Effect of α-particle irradiation on a NdFeAs(O,F) thin film

The effect of α-particle irradiation on a NdFeAs(O,F) thin film has been investigated to determine how the introduction of defects affects basic superconducting properties, including the critical temperature T _c and the upper critical field H _c2, and properties more of interest for applications, like the critical current density J _c and the related pinning landscape. The irradiation-induced suppression of the film T _c is significantly smaller than on a similarly damaged single crystal. Moreover H _c2 behaves differently, depending on the field orientation: for H//c the H _c2 slope monotonically increases with increasing disorder, whereas for H//ab it remains constant at low dose and it increases only when the sample is highly disordered. This suggests that a much higher damage level is necessary to drive the NdFeAs(O,F) thin film into the dirty limit. Despite the increase in the low temperature H _c2, the effects on the J _c(H//c) performances are moderate in the measured temperature and field ranges, with a shifting of the pinning force maximum from 4.5 to 6 T after an irradiation of 2 10¹⁵ cm^-2. On the contrary, J _c(H//ab) is always suppressed. The analysis demonstrates that irradiation does introduce point defects (PD) acting as pinning centres proportionally to the irradiation fluence but also suppresses the effectiveness of c-axis correlated pinning present in the pristine sample. We estimate that significant performance improvements may be possible at high field or at temperatures below 10 K. The suppression of the J _c(H//ab) performance is not related to a decrease of the J _c anisotropy as found in other superconductors. Instead it is due to the presence of PD that decrease the efficiency of the ab-plane intrinsic pinning typical of materials with a layered structure.