ANALYSIS OF MAGNETIC CRITICAL FIELDS IN IRON-BASED SmFeAsO0.85 HIGH-Tc SUPERCONDUCTOR

The magnetic properties of the newly discovered iron-oxypnictide SmFeAsO0.85 high-T-c superconductor with a T-c of around 55 K were investigated. Bulk SmFeAsO0.85 was prepared by a method for high-pressure synthesis. The lower critical field H-c1 was estimated from the magnetization at low fields; H-c1(0) was measured to be 212 Oe. A linear temperature dependence instead of saturation at low temperatures in H-c1(T) revealed unconventional superconductivity with a nodal gap structure in our SmFeAsO0.85 superconductor. The results showed that the well-known secondary peak in the temperature dependence of the critical current density J(c) is absent in the SmFeAsO0.85 high-T-c superconductor. The irreversibility line B-irr was fitted well by the power law dependence (1 -T/T-c)(n) with n similar to 1.5. This is indicative of the flux creep phenomena in the SmFeAsO0.85 high-T-c superconductor. In addition, within the range of measurement temperatures in this study, no crossover was observed in the temperature dependence of the irreversibility line B-irr which may be due to low anisotropy in our SmFeAsO0.85 superconductor.