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

Abstract

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 Tc and the upper critical field Hc2, and properties more of interest for applications, like the critical current density Jc and the related pinning landscape. The irradiation-induced suppression of the film Tc is significantly smaller than on a similarly damaged single crystal. Moreover Hc2 behaves differently, depending on the field orientation: for H//c the Hc2 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 Hc2, the effects on the Jc(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, Jc(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 Jc(H//ab) performance is not related to a decrease of the Jc 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.