Temperature dependence of the persistent critical current and instabilities in MgB2 thin films

Abstract

Temperature dependences of the persistent critical current density Jc(T) and its relaxation rate S=dlnJ∕dlnt were measured from 8.0K to Tc in MgB2 ring-shaped thin films. Jc(T) was observed to approach a Ginzburg-Landau-like temperature dependence with a decreasing Tc in magnesium deficient films. No correlation was found between the current density and the magnitude of the relaxation rate, suggesting percolative (filamentary) flow of the supercurrent. A suppression of the supercurrent density (instability) was observed between 8.0 and 10.5K in MgB2 thin films of Tc above 30K, but not in films of lower transition temperatures. Vacuum annealing shifts the instability to lower temperatures.