Upper critical field and thermally activated flux flow in LaFeAsO1−xFx

Abstract

The magneto-resistance, critical current density, Jc, upper critical field, Hc2, and flux pinning properties of LaFeAsO1−xFx superconductors were investigated systematically by magnetic and magneto-transport measurements in the fields up to 13 T over a temperature range of 5–35 K. It was found that the Hc2 increased with increasing fluorine concentration up to x ≤ 0.15, while with higher fluorine doping, Hc2 decreased. A peak effect in the Jc as a function of field was observed at T < 15 K for both the 5% and 15% fluorine doped samples. The broadening of the superconducting transition in magnetic field can be well understood by the thermally activated flux flow model. The pinning potential, Uo, scales as Uo/kB ∝ B−n with n = 0.13 for B < 1 T and n =− 0.68 for B > 1 T for LaFeAsO0.85F0.15.