Collective stacking faults and their effect on flux pinning in powder melting processed Y_0.6Ho_0.4Ba₂Cu₃O_y superconductors

We have fabricated the highly textured Y_0.6Ho_0.4Ba₂Cu₃O_y bulk superconductors by a powder melting process (PMP) method and investigated the microstructure and flux pinning characteristics. The microstructural observations show that there are many collective 'stair like' stacking faults in our sample, which is quite different from other melt-processed YBa₂Cu₃O_y. The formation of these stacking faults is discussed. It is found that the density of stacking faults is dramatically reduced when the sample is annealed at high temperature. After annealing, the critical current density and irreversibility line are significantly decreased in H ∥ c and H ∥ ab at various temperatures. These results clearly indicate that these stacking faults are very effective pinning centres in the PMP YBCO superconductors over a wide temperature and field range. The effect of stacking faults on the flux pining is sensitive to temperature and magnetic field. Also, the influences of stacking faults on the relaxation rate, the apparent pinning potential and the anisotropy of critical current are briefly studied.