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

Abstract

The flux-pinning-force density {ital F}{sub {ital p}} and apparent pinning potential {ital U}{sub 0} in a series of 3{ital d}-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. {ital F}{sub {ital p}} shows an interesting double-peak behavior with an additional critical field {ital H}{sup *} 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 {ital U}{sub 0} show that the thermally activated flux creeps are enhanced in these doped films. The variation of {ital U}{sub 0} and {ital VX} as a function of field and temperature are consistent with the {ital F}{sub {ital p}} characteristic which can be described by the flux-creep model in the critical state. (Here {ital V} and {ital X} are, respectively, the activation volume and the width of the pinning barrier.)