NATURE OF MAGNETIC-RELAXATION IN A SUPERCONDUCTING PR1.85CE0.15CUO4-YSINGLE-CRYSTAL

Magnetic relaxation in a Pr1.85Ce0.15CuO4-y single crystal has been st udied as a function of field and temperature. Two different regimes ar e evident in the relaxation process. The usual logarithmic decay of th e magnetization is observed at low fields. A crossover from logarithmi c to exponential relaxation takes place at higher fields. The magnetiz ation decay becomes exponential as H approaches H(irr)(T), and eventua lly any trace of relaxation disappears. The logarithmic decays have be en used to estimate the field dependences of the critical currents and the relevant activation energies at high driving currents. The functi onal dependence U(J) has been evaluated by using Maley's method. The e xtracted values for U (J, T, B) are discussed in terms of the availabl e models for flux motion; we conclude that the observed behavior might be a signature of a crossover in the nature of the thermally activate d process, from jumps of elastically correlated flux volumes to plasti c flux motion.