Enhancement of intergranular current density of Sm-based oxypnictide superconductors with Sn addition

A series of Sn-added Sm1111 sintered bulks (SmFeAs(O, F) + xSn; x = 0–0.8) was synthesized, and the influence of Sn addition on the superconducting properties of SmFeAs(O, F) was studied. The cell volume (V) slightly increases with Sn addition in the lightly F-doped Sm1111 (SmFeAsO_0.88F_0.12), suggesting a slight reduction of F concentration in the lattice, whereas there is almost no change in V for the optimally F-doped sample (SmFeAsO_0.8F_0.2). The transition temperature (T_c) of SmFeAsO_0.88F_0.12 decreased from 50 to 40 K for x = 0.35, with another maximum at x = 0.27 (T_c = 50 K). On the other hand, SmFeAsO_0.8F_0.2 remained an almost constant T_c ∼ 56 K up to x = 0.8. Microstructural analysis depicts that the impurity phases in SmFeAsO_0.8F_0.2 were reduced by Sn addition, resulting in an increase in clean and well connected grain boundaries. Remanent magnetization measurements revealed that Sn addition improved the intergrain J_c at 5 K from 1 × 10² to 1.1 × 10⁴ A cm⁻² for SmFeAsO_0.8F_0.2 and from 4 × 10³ to 9.7 × 10³ A cm⁻² for SmFeAsO_0.88F_0.12. We believe this superior intergrain J_c to be attributable to the strong intergrain coupling due to grain connectivity improved by the Sn addition. (paper)