Study of the irreversibility line of GdBa2Cu3O7−δ added with nanosized ferrite CoFe2O4

Abstract

(CoFe2O4)xGdBa2Cu3O7−δ superconducting samples were synthesized using a conventional solid-state reaction technique with 0 ≤ x ≤ 0.1 wt%. Characterization of the prepared samples was carried out using X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The effect of adding CoFe2O4 to the Gd-123 phase, which acts as a flux pinning center, was investigated by measuring the ac magnetic susceptibility at different applied dc magnetic fields. Both the superconducting transition temperature Tc and the critical current density Jc increased up to x = 0.01 wt%, and then, they decreased with further increase in x. The temperature dependence of the irreversibility line was investigated by the logarithmic plot of the irreversibility field Hirr versus (1 − Tirr/Tc(0)). This plot shows a crossover at a field of 1000 Oe, indicating 3D to 2D flux creep fluctuations. Furthermore, the Hirr–T curves are well fitted according to a theoretical model pro- posed by Matsushita, based on the depinning mechanism caused by thermally activated flux creep. © Springer Science+Business Media New York 2015.