Flux pinning in melt processed RE-Ba-Cu-O

RE-Ba-Cu-O (RE: rare earth element selected from Nd, Sm, Eu, Gd) superconductors exhibit high T_c and J_c values when they are melt processed in a reduced oxygen atmosphere (oxygen-controlled-melt-growth: OCMG process). Large J_c values in these systems can be attributed to finely dispersed RE rich 123 clusters, which can act as field-induced pinning centers, and thus leading to the second peak effect in M-H loops. In this paper, we first propose the mechanism how RE rich clusters are formed during melt processing. Second we show that flux pinning depends on oxygen annealing conditions, because the optimum oxygenation conditions are different depending on the range of Nd-Ba substition. We also report that the addition of Nd422 is effective in raising Jc, although the mechanism is rather complicated owing to the fact that Nd422 also has a RE-Ba solid solution toward Ba rich direction.