Pinning-force scaling and magnetic relaxation in doped Y-Ba-Cu-O thin films

The flux-pinning-force density Fp and apparent pinning potential U0 in a series of 3d-transition-metal (Fe, Co, and Ni) -doped Y-Ba-Cu-O thin films have been measured as a function of magnetic field, temperature, and doping concentration. Fp shows an interesting double-peak behavior with an additional critical field H* separating the peaks. It is believed that this feature is associated with the flux-line configuration in the thin film in a transverse field. A universal scaling law can be applied which accounts for all the variations with field, temperature, and doping concentration. The magnetic-relaxation measurements of U0 show that the thermally activated flux creeps are enhanced in these doped films. The variation of U0 and VX as a function of field and temperature are consistent with the Fp characteristic which can be described by the flux-creep model in the critical state. (Here V and X are, respectively, the activation volume and the width of the pinning barrier.)