The flux pinning behaviour of (Nd1/3Sm1/3Gd 1/3)Ba2Cu3O7-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. Hirr(T) extrapolated to 77 K, H irr77 K(T), was determined from the Hirr(T) plots using best fits to an empirical formula, Hirr(T) ≤ H irr(0)(1- T/Tc)a, where a is a constant. H irr77 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, Hirr77 K(T) values are above 15 T, the maximum being 243 T for the 40 wt% sample.
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