ac losses in circular disks of thin YBa2Cu3O7 films in perpendicular magnetic fields

Abstract

The ac losses at 20 and 30 Hz were measured for two disk-shaped YBa2Cu3O7 films in perpendicular peak applied magnetic fields up to ∼0.2 T in liquid nitrogen. One of the films had a significantly higher critical-current density than the other as determined from the loss measurements. Also, it exhibited a more-uniform flux penetration around the circumference of the disk than the other as observed by magneto-optical images of these films in perpendicular dc fields. The results from this film were compared with theoretical predictions of ac losses for disks of thin superconducting films in perpendicular magnetic fields using the Bean model [J. R. Clem and A. Sanchez, Phys. Rev. B 55, 9355 (1994)], and the Kim critical-current model [D. V. Shantsev et al., Phys. Rev. B 61, 9699 (2000)]. The asymptotic Bean model predictions for low and high fields were in reasonably good agreement with the data. The numerical calculation of the losses following Shantsev et al. was found to give extremely good agreement with the loss data throughout the entire field range of the measurement when the Kim model for the critical-current density was used.