MAGNETIC-RELAXATION AND FLUX CREEP IN CERAMIC (BI, PB)-2223 HTSC

The magnetic relaxation of ceramic (Bi, Pb)-2223 (T(c) = 107.2 K) has been studied in the temperature range 3.5-100 K and in fields up to mu-0H = 1 T. The relaxation of the magnetization at temperatures up to 80 K can be well described by a logarithmic time decay due to thermal activation of vortex lines. The activation energy U0 obtained from the relaxation rate S = 1/M (dM/dln t) in the single barrier height model exhibits a strong temperature. Both in applied fields and in the remanent state the activation energy increases with temperature (U0 = 20-50 meV at T = 4 K, U0 = 250-400 meV at T approximately-equals 60 K). The distribution of pinning barriers has been evaluated and exhibits a peak at U0 approximately-equals 20-30 meV in the remanent state and U0 approximately-equals 15-20 meV in an applied field of mu-0H = 1 T. These small values explain the rapid drop of the remanency at T greater-than-or-equal-to 25 K, the peak in the relaxation rate at T approximately-equals 10 K and the increase of H(cl) at T less-than-or-equal-to 25 K.