Optimization of matrix chemical ratio for high flux pinning in ternary (Nd-Eu-Gd)Ba₂Cu₃O_y

We prepared (Nd, Eu, Gd)Ba₂Cu₃O_y samples with various Nd: Eu: Gd ratios in the rare earth site. It was found that the three elements contributed to flux pinning in different ways. Nd mainly enhanced flux pinning at low magnetic fields, Eu controlled the second peak position and the irreversibility field, while Gd slightly enhanced intermediate and high-field J_c values. Scaling analyses for the pinning force density as a function of the reduced field h = H_a/H_irr (where H_irr denote the irreversibility field) showed that the highest peak was achieved at h = 0.56. This value is even higher than the theoretically predicted highest value of h = 0.5. We also show that a maximum flux pinning can be achieved in the whole magnetic field when very fine secondary phase particles are dispersed in a superconducting (Nd, Eu, Gd)Ba₂Cu₃O_y matrix with an optimum Nd: Eu: Gd ratio.