Synthesis, crystal structure, microstructure, transport and magnetic properties of SmFeAsO and SmFeAs(O0.93F0.07)

SmFeAsO and the isostructural superconducting SmFeAs(O0.93F0.07) phase were prepared and characterized by means of Rietveld refinement of x-ray powder diffraction data, scanning electron microscope observation, transmission electron microscope analysis, and resistivity and magnetization measurements. Sintering treatment strongly improves the grain connectivity, but, on the other hand, induces a competition between the thermodynamic stability of the oxy-pnictide and Sm2O3, thus affecting the sample purity. In the pristine sample both magnetization and resistivity measurements clearly indicate that two different sources of magnetism are present: the former related to Fe ordering at T similar to 140 K and the latter due to the Sm ions that order antiferromagnetically at low temperature. The feature at 140 K disappears in the F-substituted sample, and a superconducting transition appears, at low temperatures. The magnetoresistivity curves of the F-substituted sample probably indicate very high critical field values.