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

The effect of a-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 Tc 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 x 10(15) 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 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.