Flux pinning sites in melt-processed (Nd_0.33Eu_0.33Gd_0.33)Ba₂Cu ₃O_y superconductors

(Nd_0.33Eu_0.33Gd_0.33)Ba₂Cu ₃O samples are prepared with extremely fine insulating 211 particles, which are uniformly dispersed in the superconducting matrix. Even though (Nd_0.33Eu_0.33Gd_0.33)₂ BaCuO₅ powders about 3 μm in diameter were added to the precursor, we have found that Gd₂BaCuO₅ (Gd-211) inclusions about 0.1 μm in diameter were finely distributed in the (Nd_0.33Eu_0.33Gd_0.33)Ba₂C ₃O_y matrix. The formation of such fine 211 particles has not been achieved in any other bulk superconductors like YBa₂Cu₃O_7-δ (YBCO) or NdBCO. As a function of the concentration of the 211 particles, the shape of the J_c(H_a) curves at T =77 K can be changed from a very pronounced fishtail behaviour to a plateau-like behaviour, yielding a nearly constant current density up to 2.5 T. The presence of small Gd-211 particles does not inhibit the function of flux pinning provided by composition fluctuations leading to a spatial scatter of the superconducting transition temperature, T_c, as long as the 211 concentration is less than 40 mol%. Therefore, two different pinning centers are simultaneously active in one sample, which is also confirmed by the pinning analysis based on the scaling formalism, where we find peak positions even above 0.5 in the temperature range 60 ≥ T ≥ 93 K. The presence of the composition fluctuations is directly confirmed by means of field-cool cooling and -warming experiments in fields up to 7 T.