Structural and critical current properties in polycrystalline SmFeAsO1-xFx

A series of polycrystalline SmFeAsO1-xFx bulks (x = 0.15, 0.2, 0.3 and 0.4) was prepared by a conventional solid state reaction. The resistivity, susceptibility, magnetic hysteresis, critical current density, and microstructure of these samples have been investigated. The resistivity result shows that the critical transition temperature T-c increases steadily with increasing fluorine content, with the highest onset Tc = 53 K at x = 0.4. On the other hand, a study on the effect of lattice constants on the critical transition temperature and resistivity at 60 K (rho(60) K) presents a possible correlation; that is, T-c increases with the shrinkage of the a-axis while rho(60) K increases with the enlargement of the c-axis. A global critical current density of 1.1 x 10(4) A cm(-2) at 5 K in self-field was achieved in the purest sample. An inter-grain J(c) of about 3.6 x 10(3) A cm(-2) at 5 K in self-field was estimated by comparing the difference in magnetization between powder and bulk samples, in contrast to the intra-grain J(c) of 10(6) A cm(-2). A weak field dependence was observed in the estimated inter-grain J(c), and the effect of composition gradients on the inter-grain J(c) is discussed.