Macroscopic Persistent Currents in Laser Ablated YBa 2 Cu 3 O 7-X Films
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MACROSCOPIC PERSISTENT CURRENTS IN LASER ABLATED YBa 2 Cu 3 O7 _x FILMS B. STRITZKER+, P. LEIDERER*, R. FEILE**, U. KRtIGER+ AND W.ZANDER+, SInstitut fUr Schicht-und Ionentechnik, KFA, D-5170 Jilich, Fakult~t fUr Physik, Universit&t Konstanz, FRG Institut fUr Physik, Universit~t Mainz, FRG
ABSTRACT Thin films of YBa2Cu3O7-x have been produced in situ by laser ablation. As the condensation of the superconducting film is done onto hot substrates (600 - 780 0 C) at high oxygen pressures (- 1 mbar), this techniques does not require any special postannealing step. Thus the total process time amounts to less than 10 min. Films were found to be polycrystalline with the c-axis preferentially oriented normal to the substrate plane on single crystalline (001) SrTiO3 and (random) Zr0 2 , as measured by ion channeling (Xmin = 5.5 % on SrTiO3), x-ray scattering and TEM. Complete superconducting transitions above 90 K with transition widths of about 1 K have been observed even on YSZ substrates. Critic~l cuirent densities, jc (77 K), of more than 2 x 10u A/cm could be obtained on SrTiO -substrates. The high quality of the thin films over Large areas was demonstrated by the measurement of persistent suerconjucting current densities at 77 K in the order of 10 A/cm in a 1.5 mm broad ring of 9 mm diameter. No dissipation of this persistent current could be observed during one hour. Measurements of the same film after 8 months in normal air showed a 40 % decrease of jc and a stronger decrease of jc with an externed magnetic field, suggesting that in the meantime the macroscopic ring current density is dominated by intergranular weak links. EXPERIMENTAL Thin film preparation by laser ablation The commonly used set-up for laser ablation [1] as described earlier [2] was modified in order to achieve homogenous films growth over a larger area. As can be seen from Fig.1 we took advantage of the fact that the laser causes ablation perpendicular to the surface of the target. Thus a cylindrical, rotating target is used. The curved target surface causes ablation into a wider angle. The rectangular laser beam is focussed by a cylindrical lens into a line-shaped area parallel to the axes of the cylinder. Thus the broadness of the beam assures homogenous ablation also in the other lateral dimension of the substrate. The substrates themselves are lying on a heating stage which is located in the middle of the homogenous ablation cloud. The oven can be heated up to 800 0 C. The surrounding oxygen pressure amounts to several mbar.
Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
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Laser-beam
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Fig.
1: Experimental set-up for laser ablation.
Fig.
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2: Sketch of the sample with the vibrating coil geometry.
Cylindrical pellets (• = 15 mm) made from stochiometric sintered YBa 2 Cu 3 O7 were used as targets. In some experiments we employed targets prepared by plasma spraying. The resulting films did not yield as good results as from sintered and more dense targets. In most cases the thin films were condensed ont
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