High critical-current density and ultra high-voltage TEM study of filamentary 0.1 at% Zr-doped (Nd_0.33Eu_0.38Gd _0.28)Ba₂Cu₃O_x superconductors

We prepared filamentary 0.1 at% Zr-doped (Nd_0.33Eu _0.38Gd_0.28)Ba₂Cu₃O_x superconductors by solution spinning and partial melting in flowing 0.1% O ₂ + Ar gas. The transport critical-current density (J_c) was measured at 77 K in applied magnetic fields up to 14 T by rotating the sample along a direction perpendicular to the filament length. Anisotropic behavior of the field dependence of J_c was detected by applying a field of more than 4 T. The J_c values for the Zr-doped sample were higher than those for the undoped sample on the application of fields of up to 11 T. The J_c values measured at the optimized angle for the doped sample were more than 10⁵ A/cm² at 77 K by applying fields of up to 6 T. Transmission electron microscopy (TEM) of the sample was performed using an ultra high-voltage TEM to clarify the pinning centers. Intergrowth of 124 phase and stacking faults in the oriented 123 matrix were observed both for the doped and undoped samples. Zr doping resulted in the fluctuation of the structure with a short disorder range of 10-30 nm. A modulated structure and fine twin planes crossing each other were partly observed for the doped sample. The presence of such small-scale disorder could improve flux pinning in the middle field region.