Observation of magnetic relaxation with vortex creep by the magneto-optical image method for high Tc superconducting films

Abstract

We observed the relaxation of magnetic flux in high Tc superconducting thin films(HTSC) by using magneto-optical image (MOI). Examined samples are thin film tapes made of $Gd_{0.6}Y_{0.5}Ba_{1.8}Cu_{2.7}O_{7-\delta}$ ((Gd,Y)BCO) which involves artificial $BaZrO_{3}$ -rod pinning centers. After the samples were cooled about 78K in various external magnetic fields, the external magnetic field was turned off. And then the distributions of remaining magnetic flux were changed. The induced current density is decreased with the logarithmic-time dependence associated with the creep motions of vortices. It was observed that the induced current density decreases when the external magnetic field is increased within the range of the small magnetic field. This phenomenon could be explained by taking into account the formation of meandering shapes of vortices which develop during the transition period from the flow mode to the creep mode. The results of the numerical simulation considering this effect are in good agreement with the experimental results.