Enhanced critical current density in the carbon-ion irradiated MgB₂ thin films

We report large improvement of the critical current density (J_c) of 400-nm-thick MgB₂ thin films through carbon (C) ion irradiation. When 350 keV C ions with various dose levels ranging from 1 × 10¹³–1 × 10¹⁵ C atoms/cm² are irradiated into the films, the c-axis lattice constant gradually expands with increase in the number of irradiated C ions and superconducting transition temperature (T_c) of the MgB₂ films is suppressed. On the other hand, the field performance of J_c for the MgB₂ films remarkably improves after irradiations, which is accompanied by the crossover of the pinning mechanism from weak collective to strong pinning. In addition, scaling behavior of flux pinning force for irradiated MgB₂ films shows a dominance of normal point pinning, indicating that the improved field performance of J_c mainly originates from the point defects, such as vacancies and interstitials, created by C-ion irradiation. Our results suggest that carbon acts as an effective irradiating source as well as a doping source to improve J_c in high magnetic fields. (author)