic Study of Twin Boundaries in YBa₂Cu₃O₇ Thin Films

Wei Liu 1, Lie Chen 1,2, and Lin Li 1,2. 1 Institute of Physics & Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, China. 2 National Laboratory for Superconductivity, Institute of Physics & Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, China.

Presenting Author: W. Liu

Microstructural features of the superconducting YBa₂Cu₃O₇ (YBCO) bulk/thin film samples contain many twin boundaries; such as 90° twins, 110 twins and various mosaic structures formed by twin boundaries. The effect of twin boundaries on the superconducting properties of YBCO thin films depends on their morphology. Generally, twin boundaries of regular pattern do not effect much the superconducting properties, but twin boundaries of irregular pattern effects the superconducting properties markedly. Therefore, to obtain high quality YBCO thin films, it is essential to investigate the microstructure of the films by transmission electron microscopy (TEM), in order to achieve the optimum thin film fabrication process. In other words, TEM investigation is an important link between film fabrication and film properties. In this paper, five different types of twin boundaries are reported, which have been observed commonly by our TEM study. The YBCO thin films were deposited on single crystal LaAlO₃ and SrTiO₃ substrates by magnetron sputtering; their superconducting transition temperatures T_C0 are about 88K. Cross-section and plane view TEM specimens were prepared by Ar ion beam thinning method. Selected area diffraction (SAED), microscopic images and high-resolution images of the specimens were observed under the JEM-200CX TEM. Results of the observation displayed five types of twin boundaries in the YBCO thin films. (a) 110 twins in the a-axis oriented thin film. (b) Twin boundaries parallel to the c-axis of the film. (c) Twin boundaries perpendicular to the c-axis of the film. (d) C-axis oriented twin boundaries of angles 30° , 45° , 60° etc. (e) Mosaic structure formed by twin boundaries perpendicular to each other. For an example, the mosaic structure formed by 90° twins consists of many a-axis oriented YBCO films 150-200nm in length, and 30nm in width, which was grown within the c-axis oriented YBCO film matrix. The boundaries between the 90° twins and c-axis oriented film form regularly coherent boundaries. This type of twin boundary will not effect the superconducting properties of the thin films. Another example showed that the boundary between 90° twin and the c-axis-oriented film is only 10° , consequently, the twin boundary is not fully coherent. As a result, 110 twins occur in the c-axis oriented thin film. This type of boundary will intercept the continuity of the Cu-O plane, which might decrease the T_C0 or critical current density of the YBCO thin film.

4PO6-13 BiSrCaCuO intergrowth nanostructures grown by molecular beam epitaxy

Michel Laguës, Jean-Baptiste Moussy, Jean-Yves Laval, Florin C. Beuran, Xiang-Zhen Xu, Catherine Deville Cavellin, Surfaces et Supraconducteurs, Laboratoire Spectroscopie en Lumière Polarisée CNRS-UPR5, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France

Presenting Author: M. Laguës

BiSrCaCuO thin films compounds were grown by molecular beam epitaxy (MBE) in order to induce intergrowth structures at nanometer scale. The 2D growth mechanism was controlled in real time by monitoring the RHEED intensity. The resulting high density of stacking faults is in contrast with a very low roughness, in the range of 1 nm as measured by atomic force microscopy, a strong c-axis texturation and a full epitaxy within the ab-plane as confirmed by four circle X-ray diffraction. The local structure of the films was examined by high resolution transmission electron microscopy (HRTEM) in correlation with the superconducting properties. Both structural and transport properties seem to be deeply affected by the percolation of 2212 domains as their concentration in the film is changed. STM spectroscopy at 4K on the surface of these films, shows continuous changes on very short scales (1 nm) from the superconducting gap to the pseudogap structure. This is supposed to be related to the intergrowth induced disorder.

4PO6-14 Growth-Induced In-Plane Anisotropy of Flux-Line Pinning in YBa₂Cu₃O_7-x Thin Films

Hanns-Ulrich Habermeier 1, Tristan Haage 1, Jörg Zegenhagen 1, R. Warthmann 2, and Ch Jooss 2. 1 MPI für Festkörperforschung Heisenbergstr. 1 D -70569 Stuttgart. 2 MPI für Metallforschung, Heisenbergstr. 1 D -70569 Stuttgart.

