Galvanomagnetic effect has been studied at 77 K on Bi-Sr-Ca-Cu-O films as a function of d.c. bias current. These films were prepared by spray pyrolysis and scrcenprinting techniques, with 7~ (R = 0) 80 K and 100 K respectively. Magnetic field dependence of Ic of sprayed-film showed very small hysteresis whereas screen-printed film showed greater hysteresis. Application of small magnetic field on these films destroyed the zero-resistance state and a finite resistance was developed. The slope of the resistance vs magnetic field curve AR/AH depended on the current flowing through the sample. To increase AR/AH, meander-shape pattern was prepared on the sprayed film. In general AR/AH increased by a factor of 103 after sample patterning. The possibility of using these films as a sensitive magnetic field sensor is discussed.
A simple method of making low-resistivity contacts ( approximately 10-7 Omega cm2) is reported for the first time on Bi-Sr-Ca-Cu-O films. The method basically involves the laying of silver-paste pads on the surface of as-deposited film, their annealing and the subsequent attachment of copper leads. The annealing step thus serves a dual purpose. It not only renders the film superconducting with requisite oxygenation but also helps in the preparation of low-resistivity contacts. The contacts on the film are quite rugged and could withstand several thermal cyclings between 4.2 and 300 K over a period of one month.
Superconducting films of YBa2Cu3O7-x with thickness ranging from 5 to 15 mu m have been grown on MgO(100) substrate by the spray pyrolysis technique followed by suitable annealing in flowing oxygen. A systematic study of the temperature dependence of Ic of the microbridges patterned on these films indicates that the grain boundary weak links behave as S/N/S junctions in microbridges having high Jc, while they behave as S/I/N/S or S/I/S junctions as the Jc of the microbridge is gradually decreased. Shapiro steps in I-V curves as well as SQUID behaviour in V- Omega curves are clearly observed in microbridges having low Jc but not in microbridges having high Jc.
Superconducting thick films of YBaCuO are made by spray pyrolysis technique on single crystal SrTiO 3 and MgO substrates. The thickness of the film is about 10 µm. The best film shows T c (R = 0) of the order of 86 K. No marked degradation in superconducting transition temperature is observed when the films are thermally cycled between room temperature and 4.2 K inside a closed liquid helium container. Temperature dependence of the critical current of microbridges fabricated on this film indicates the presence of SNS type weak links. dc SQUID configuration has been made in this film by scribing two bridges in parallel. V - Φ curves show periodic behavior superposed with additional structures.
Superconducting Bi-Sr-Ca-Cu-O (22121 films were prepared by spraying stoichiometric aqueous solutions of nitrates of bismuth, strontium, calcium and copper on heated MgO (100) substrates and subsequent annealing in air. The R-T curves of the films show metallic behaviour above the superconducting transition temperature. Tc (R = 0) is observed between 80 and 85 K. Annealing temperature has a profound effect on Tc (R = 0) and on the orientation of the film. Annealing in air in near-melting region yields highly oriented films with c-axis perpendicular to the substrate. These films show a sharp superconducting transition with zero resistance at 85 K. Microbridges of the dimensions of 50/~m × 50#m have been patterned photolithographically followed with chemical etching. The I V characteristics of the microbridges show Josephson efl,~cts due to the presence of grain boundary weaklinks. The temperature dependence of the critical current for these microbridges suggest formation of superconductor-normal-superconductor type weaklinks.
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