Local magnetic relaxation in high-temperature superconductors

A novel Hall probe array technique is used to measure the spatial distribution and time dependence of the magnetic induction in YBa2Cu3O7-δ crystals. Analysis of the data based on the flux diffusion equation allows a direct, model-independent determination of the local activation energy U and the logarithmic time scale t0 for flux creep. The results indicate that the spatial variations of U are small (±kT) and that U increases logarithmically with time. The time t0 is inversely proportional to the field and it exhibits a nonmonotonic temperature dependence. These results confirm theoretical predictions based on the logarithmic solution of the flux diffusion equation.