Study of Flux Pinning in Zone-melted YBCO by SQUID

Magnetic hysteresis loops arid magnetization relaxation of a zone-melted YBa₂Cu₃0_6+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 arid the gradual deviation as the temperature is increased demonstrated important role of nonlinear current density dependence of effective flux pinning potential U( J) in high temperature superconductors.