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 E2g 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 (Tc) with increasing Ni-ion dose was correlated with these spectroscopic changes. The self-field critical current density (Jc) 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 Jc was observed to clearly enhance and reach a maximum at a Ni-ion dose of 5 × 10¹³ ions/cm2, and Jc 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.