Flux pinning sites in melt-processed (Nd0.33Eu0.33Gd0.33)Ba2Cu3Oy superconductors

(Nd0.33Eu0.33Gd0.33)Ba2Cu3Oy samples are prepared with extremely fine insul ating 211 particles, which are uniformly dispersed in the superconducting m atrix. Even though (Nd0.33Eu0.33Gd0.33)(2)BaCuO5 powders about 3 mu m in di ameter were added to the precursor, we have found that Gd2BaCuO5 (Gd-211) i nclusions about 0.1 mu m in diameter were finely distributed in the (Nd0.33 Eu0.33Gd0.33)Ba2Cu3Oy matrix. The formation of such fine 211 particles has not been achieved in any other bulk superconductors like YBa2Cu3O7-delta (Y BCO) 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 prono unced fishtail behaviour to a plateau-like behaviour, yielding a nearly con stant current density up to 2.5 T. The presence of small Gd-211 particles d oes not inhibit the function of flux pinning provided by composition fluctu ations leading to a spatial scatter of the superconducting transition tempe rature, T-c, as long as the 211 concentration is less than 40 mol%. Therefo re, two different pinning centers are simultaneously active in one sample, which is also confirmed by the pinning analysis based on the scaling formal ism, where we find peak positions even above 0.5 in the temperature range 6 0 less than or equal to T less than or equal to 93 K. The presence of the c omposition fluctuations is directly confirmed by means of field-cool coolin g and -warming experiments in fields up to 7 T. (C) 2000 Elsevier Science B .V. All rights reserved.