Some Physical Properties of Suspendable Superconducting 123-Ag Composites
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SOME PHYSICAL PROPERTIES OF SUSPENDABLE SUPERCONDUCTING 123-Ag COMPOSITES C. Y. HUANG,* H. H. TAI,* Y. D. YAO,+ T. J. LI,+ AND M. K. WU# Lockheed Palo Alto Research Laboratory, Palo Alto, CA 94304 + Institute of Physics, Academia Sinica, Taipei, TAIWAN # Dept. of Physics, Tsinghua University, Hsinchu, TAIWAN *
ABSTRACT We have measured the M-H hysteresis loops of nYBa 2Cu 3Oy:Ag (n = 3, 5, and 7), and 3RBa 2Cu 3Oy:Ag (R = rare earth) as a function of temperature. We have found that the residual magnetization and, hence, pinning, is strong and is independent of n and R, but dependent on the metallurgy. The giant creep rates have been measured and are greater than those of pure 123 samples. Scanning electron microscopy and polarized light microscopy have shown that the presence of silver gives rise to the growth of large grains (_1 mm) in the 123-Ag samples. The field dependence of the critical current density is not strong at high field. We have also measured the resistance in field up to 200 kOe. The resistance remains zero even at 80 K and 200 kOe. The field dependence of the superconducting transition is discussed in terms of phase slippage. INTRODUCTION Recently, some of us have shown that the suspension of a YBa 2Cu 3Ox:Ag composite superconductor below a permanent magnet arises from strong pinning of the sample [1,21. From their observations, these authors showed that the suspension effect is a general phenomenon and should be observable in all superconductors with strong pinning. Very soon afterwards, this effect was observed in all superconductors, including the other 123-systems [3,4], the Bi-system [31, and the TI-system [3,5], as well as the conventional lowtemperature A-15 superconductors [6]. The pinning force Fp can be obtained from the magnetization (M)-field (H) hysteresis loop. In order to find the effect of the magnetism from the rare earth in RBa 2 Cu 30, (R = rare earth), we have measured the M-H hysteresis loops of these samples at various temperatures. In this paper, we report how the residual magnetization varies with the silver content in each sample. The mechanisms of pinning in conventional superconductors have been studied intensively [7-91. It is well established that defects and impurities are responsible for pinning. It is thus useful to investigate the microstructures of these composites. The results of our microscopic studies are presented. Furthermore, for the purpose of understanding the nature of the transition between the superconducting and normal states, we have measured the resistance near the transitions in high fields. EXPERIMENTAL RESULTS 1. Magnetization The samples, nRBa 2 Cu 3Ox:Ag (R = Y and rare earth), where n is the weight ratio of the 123 material to AgO before processing, were prepared according to the methods deMat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
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scribed in Ref. 10. The magnetization was measured with a PAR vibrating-sample magnetometer. We measured the magnetic moments of our samples cooled in magnetic field with a SQUID m
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