Study Of Flux-Pinning In Zone-Melted Ybco By Squid

Magnetic hysteresis loops and magnetization relaxation of a zone-melted YBa2Cu3 O6+x sample were measured by superconducting quantum interference device (SQUID) at temperature from 4.5 to 70 K. The logarithmic behavior of magnetic decay at 4.5 K and the gradual deviation as the temperature is increased demonstrated the important role of nonlinear current density dependence of effective flux pinning potential U(J) in high temperature superconductors.