Presenting Author: H.-U. Habermeier

Recently, methods have been devloped to microscopically modify substrate surfaces in order to study the modification of film growth and consequently flux line pinning on tailored substrate structures. Progress in tailoring periodic step structures in vicinal SrTiO₃ single crystal substrate surfaces enables a systematic study of the interplay of growth mediated defect structures and transport properties in HTS thin films. UHV annealed (001)-oriented SrTiO₃ substrates with a miscut angle up to 10° towards the [010] direction were characterized by UHV-STM and subsequently Y₁Ba₂Cu₃O_7-x films were deposited and analyzed with the STM. A regular nanoscale step structure on the substrate generates an almost periodic surface structure of the film. Planar view as well as cross-sectional TEM analysis reveals an anisotropic defect structure arising from the substrate surface. Transport measurements and magnetooptic analysis of the flux penetration show an anisotropic enhancement of the critical current, j_c, along the step edges Quantitatively, j_c ( 5K ) measured along the step edge is determined to 8x10⁷ A/cm² , perpendicular to the steps, j_c is comparable to epitaxially grown c-axis oriented Y₁Ba₂Cu₃O_7-x films. In this paper we discuss the elementary steps necessary for the transition from Stranski-Krastanov-type film growth to step flow growth and correlate the growth mode with the planar defect arrangement and flux-line pinning.

4PO6-15 Influence of substrate irradiation on critical current density and microstructure in YBCO thin films

S. Leonhardt 1, R. Warthmann 2, J. Albrecht 2, H. Kuhn 2, H. Kronmueller 2, and H.-U. Habermeier 1. 1 Max-Planck-Institut fuer Festkoerperforschung, D-70569 Stuttgart, Germany. 2 Max-Planck-Institut fuer Metallforschung, D-70569 Stuttgart, Germany.

Presenting Author: S. Leonhardt

Strontiumtitanate substrates have been irradiated with a focused ion beam (FIB) of gallium ions, choosing different irradiation times and ion currents. This leads to a change of the surface morphology of the substrates. Afterwards YBCO was grown epitactically on these substrates by pulsed laser deposition.

The effect of the modification on the flux density distribution in the YBCO films has been pictured by the magnetooptical Faraday effect. Using a numerical inversion of the Biot-Savart law, the corresponding current density distribution has been calculated. We find a strong dependence of the critical currents on the irradiation parameters of the FIB. By applying small irradiation times we can achieve an enhancement of j_c of about 30 percent. The correlation between the change in j_c and the microstructure of the YBCO samples is investigated by transmission electron microscopy and a more ordered microstructure in samples with enhanced critical currents can be found.

4PO6-16 The non-epitaxial growth of c-axis YBa₂Cu₃O_x films on high-index planes of silver

Daxiang Huang, Yasuji Yamada, and Izumi Hirabayashi, Superconductivity Research Laboratory, ISTEC, 2-4-1 Nutsuno, Atsuta, Nagoya 456-8587, Japan

Presenting Author: D.X. Huang

YBa₂Cu₃O_x (YBCO) thin films were deposited by the pulsed-laser ablation on polycrystalline Ag substrates. Just because the substrate is polycrystalline, it provides us a chance to observe the YBCO film growth on many different kinds of crystal planes. The cross-sectional transmission electron microscopy investigation indicates that the YBCO film just grows with its a-b plane parallel to the substrate surface plane, no matter the surface plane is a low index plane or a high index plane. Under this main feature of the film/substrate orientation relationship, if the surface plane is a low-index plane, such as (110) plane of the Ag crystal, a three-dimensional (3D) film/substrate epitaxial relationship can be kept. When the surface plane is a high index plane, however, the a- and b-axis of YBCO grains are usually oriented randomly, and no epitaxial relationships can be found between the YBCO film and the substrate. As a result, on the high-index surface planes, the c-axis YBCO crystal grains with high-angle grain boundaries are the main microstructure feature in the grown YBCO films, which can produce the weak-links to be harmful for the transition of super-current of the YBCO films.

4PO6-17 Engineering stacking fault configurations for high critical current melt textured YBCO

J. Plain 1, T. Puig 1, F. Sandiumenge 1, X. Obradors 1, and J. Rabier 2. 1 Institut de Ciencia de Materials de Barcelona (CSIC), Campus de la UAB, 08193 Bellaterra, Spain. 2 Universite de Poitiers, Lab de Metallurgie Physique, Poitiers, France.

