The critical current density j c (B,T,0) of epitaxial, c-axis-oriented Yba2Cu30 x films was measured between 4.2 and 77 K in magnetic fields up to 8 T as a function of the field direction 0. Strongly enhanced critical currents are observed when the flux lines are aligned along the CuO planes, but there is also a maximum in j c when the magnetic field is adjusted parallel to the c axis. We relate these effects to an intrinsic pinning between CuO layers, an anisotropy of the shear modulus C66, and pinning effects of twin boundaries or stacking faults perpendicular to the film plane, respectively. PACS numbers: 74.60.Jg, 74.60.Ge, 74.75.+t Ceramic superconductors are well known to show a strong anisotropy in resistivity, critical current, and upper critical field B C 2. x A single-crystalline material is required to study the anisotropic behavior of the superconducting properties in detail. In this work the critical current density j c in epitaxial, c-axis-oriented films is investigated as a function of the magnetic field direction up to 8 T for various temperatures.The samples were prepared by laser deposition in an in situ process using a Siemens XP 2020 excimer laser (XeCl, 308 nm). 2 Epitaxial growth and c-axis orientation were achieved by heating the <100> SrTiC>3 substrates to about 800 °C and applying a 0.4-mbar oxygen atmosphere during the deposition. The 200-to 250-nmthick films were patterned to 10-/xm-wide and 180-jumlong strip lines using photoresist and wet-etching techniques. 3 The strip lines were randomly oriented with respect to the a-and &-axis directions in the YBa2Cu30 x films. Silver was evaporated as a contact material after sputtering the film surface for 40 sec with Ar ions. The contacts had to withstand currents of more than 1 A passing the strip line at 4.2 K and corresponding to y c 's up to 5x 10 7 A/cm 2 . j c , always measured perpendicular e^ A axis of rotation -CuO -layer -* flux line (b) Lorentz force _FL c-axis (a) flux t -CuO -layer Lorentz "* force _FL (c) twins, stacking faults FIG. 1. (a) Directions of current, magnetic field, and film normal and definition of the tilt angle 0. (b) Direction of the Lorentz force in the B-Lc case, (c) Direction of the Lorentz force in the Bile case.to the magnetic field direction for all angles 0, was determined by the four-probe technique with a voltage criterion of 0.5 ^V. The films were oriented with respect to the magnetic field direction from 0° to 180° as shown in Fig. 1(a). The current was reversed to simulate the range from 180° to 360°. The T c Gtf-0) of the film whose j c values are presented in this work was 89.1 K, the resistivity was 76 n O. cm at 100 K and 242 JJ, n cm at 300 K. Fig. 2. At T < 60 K a maximum of the critical current density is obtained when the magnetic field is aligned along the film plane. However, at 77 K and B < 2 T we observe higher j c values when B is perpendicular and parallel to the film plane. There is a crossover of the j c (B) curves perpendicular and parallel to the film plane. j c (B\\c) drops to zero b...
We report on YBa2Cu3O7−δ thin-film preparation by a new laser deposition geometry, the so-called off-axis laser deposition. Combined with radiation-based substrate heating, this results in c-axis oriented, epitaxial YBa2Cu3O7−δ thin films with critical current densities above 106 A/cm2 at 77 K and zero field and an exceptionally good surface quality, with complete absence of the otherwise observed laser droplets. As proved by atomic force microscopy, the surface roughness is <80 Å over an area of at least 10×10 μm2. Using this off-axis geometry it is possible to coat both sides of a substrate simultaneously, providing a one-step process in double-sided thin-film deposition. Both YBa2Cu3O7−δ films on the substrate show identical superconducting properties.
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