Effects of ion irradiations on crystal structure and superconducting properties of MgB2 films

In this study, variations in the crystal structure and improvements in the superconducting properties of MgB₂ films were investigated. The almost pure crystal MgB₂ films, about 800 nm thick, were successfully created using the hybrid physical vapor deposition (HPCVD) method. The irradiations of Nb and Ni ions were carried out by using the accelerator. The irradiated conditions were set up to have an ion energy of 2 MeV and an ion dose of 5 × 1013 ions/cm2. Crystallinity of the pristine and ion-irradiated MgB₂ films was examined by using the X-ray diffraction (XRD) technique. The temperature-dependent magnetization results showed the degradation of the critical temperature (Tc) of the ion-irradiated MgB₂ films. Interestingly, the flux pinning properties of both Ni- and Nb-irradiated MgB₂ films were found to improve compared to that of the pristine one; those were revealed by the increases in value of irreversibility field (Hirr) and the enlargements of the area enclosed by the half of hysteresis loops of the ion-irradiated MgB₂ films. The value of critical current density (Jc) deduced from the hysteresis loop of the ion-irradiated MgB₂ films was clearly enhanced, especially at high-field regions. The drop in Tc and the increase in Jc might be due to the creation of disorder defects caused by the ion tracing that happens during ion irradiations.