Effects of solid solutions on the superconducting properties of Gd-Ba-Cu-O superconductors

The superconducting properties of the nominal Gd1+2xBa2+xCu3+xO7-δ (x = 0, 0.1, 0.2, 0.3, 0.4) single-domain bulks which consist of GdBa2Cu3O7-δ (Gd123), Gd2BaCuO5 (Gd211) and Gd1+yBa2-yCu3O7-δ, have been investigated through magnetization and resistivity measurements. With increasing Gd211 content x, the trapped magnetic field reaches a maximum for the sample x = 0.2. The second peak in the critical current density Jc versus magnetic field B curve shifts toward the high magnetic regime for the sample x = 0.2 compared with that for the sample x = 0, but decreases to low magnetic field for the samples x>0.2. Two superconducting transitions in resistivity ρab indicate that the low Tc clusters, i.e. Gd1+yBa2-yCu3O7-δ solid solutions, are interspersed in the GdBa2Cu3O7-δ superconducting matrix. The δTc pinning resulting from the compositional fluctuations should be responsible for the enhancement of Jc and the trapped field for the sample x = 0.2