Magnetic measurements in NdBa₂Cu₃O_7-δ single crystals: Magnetization, flux pinning and creep

In this paper, we present the results of various magnetic measurements on an NdBa₂Cu₃O_7-δ (NdBCO) single crystal. Special features observed in the magnetization loops (MHLs) are discussed. From these loops, critical current densities, j_c as a function of the applied field, H_a, and volume pinning forces, F_p =j_c × B, are derived. The samples exhibit a strong fishtail or peak effect at temperatures above 30 K. In a small temperature window between 50 K and 74 K, even three peaks are seen in the j_c(T,H_a) curves. A good scaling of the j_c (T,H_a) curves is obtained, which allows the identification of the peaks. The scaled volume pinning forces (vs the reduced field, h = H_a/H_iir), in the temperature regime between 60 and 90 K give experimental evidence for a strong pinning at extended superconducting defects (interaction volume V_pin ∼ξ²d). These defects are ascribed to spatial composition fluctuations found in light rare earth superconductors, providing a scatter of the transition temperature, T_c. Furthermore, the scaling analysis of F_p is extended towards low temperatures. It is shown that at 5 K another peak in F_p is found, which is due to pinning at twin boundaries. Using this scaling analysis of both j_c and F_p, the irreversibility line (IL) is determined in the entire temperature range 0-92 K. Magneto-optic (MO) imaging is employed to investigate the flux distributions of various NdBCO single crystals. These images confirm the important role of twin boundaries as pinning sites at low temperatures and low applied fields.