Effects of Ni irradiation on electron-phonon coupling and flux pinning in MgB₂ film via Raman spectroscopy

Effects of Ni-ion irradiation on the structural and flux-pinning properties of MgB₂ films were investigated with particular importance on the role of Raman spectroscopy. MgB₂ films (∼400 nm thick) were fabricated on c-cut Al₂O₃ substrates using a hybrid physicochemical vapor deposition technique and irradiated with 1 MeV Ni ions at doses ranging from 3 × 10¹³ to 9 × 10¹³ ions/cm². X-ray diffraction (XRD) and Raman spectroscopy were examined to characterize the crystallinity and electron-phonon coupling (EPC) in the films, while magnetization measurements provided insights into the superconducting properties. The XRD results confirmed that all films were highly c-axis oriented, and the c-axis lattice constant slightly increased with increased Ni-ion dose. Raman spectra revealed significant changes in the E_2g phonon mode, including shifts in peak positions and broadened FWHM (full-width at half-maximum), indicative of EPC degradation and the introduction of lattice disorder at an atomic scale. The decrease in the critical temperature (T_c) with increasing Ni-ion dose was correlated with these spectroscopic changes. The self-field critical current density (J_c) of the pristine MgB₂ film was well maintained to be 4 × 10⁶ A/cm² at 20 K. Above the field of ∼2.5 T, the J_c was observed to clearly enhance and reach a maximum at a Ni-ion dose of 5 × 10¹³ ions/cm², and J_c was maintained ∼10³ A/cm² at the field of ∼3.7 T. The observations might reveal the dominance of core point pinning, which resulted from the Ni-ion irradiation.