DEPENDENCE OF FLUX-CREEP ACTIVATION-ENERGY UPON CURRENT-DENSITY IN GRAIN-ALIGNED YBA2CU3O7-X

We have investigated magnetic relaxation on a grain-aligned sample of YBa2Cu3O7-x powder embedded in epoxy with the magnetic field along the c axis. Through a thermal-cycling procedure and through variations in the measuring temperature over the range 10 30 K, we manage to commence magnetic relaxation from a range of values of the magnetization, M, or current density J. Analysis of relaxation measurements at a magnetic field of 1.0 T over the temperature range 10 30 K reveals a nonlinear dependence of the effective activation energy Ue on M or J that is consistent with the logarithmic dependence obtained by Zeldov et al. from transport measurements. This demonstration of a nonlinear dependence of Ue(M) is capable of resolving anomalies in the magnitudes and temperature dependence of flux-creep activation energies obtained from magnetic-relaxation studies. © 1990 The American Physical Society.