Nonlinear ac susceptibility studies of high-$T_c$ rings: Influence of   the structuring method and determination of the flux creep exponent
  We have studied the influence of the patterning procedure on the critical current density of high-$T_c$ YBa$_2$Cu$_3$O$_{7-\delta}$ thin rings using the nonlinear ac susceptibility method. At no applied dc magnetic field we have found that laser ablation degrades strongly the critical current density whereas ion beam etching has only a weak influence on it. From the measurements at different frequencies and dc magnetic fields we analyzed the influence of flux creep and obtained the field dependence of the flux creep exponent. Our data reconfirm the recently observed scaling relation for the nonlinear susceptibility response of type-II superconductors. 
S. Streubel, F. Mrowka, M. Wurlitzer, P. Esquinazi and K. Zimmer