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

Temperature dependences of the persistent critical current density J(c)(T) and its relaxation rate S=d ln J/d ln t were measured from 8.0 K to T-c in MgB2 ring-shaped thin films. J(c)(T) was observed to approach a Ginzburg-Landau-like temperature dependence with a decreasing T-c 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 10.5 K in MgB2 thin films of T-c above 30 K, but not in films of lower transition temperatures. Vacuum annealing shifts the instability to lower temperatures. (C) 2007 American Institute of Physics.