Flux creep in a thin disc of YBa2Cu3O7-deltasuperconductor

We investigated the flux-creep behaviour of a thin disc of YBa 2 Cu 3 O 7- 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 for the whole temperature range. The field and temperature dependences of the critical current j c and the pinning potential U 0 can be derived accurately by fitting the relaxation data. The results are discussed within collective pinning theory. The critical current j c shows a power law field dependence of the form j c ( H ) H - b with b 0.50, which is kept during the relaxation process. The E versus j characteristics are quantitatively derived from the relaxation data at the circumference of the disc and show a very steep E ( j ) relation described by a power law 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 relations for a thin disc and this gives an exponent µ 0.68 for H = 20 kG.