Irreversibility line and flux pinning properties in iron-based high-Tc superconductor SmFeAsO0.85

The irreversibility line and flux pinning properties of high-Tc superconductor SmFeAsO0.85 were studied using DC magnetization data. Polycrystalline SmFeAsO0.85 was prepared in a high pressure synthesis apparatus under the pressure of 6 GPa. The results of DC susceptibility showed the superconducting transition at about 55 K. A critical current density Jc(B) was calculated using Bean's critical state model. At low temperatures (≤20 K), Jc(B) showed a relatively high value with weak dependence on an applied magnetic field. At higher temperatures, a stronger dependence of the magnetic field was observed, which resulted from decrease in a critical current density probably due to the flux creep effect. The irreversibility line (IL) agreed well with the flux creep theory of Matsushita et al. A comparison of normalized pinning force density with the theoretical models showed that the irreversible behavior in SmFeAsO0.85 is dominated mainly by normal point pinning (δTc) and surface pinning mechanisms.