GEOMETRIC ASPECT OF INTERGRAIN FLUX-PENETRATION AND PINNING IN YBA2CU3O7 CERAMICS

We report on studies of the magnetic properties of ceramic YBa2Cu3O7-delta modified by the sample geometry (shape). The measurements of the diamagnetic shielding, the Meissner effect, the trapped field, and the relaxation phenomena were performed at 77 K and over a range of applied magnetic fields 0-1200 G for the sintered sample (whose shape was changed from a disk to a ring and finally to a broken ring) and for the pressed-powder disk-shaped sample. These studies allowed us to separate the intergranular magnetic properties of the ceramic sample from the intragranular ones. The studies revealed the effect of the macroscopic shielding currents, weak links, grain-decoupling process, and intergrain vortex-field gradients penetrating the sample on the shielding properties of the ceramic sample at low applied magnetic fields. The relaxation rate of the trapped intergrain flux depends on the sample geometry. An effective activation energy for the intergranular flux creep equal to 1.25 +/- 0.3 eV at 77 K was found in the disk-shaped sintered sample. An effective activation energy for the intragranular flux creep equal to 0.05 +/- 0.01 eV (the zero-field-cooled case) and 0.09 +/- 0.02 eV (the field-cooled case) at 77 K was found in the pressed-powder disk-shaped sample of the same dimensions. We applied the critical-state model of Daumling and Larbalestier [Phys. Rev. B 40, 9350 (1989)], who calculated diamagnetic shielding in disk-shaped conventional superconductors, to explain the dependence of the intergrain component of the diamagnetic shielding in the ceramic disk and rings on an applied magnetic field. Good agreement between this model and the experimental data was obtained for applied magnetic fields above 100 G corresponding to the full critical state in the 2-mm-thick ceramic disk and ring.