A reduced oxygen atmosphere during melt processing turned out to be critical for the fabrication of NdBa2Cu3Oy (Nd123) superconductors possessing high superconducting transition temperature (Tc) with a sharp transition and large critical current density (Jc) at 77 K. In a dc magnetization measurement, Nd123 superconductors melt processed in flowing a mixture gas of 1% O2 in Ar exhibited the Tc of about 95 K and the transition width of 1.5 K with the applied field of 10 Oe. A four-probe measurement showed the zero resistive transition Tc (R=0) of about 95 K. An anomalous peak effect in the magnetization hysteresis (M-H) loops was commonly observed and lead to large magnetic Jc of 2×104 A/cm2 at 77 K and 2 T for the applied field H parallel to the c axis of a sample (H∥c). This achievement is attributable to a preferential formation of high Tc phase (x<0.1) among the Nd1+xBa2−xCu3Oy solid solutions in a reduced oxygen atmosphere.

REBa₂Cu₃O _y superconductors with rare earth (RE) ions with large radii (RE: La, Nd, Sm) exhibit relatively low T _c due to the presence of RE-Ba solid solution. We have found that this solid solution can be suppressed if these superconductors are melt processed in a reduced oxygen atmosphere. We have also found that critical current densities of these superconductors are higher than those of melt processed YBa₂Cu₃O _y with fine Y₂BaCuO₅ inclusions in a high field region. The irreversibility line was also shifted toward the higher H-T region. We believe that flux pinning in these superconductors is ascribable to a finely distributed RE(Ba_{1- x} , RE _x )₂Cu₃O _y phase in a good superconductive matrix.

We report on a new type of correlated nanometer-scale pinning structure observed in a melt-processed $({\mathrm{N}\mathrm{d}}_{0.33}{\mathrm{E}\mathrm{u}}_{0.38}{\mathrm{G}\mathrm{d}}_{0.28}){\mathrm{B}\mathrm{a}}_2{\mathrm{C}\mathrm{u}}_3{\mathrm{O}}_y$ (NEG-123). It consists of $\mathrm{N}\mathrm{E}\mathrm{G}/\mathrm{B}\mathrm{a}$-rich clusters in the stoichiometric NEG-123 matrix forming a lamellar array with a period of a few nanometers. These lamellas appear within regular twins, thus representing their fine substructure—sometimes straight, sometimes wavy. This new material structure correlates well with the significant enhancement of pinning at high fields, represented by irreversibility field above 14 T at 77 K ($B\parallel c$). We believe that the new pinning medium enables one to significantly broaden the limits for high-field applications.

The effects of various oxygen partial pressures (P _O2) during melt processing on the superconducting properties of fabricated NdBa₂Cu₃O _y (Nd123) superconductors have been systematically investigated. Unlike in air or O₂, samples melt-processed in reduced oxygen atmospheres exhibited unprecedentedly high T _c's of 95–96 K with a sharp superconducting transition. Especially, for a sample melt-processed in 0.1% O₂ in Ar, the magnetization hysteresis ( M-H ) loop was kept open up to 7 T at 77 K and for the field parallel to the c-axis of the sample (H//c), implying that very effective flux pinning can be achieved in high magnetic fields.