Critical currents and relaxation in Bi-based ceramic superconductors

• Published in: Cryogenics (Guildford) 1992, Vol.32 (11), p.1056-1060
• Main Authors: Zhukov, A.A., Komarkov, D.A., Mirkovic, J.
• Format: Article
• Language: English
• Subjects: ceramics; Condensed matter: electronic structure, electrical, magnetic, and optical properties; critical currents; Exact sciences and technology; magnetization; Physics; Properties of type I and type II superconductors; Superconductivity
• ISSN: 0011-2275; 1879-2235
• DOI: 10.1016/0011-2275(92)90027-8

The temperature and magnetic field dependences of inter- ( j J c) and intragrain ( j G c) critical current densities obtained from magnetic measurements, as well as the relaxation of the intragrain magnetization, were studied on Bi-based ceramics. The best fitting for j J c is given by j J c ( B) = G( T)/(1 + B 2/ B 2 o) with the G( T) and B o( T) dependences being close to linear. The magnetic field behaviour of j G c fits the exponential law j G c (B) = j G c (T) exp(−B/B ∗ (T)) with nearly exponential j G c( T) and B ∗(T) . Values of j G c at intermediate temperatures (∼ 10 5 A cm −2) are an order of magnitude higher than in perfect single crystals, though at T- O they are close (∼ 10 6 A cm −2). At T ≈ 20 K j G c( T) shows a kink accompanied by a drop in the normalized relaxation rate (d(ln M/d(ln t) that was previously observed in Bi single crystals and was related to the transition from a low to a high temperature pinning mechanism. The hysteretic and time-dependent transport critical current j tr c was measured at T = 77 K. The relaxation rate of j tr c exhibits complex, hysteretic behaviour in magnetic fields with the virgin j tr c( H) curve showing no relaxation up to fields of 100 Oe.