Presenting Author: F. Sandiumenge

YBCO (1:2:3) is metastable under a wide area of P(oxygen)-T space covering typical processing conditions. Under these conditions, extrinsic Cu-O stacking faults (SFs) are nucleated due to a local phase transformation of the 1:2:4 phase. Motivated by the high potential of dislocations as pinning centers, we explore different routes to develop SF configurations exhibiting a high perimeter to surface ratio in order to maximize the length of the 1/6<301> partial dislocation: Cold Isostatic Pressing (CIP) and High Oxygen Pressure Processing (HOPP). Two forces are required for the nucleation and growth of such SFs. (1) A thermodynamic one, F1, which measures the distance from the stability line of 1:2:3 at which the sample is processed, and (2) a mechanical one, F2, necessary to move the bounding partials on (001). In the CIP experiments F2 results from an isostatic pressure due to the anisotropy of the composite. Under 200 MPa in Ar at 300ºC it is found that F2 is dominant, the total SF area remains almost constant, but owing to the different SF energies on both twin domains, a selective re-organization of pre-existing SFs to a low energy configuration in one twin domain occurs resulting in an increase of the total partial dislocation length. A 100% increase of the critical current is achieved by this procedure. On the other hand, by HOPP (400ºC and 10 MPa oxygen) F1 dominates over F2. Accordingly the SF area is increased through the nucleation of new SFs at 2:1:1 interfaces, which owing to the strong non-equilibrium conditions, expand across twin walls and display irregular dendritic-like shapes. In this case critical currents are augmented by 150%. Pinning mechanisms are discussed on the basis of transmission electron microscopy observations and inductive magnetization (SQUID) measurements.

4PO6-18 YBa₂Cu₃O_7-d Films with Self-Organized Natural Linear Defects

J.M. Huijbregtse, B. Dam, F.C. Klaassen, J.H. Rector, and R. Griessen, Faculty of Sciences, Division of Physics and Astronomy, Vrije Universiteit, De Boelelaan 1081, NL-1081 HV Amsterdam, The Netherlands

Presenting Author: J.M. Huijbregtse

Recently [1], we have shown that dislocations act as strong pinning sites for vortices in YBa₂Cu₃O_7-d films. Using a wet-chemical etching technique in combination with Atomic Force Microscopy, we investigate the distribution of these natural linear defects in laser ablated films. We find that:

i) dislocations are induced in the early stages of film growth at the substrate-film interface and persist up to the film surface, resulting in a uniform length distribution;

ii) the in-plane defect distribution exhibits short-range ordering.

This distribution of pinning centers results in critical current densities of 10¹² A/m² that remain constant up to the matching field.

The mechanism of defect formation is closely related to the growth mechanism of YBa₂Cu₃O_7-d films. Consequently, by manipulating the growth conditions, we can reproducibly tune the number of linear defect between ~1 - 100 per m m². As both the length and radial defect distribution are independent of the defect density, we now have a very attractive system of self-organized natural strong pinning centers with a non-random distribution to study vortex matter. This self-organization makes films completely different from e.g. single crystals with artificial columnar defects.

[1] B. Dam, J.M. Huijbregtse, F.C. Klaassen et al., Nature 399 (1999) 439

4PO6-19 Study of Microstructures of Pb(Zr_xTi_1-x)/YBa₂Cu₃O_7-x Heterostructural Films

Z.H. Mai 1 B.T. Liou 1, J.H. Li 1, Z. Hao 1, B. Xu 1, B.R. Zhao 1, C. Giannini 2, S.A. Serinelli 2, and L. Tapfer 2. 1 Institute of Physics, Chinese Academy of Sciences, Beijing, 100080 China. 2 Centro Nazionale Ricerca e Sviluppo Materiali, Strada Statale 7 Appia km. 721, I-72100 Brindisi, Italy.

Presenting Author: Z.H. Mai

The structures of Pb(Zr_xTi_1-x)O₃/YBa₂Cu₃O₇ (PZT/YBCO) bilayer heterostructure with different thickness of PZT sublayer were investigated by X-ray high resolution diffraction, small angle reflectivity and reciprocal space mapping. The epitaxial films of YBCO and PZT are highly oriented with the STO substrate. The lattice parameter, c, of YBCO layer is obtained to be c~0.722 ± 0.001Å, being of 0.5% larger than that of the bulk one. The lattice parameter, c, of PZT layer is c= 4.108 ± 0.001Å, indicating that the composition of the samples is Pb(Zr_0.51Ti_0.49)O₃. The interface roughness of the YBCO/STO and YBCO/PZT, as well as the surface roughness are obtained. It is found that there exists a surface layer on the top of the PZT layer. The effect of the crystalline quality of the STO substrate on the quality of the YBCO and PZT epitaxial films was discussed.