A. Katsaros

Growth and evolution of microstructure of epitaxial YBa2Cu3O7−x ultrathin and thin films on MgO

Abstract The nucleation and early stages of growth, and the evolution of surface microstructure of epitaxial c -axis YBa 2 Cu 3 O 7− x thin films are studied as a function of film thickness and growth temperature. The films were grown in situ on polished MgO (100) substrates by unbalanced DC magnetron sputtering at a growth rate of 3 nm/min. We use a combination of high-resolution scanning electron microscopy (SEM), scanning tunneling microscopy (STM), atomic force microscopy (AFM), X-ray diffraction, and transport-property measurements. The results from ultrathin films, 3 to 50 c ⊥ unit cells (1 c ⊥ unit cell equals 1.17 nm) indicate that epitaxial growth proceeds by the classical screw dislocation mediated three-dimensional island growth mode, i.e. each island grows both vertically and laterally by the incorporation of adatoms at a spirally expanding step emanating from one or more screw dislocations. The initial adatom clustering leads to a high density of nuclei (≈2×10 11 cm −2 ). The density of screw dislocations, ϱ screw , determined from STM images of the film surface, is found to depend on both the thickness of films and the growth temperature. At moderate growth temperatures (720–760°C) the early stage of coalescence of nuclei and island growth is characterised by a high density of screw dislocations, (3–9)×10 9 cm −2 , decreasing to (2–3)×10 9 cm −2 for 200 c ⊥ unit-cells films. Higher growth temperatures result in smooth films with a low density of screw dislocations, e.g.≤0.3×10 9 cm −2 at 850°C for 200 c ⊥ unit-cells films. A correlation is found between high values of ϱ screw and high values of the critical current density. The ultrathin films exhibit suppression of superconductivity and broadening of the resistive transition with decreasing film thickness, so that T c0 =55 K for a 3 c ⊥ unit-cells film.

Biaxially aligned buffer layers of cerium oxide, yttria stabilized zirconia, and their bilayers

Biaxially aligned cerium oxide (CeO2) and yttria stabilized zirconia (YSZ) films were deposited on Ni-based metal (Hastelloy C276) substrates held at room temperature using ion beam assisted (IBAD) magnetron deposition with the ion beam directed at 55° to the normal of the film plane. In addition, we achieved, room-temperature epitaxial growth of CeO2 by bias sputtering to form biaxially aligned CeO2/YSZ bilayers. The crystalline structure and in-plane orientation of films was investigated by x-ray diffraction techniques. Both the IBAD CeO2 and YSZ films, and the CeO2/YSZ bilayers have a (111) pole in the ion beam direction.

Epitaxial growth of cerium oxide thin film buffer layers deposited by d.c. magnetron sputtering

Abstract We studied the epitaxial growth of CeO2 thin films as a function of deposition temperature (300–850°C) and film thickness (100–900 nm). The films were grown on YSZ(100), MgO(100) and Al2O3( 1 1 02 ) (r-plane sapphire) substrates by reactive d.c. magnetron sputtering of a cerium metal target in an Ar/O2 plasma. The crystalline quality and biaxial alignment of the films were investigated using X-ray diffraction techniques (θ−2θ, ω-scans, pole figures, φ-scans) to determine the degree of c-axis (out-of-plane) and in-plane alignments given by the FWHM Δω and Δφ, respectively. The CeO2/YSZ(100) heteroepitaxy occurred below 300°C while deposition at higher temperatures resulted in single-crystal quality CeO2 films with Δω=0.1° and Δφ=0.2° at 650–750°C. The CeO2/MgO(100) and CeO2/Al2O3( 1 1 02 ) heteroepitaxy was evident at 600°C and developed to nearly perfect biaxial alignment at 850°C with Δω=1° and Δφ=5° for CeO2/MgO(100), and Δω=5° and Δφ=9° for CeO2/Al2O3( 1 1 02 ).

Temperature, field and frequency dependence of intergranular AC loss in high-temperature superconductors

Abstract AC susceptibility studies have been performed to determine the dependence on temperature (4.2.–120 K), AC magnetic field amplitude (0.01–10 G), and frequency (0.02–10 kHz) of the intergranular component χ″ J , of the imaginary part of the susceptibility of polycrystalline YBa 2 Cu 3 O 7 and Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O 10 specimens. The intergranular loss peak temperature, T p , shifts significantly to low temperatures with increasing field, and marginally to higher temperatures with increasing frequency. From the field and frequency dependence of T p we determine the intergranular critical current density, J cJ , and the intergranular pinning potential U 0 9 = 0, H ac = 0. From the onset of the hysteresis loss we determine the irreversibility line.

