The effect of Gd₂BaCuO₅ nanoparticles on irreversibility fields of (Nd-Sm-Gd)Ba₂Cu₃O _7-δ

The flux pinning behaviour of (Nd_1/3Sm_1/3Gd _1/3)Ba₂Cu₃O_7-δ samples, having different concentrations x of Gd₂BaCuO₅ secondary-phase nanoparticles, was studied using magnetoresistivity data collected in the presence of 0, 0.5, 1, 2, 4, 6, 8 T magnetic fields (H) along and perpendicular to the c axis, with the transport current lying in the ab plane. Significant enhancement of the flux pinning and hence also the irreversibility field H _irr(T) was found. H_irr(T) extrapolated to 77 K, H _irr^{77 K}(T), was determined from the H_irr(T) plots using best fits to an empirical formula, H_irr(T) ≤ H _irr(0)(1- T/T_c)^a, where a is a constant. H _irr^{77 K}(T) for H|| c axis (a) remains almost constant up to x ≤ 0.2, (b) increases beyond x ≤ 0.2, (c) peaks at x ≤ 0.4 and (d) falls beyond x ≤ 0.4. Further, H_irr^{77 K}(T) values are above 15 T, the maximum being 243 T for the 40 wt% sample.