Anisotropic structural and magnetic properties of the field-aligned superconducting system SmFeAsO_1-xF_x (x = 0, 0.1, 0.2, 0.25 and 0.3)

Anisotropic structural and magnetic properties of the field-aligned superconducting system SmFeAsO_1-xF_x (x = 0, 0.1, 0.2, 0.25 and 0.3) are reported. Due to the Fe spin-orbital related anisotropic exchange coupling, all the tetragonal microcrystalline powders in epoxy were aligned at room temperature using the field-rotation method where the tetragonal ab-plane is parallel to the magnetic alignment field B_a of 0.9 T and the c-axis parallels to the rotating axis. Anisotropic magnetic properties are studied through low temperature magnetic measurements along the c-axis and paralleled to the ab-plane of aligned samples in both zero-field-cooled (ZFC) and field-cooled (FC) modes. The under-doped compound (x = 0.1) is not superconducting with an antiferromagnetic Néel temperature T_N ∼ 40 K, while the two optimum-doped compounds (x = 0.2 and 0.25) show high superconducting transition temperatures T_c of 49K and 50K, respectively. The variation of anisotropic structural and magnetic properties for this system are discussed and compared with the previously reported 52 K anisotropic superconductor Sm_0.95La_0.05FeAsO _0.85F_0.15.