New techniques for fabricating step-edge junctions for high-T/sub c/ SQUIDs on MgO substrates

The authors describe two methods used to produce step-edge junctions on MgO substrates suitable for superconducting quantum interference devices (SQUIDs). In both processes a titanium mask has been used to produce a straight, well-defined step which assists in minimizing the occurrence of multiple junctions. They describe two ion-milling processes which produce differently connected step-edge junctions as indicated by scanning electron microscope (SEM) micrographs, and I-V and I/sub C/-B characteristics. Predictable relationships between the ratio of the film thickness to step height and the junction critical-current density are demonstrated; these enable the creation of step-edge junctions with appropriate values of I/sub C/ for particular SQUID designs.<<ETX>>

Josephson behaviour and flux penetration effects in YBCO double tilt-angle step-edge junctions

YBCO thin-film step-edge junctions have been prepared whose characteristics show features corresponding to the two tilt-angle grain-boundary junctions formed at the step. The I-V characteristics of these junctions are fitted to high accuracy by a model consisting of two noisy resistively-shunted Josephson junctions. In applied magnetic fields up to a few hundred microtesla, both junction critical currents show reversible flux modulation, in one case indicating excellent uniformity of the critical current distribution along the step. In stronger magnetic fields, the junction critical current modulation shows evidence of flux penetration into the film, i.e. the mixed state. Taking demagnetising effects into account the lower critical field of YBCO at 77 K, /spl mu//sub 0/H/sub e1,e/, is found to be 12 mT.<<ETX>>

YBCO coated tapes fabricated by IBAD and magnetron sputtering techniques

Abstract YBCO coated tape conductors (YBCO/YSZ/Has) were fabricated using magnetron sputtering to deposit YBa 2 Cu 3 O 7 thin films onto Hastelloy metal substrates that were pre-coated with a biaxially aligned YSZ buffer layer deposited by magnetron ion-beam-assisted deposition (IBAD). X-ray Φ-scans and pole figures showed the YSZ buffer layers to be highly aligned with a minimum ΔΦ = 10°. The best YBCO tapes had a high degree of biaxial alignment with a minimum ΔΦ = 7° and achieved J c (77K) up to 2.1×10 6 A cm −2 .

Environmental degradation of YBa2Cu3O7 − x thin films. Analysis by atomic force microscopy

Abstract YBa 2 Cu 3 O 7 − x thin films deposited on polished MgO substrates have been exposed to dry and wet steam at 65°C for periods up to 120 min. The resultant degradation of the film and the substrate has been studied by atomic force microscopy. It was found that wet steam was much more aggressive than dry steam, and that there was a synergistic relationship between the film and the substrate. The early stages of degradation of the film can be described adequately by the prevailing model based on observations on bulk specimens.

AC losses of YBCO strips on YSZ/hastelloy substrates

Abstract The effect of subdividing superconducting YBCO films on YSZ-buffered hastelloy substrates into arrays of parallel strips on the AC loss was investigated. Measurements of the loss component of the AC susceptibility, χ″, revealed that the frequency-independent hysteretic contribution to χ″, which is dominant at low frequencies ( f ≲ 1 kHz), decreased by a factor 1/ N when the film was subdivided into N parallel strips. As the strips were made wider or the separation between them made smaller, the hysteretic χ″ increased. The eddy current contribution to χ″, which originates from the metallic substrate, was found to become dominant with increasing frequency and approached a linear frequency dependence at high frequencies. The experimental findings are in close agreement with theoretical predictions of the AC loss in x -arrays of YBCO strips on metallic substrates.

Critical current and magnetic field performance of Bi-2223/Ag composite superconducting tapes

We report measurements of the electrical transport properties of ((BiPb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10/-Ag) Bi-2223/Ag composite conductors produced as part of the long-length product development in an industrial plant. The powder-in-tube (PIT) process was used to manufacture conductors containing various filament configurations including monofilament and multifilament tapes with untwisted and twisted filaments, and round wires. The transport critical current I/sub c/ was measured at various temperatures (T=4-80 K) and magnetic fields (B=0-9 T) for different orientations of the tapes with respect to the field. Self-field transport ac losses were determined at 77 K and 60 Hz as a function of ac current amplitude (0-100 A rms). The strain performance was evaluated at 77 K for applied bend strains from zero to 1.5%.