Vortex motion noise in YBa2Cu3Ox films before and after heavy-ion irradiation

Abstract

We have studied vortex motion noise in epitaxial YBa2Cu3Ox films in magnetic fields before and after introducing columnar defects into the film by heavy-ion irradiation. Vortex motion exhibited a noise peak, which occurred at a field-dependent temperature. We found an increased noise peak magnitude after irradiation, and interpreted it as an increased bundle size, probably due to an increased length of vortex segments due to the interaction with pinned vortices in columnar defects. The magnetic field and the current dependence of the noise provided additional insights into the effects of columnar defects. After irradiation, the noise peak was almost independent of magnetic field, B, when it is less than about half of the matching field B-phi but dropped by a factor of 3 for B similar to 2B(phi). On the other hand, for B < 0.5B(phi), the bundle size was strongly current dependent at small current in the irradiated sample, but such a current dependence disappeared at higher currents or fields and was absent in the unirradiated sample. [S0163-1829(99)05829-4].