Relaxation of persistent current and the energy barrier Ueff(J) close to Tc in a grain-aligned YBa2Cu3O7-δ ring

Abrikosov flux-creep-induced relaxation of persistent current was investigated close to Tc in a ring-shaped grain-aligned MPMG-processed YBa2Cu3O7-δ (with the c axis perpendicular to the ring's plane). The measurements were performed for a wide range of current (0<J<Jc), using zero-field-cooling procedure over a temperature range of 80-90 K. A scanning Hall probe system was used to measure profiles of the magnetic field (Hc) generated by the persistent current circulating in a ring. The magnitude of the current and its decay were inferred from the magnitude and decay of the magnetic field at the ring's center. Relaxation measurements were performed over a time scale between 30 s and 3×104 s. The results revealed two distinct relaxation regimes: (1) a steady-state logarithmic relaxation of the persistent current from an initial value J0 close to Jc and (2) a slow nonsteady nonlogarithmic initial relaxation at low values of J0, which eventually converges to a long-time steady-state logarithmic relaxation. The logarithmic decay of the persistent current is consistent with the Anderson flux-creep model. The slow nonlogarithmic initial relaxation of the persistent current for low values of J0 is due to a transient redistribution of magnetic flux over the sample volume, consistent with a theoretical analysis of nonlinear flux diffusion [Gurevich and Küpfer, Phys. Rev. B 48, 6477 (1993)]. At fixed value of time, Ueff(J) calculated from the Beasley, Labusch, and Webb [Phys. Rev. 181, 682 (1969)] rate equation for thermally activated motion of flux, is current independent up to about 0.6-0.7Jc and at higher currents it drops linearly with increasing current. Ueff(J,T) for a steady-state logarithmic relaxation provides a high-temperature extension of Ueff(J) measured at low temperatures in grain-aligned YBa2Cu3O7-δ with Maley's method [Phys. Rev. B 48, 13 992 (1993)], suggesting a gradual conversion of the flux-creep process from a vortex-glass or collective flux creep at low temperatures to Anderson single-barrier vortex creep at high temperatures.