Refinement of secondary phase particles for high critical current densities in (Nd,Eu,Gd)-Ba-Cu-O superconductors

We have prepared (Nd_0.33Eu_0.33Gd_0.33)Ba₂Cu ₃O_y ('NEG') superconductors by means of melt-processing under a partial oxygen pressure of 0.1%. In these samples, we achieved a large critical current density, J_c, of 6.8 × 10⁴ A/cm² at T = 77 K and in an applied magnetic field, H_a = 2.5 T. Furthermore, the irreversibility field, H_irr, exceeds 7 T at 77 K for fields applied parallel to the c-axis of the sample. As additional pinning sites, we introduced up to 40 mol% (Nd,Eu,Gd)₂BaCuO₅ (NEG-211) particles plus Pt in analogy to YBa₂Cu₃O_y (YBCO) melt-processed superconductors. Microstructural observations clarified that the Pt addition was effective in reducing the size of NEG-211 second-phase particles, which led to a dramatic increase in J_c as compared to melt-processed YBa₂Cu₃O_y and NdBa₂Cu₃O_y (NdBCO). A pronounced secondary peak effect was observed in all magnetization curves obtained at 77 K, however, there is a considerable change in the shape of the magnetization loops as a function of NEG-211 content, which demonstrates the pinning provided by the NEG-211 particles in addition to the δT_c-pinning caused by fluctuation in the transition temperature, T_c, like in ordinary NdBa₂Cu₃O_y superconductors.