MAGNETIC-RELAXATION CLOSE TO TC IN YBA2CU3O7 CRYSTAL

Using a novel local Hall probe technique, we extend earlier magnetic relaxation studies of YBa2Cu3O7 crystals to the temperature range approaching within a degree or two of T(c), up to 91K for H parallel-to c and 90K for H parallel-to ab. Long time relaxations (1-10000 sec) are close to logarithmic but exhibit deviation from logarithmic behavior which can be well fit to the vortex glass or collective pinning theory. The plateau in normalized relaxation rate partial derivative lnM/partial derivative lnt appears to extend to the highest temperatures we measure. Effective attempt times deduced from the data are surprisingly large, with lower limits in the msec range.