Upper critical field and thermally activated flux flow in LaFeAsO <inf>1-x</inf>F<inf>x</inf>

The magneto-resistance, critical current density, Jc, upper critical field, Hc2, and flux pinning properties of LaFeAsO 1-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 J c 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/k B B-n with n 0.13 for B 1 T and n - 0.68 for B 1 T for LaFeAsO0.85F0.15. © 2011 American Institute of Physics