Effects of proton irradiation on flux-pinning properties of underdoped Ba(Fe0.96Co0.04)2As2 pnictide superconductor

Abstract We study the effect of proton irradiation on Ba(Fe 0.96 Co 0.04 ) 2 As 2 superconducting single crystals from combined magnetisation and magnetoresistivity measurements. The study allows the extraction of the values of the apparent pinning energy U 0 of the samples prior to and after irradiation, as well as comparison of the values of U 0 obtained from the flux-flow reversible region with those from the flux-creep irreversible region. Irradiation reduces T c modestly, but significantly reduces U 0 in both regimes: the critical current density J c is modified, most strikingly by the disappearance of the second magnetisation peak after irradiation. Analysis of the functional form of the pinning force and of the temperature dependence of J c for zero field, indicates that proton irradiation in this case has not changed the pinning regime, but has introduced a high density of shallow point-like defects. By considering a model that takes into account the effect of disorder on the irreversibility line, the data suggests that irradiation produced a considerable reduction in the average effective disorder overall, consistent with the changes observed in U 0 and J c .