A comparative study of Sr_1−xK_xFe₂As₂ and SmFeAsO_1−xF_x superconducting tapes by magneto-optical imaging

Using the magneto-optical imaging (MOI) technique, the intergranular critical current density J_c at various temperatures and the homogeneity of the local structure of the superconducting cores for the powder-in-tube (PIT) Sr_1−xK_xFe₂As₂ and SmFeAsO_1−xF_x tapes are systemically investigated. These two tapes have large transport J_c over 10⁴ A cm⁻² in self-field at 20 K and 4.2 K, respectively, but the J_c of the SmFeAsO_1−xF_x tape decreases rapidly with increasing magnetic field. The MOI characterization indicates large bulk currents circulating through the whole sample for both Sr_1−xK_xFe₂As₂ and SmFeAsO_1−xF_x tapes, but also reveals the inhomogeneity inside the SmFeAsO_1−xF_x sample. The results obtained from the MO measurements can be confirmed by the magnetic hysteresis measurements M(H) and the SEM examination. The weak high-field performance of the SmFeAsO_1−xF_x tape may be ascribed to its short-time heat treatment. (paper)