Flux creep in a thin disc of YBa2Cu3O7-delta superconductor

We investigated the flux-creep behaviour of a thin disc of YBa2Cu3O7-delta superconductor in the framework of an analytical model, in the temperature range T = 10-60 K and for a perpendicular applied field H-a = 5-50 kG. The relaxation of the magnetic moment shows a logarithmic dependence on time fo r the whole temperature range. The field and temperature dependences of the critical current j(c) and the pinning potential U-0 can be derived accurat ely by fitting the relaxation data. The results are discussed within collec tive pinning theory. The critical current j(c) shows a power law field depe ndence of the form j(c)(N) proportional to H-b with b similar or equal to 0 .50, which is kept during the relaxation process. The E versus j characteri stics are quantitatively derived from the relaxation data at the circumfere nce of the disc and show a very steep E(j) relation described by a power la w E(j) = E-c(j/j(c))(n) for the whole temperature range, while a change of this behaviour is observed for T = 60 K. The activation barrier U(j, T = 0, H) is derived with the application of Maley's method based on the exact re lations for a thin disc and this gives an exponent mu similar or equal to 0 .68 for H = 20 kG.