Effect of $\alpha$-particle irradiation on a NdFeAs(O,F) thin film

Abstract

The effect of $\alpha$-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 T to 6 T after an irradiation of $2\times10^{15} cm^{-2}$. On the contrary, $J_c$(H//ab) is always suppressed. The analysis demonstrates that irradiation does introduce point defects 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 point defects that decrease the efficiency of the ab-plane intrinsic pinning typical of materials with a layered structure.