Excess of Eu in the (Nd_0.33Eu_0.33+xGd_0.33-x)Ba₂Cu₃ O_y system: The way to a high irreversibility field at 77 K

With the aim of raising the irreversibility field, we have fabricated (Nd_0.33Eu_0.33+xGd_0.33-x)Ba₂Cu₃ O_y (NEG-123) bulk superconductors with x = 0.05, 0.07 and 0.1, with the addition of 0, 5 and 10 mol% of (Nd_0.33Eu_0.33Gd_0.33)₂BaCuO₅ (NEG-211) secondary phase particles. The magnetization loops of all the samples measured with a SQUID magnetometer were open, even at 7 T at 77 K for H//c. The highest irreversibility field of over 14 T, as measured with a vibrating sample magnetometer, was achieved in the sample with x = 0.05 containing 5 mol% NEG-211. Dark-field transmission electron microscopy (TEM) showed a significant structural modulation in this sample and a fine twin-like substructure in the NEG-123 matrix. High-resolution TEM observation further confirmed the presence of a fine twin-like substructure, and also the presence of anti-phase boundaries. TEM and energy dispersive x-ray analyses have clarified that the chemical fluctuations in the NEG-123 matrix grow with increasing secondary phase content. The specific material structure supported by the particular matrix chemical composition is evidently a key parameter for the observed increase of the irreversibility field.