Irreversible field determined by pulsed magnetization and compositional fluctuation of melt-processed (Sm, Eu, Gd)Ba₂Cu₃O _7-δ superconductors

Irreversible fields B_irr of melt-processed (Sm, Eu, Gd)Ba ₂Cu₃O_7-δ bulk superconductors with varying (Sm, Eu, Gd)₂BaCuO₅ concentrations have been measured with pulsed magnetic fields up to 28 T in a temperature range from 55 to 85 K. The temperature dependence of B_irr was studied by taking account of the effect of heat generation. In comparison with that including 5 mol% (Sm, Eu, Gd)₂BaCuO₅ phase the samples with 40 mol% (Sm, Eu, Gd)₂BaCuO₅ inclusion displayed a lower B _irr value for temperatures above 75 K, while B_irr was enhanced below 75 K. TEM observation unveiled a sharp contrast between a uniform nano-scaled compositional modulation in compositions with 40% (Sm, Eu, Gd)₂BaCuO₅ inclusion and an inhomogeneous distribution with very low (Sm, Eu, Gd)₂BaCuO₅ concentration. The present result could thus be understood in the context of different defect states induced by locally compositional fluctuation.