Influence of carbon-ion irradiation on the superconducting critical properties of MgB₂ thin films

We investigate the influence of carbon-ion irradiation on the superconducting (SC) critical properties of MgB₂ thin films. MgB₂ films of two thicknesses, 400 nm (MB400nm) and 800 nm (MB800nm), were irradiated by 350 keV C ions having a wide range of fluence, 1 × 10¹³-1 × 10¹⁵ C atoms cm^-2. The mean projected range (R _p) of 350 keV C ions in MgB₂ is 560 nm, thus the energetic C ions will pass through the MB400nm, whereas the ions will remain into the MB800nm. The SC transition temperature (T _c), upper critical field (H _c2), c-axis lattice parameter, and corrected residual resistivity (ρ _corr) of both the films showed similar trends with the variation of fluence. However, a disparate behavior in the SC phase transition was observed in the MB800nm when the fluence was larger than 1 × 10¹⁴ C atoms cm^-2 because of the different T _cs between the irradiated and non-irradiated parts of the film. Interestingly, the SC critical properties, such as T _c, H _c2, and critical current density (J _c), of the irradiated MgB₂ films, as well as the lattice parameter, were almost restored to those in the pristine state after a thermal annealing procedure. These results demonstrate that the atomic lattice distortion induced by C-ion irradiation is the main reason for the change in the SC properties of MgB₂ films.