MAGNETIZATION RELAXATION IN YBA2CU3O7-DELTA

The time and field sweep rate dependencies of magnetization in high temperature superconductors are derived from a combined view based on the thermally activated flux creep. The tau(0) in the magnetization relaxation expression M=M(0)+M(1) ln(1+t/tau(0)) is a macroscopic quantity related primarily to the sample geometry and other experimental conditions via the effective activation energy U(J(0)). This activation energy provides an alternative interpretation to the nearly inverse proportionality of tau(0) to the field sweep rate. The values of tau(0), in the magnetization relaxation of single crystal YBa2Cu3O7-delta are consistent with the calculated ones. As the nonlinear U(J) is taken into account, the curvature in the \M\-ln t plot may change from negative to positive over a sufficiently long time window, and the magnetization hysteresis M(H) depends on the field sweep rate H by const+A ln H+B ln(2) H, where A is slightly larger than M(1) and B>0. A discussion of the temperature dependency of the normalized flux creep rate is at the conclusion of the article.