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 (SmFeAsO0.88F0.12), suggesting a slight reduction of F concentration in the lattice, whereas there is almost no change in V for the optimally F-doped sample (SmFeAsO0.8F0.2). The transition temperature (T-c) of SmFeAsO0.88F0.12 decreased from 50 to 40 K for x = 0.35, with another maximum at x = 0.27 (Tc = 50 K). On the other hand, SmFeAsO0.8F0.2 remained an almost constant T-c similar to 56 K up to x = 0.8. Microstructural analysis depicts that the impurity phases in SmFeAsO0.8F0.2 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 x 10(2) to 1.1 x 10(4) A cm(-2) for SmFeAsO0.8F0.2 and from 4x10(3) to 9.7 x 10(3) A cm(-2) for SmFeAsO0.88F0.12. We believe this superior intergrain Jc to be attributable to the strong intergrain coupling due to grain connectivity improved by the Sn addition.