Enhancing the magnetic moment of ferrimagnetic NiCo₂O₄ via ion irradiation driven oxygen vacancies

Ion irradiation has emerged as a powerful tool for the efficient control of uniaxial lattice expansion to fine tune and modulate the otherwise inaccessible complex correlated phases in oxide thin-films. We report the fine tuning of the magnetic moment, ferromagnetic-paramagnetic and metal-insulator transition temperatures in the NiCo₂O₄ inverse-spinel oxide by creating oxygen deficiencies, employing high energy He-ion irradiation. Tailoring of oxygen vacancies and consequently a uniaxial lattice expansion in the out-of-plane direction drives the system toward the increase of the magnetic moment by two-times in magnitude. The magnetic moment increases with the He-ion irradiation fluence up to 2.5 × 1016/cm2. Our results are corroborated well by spin-polarized electronic structure calculations with density functional theory and X-ray absorption spectroscopic data, which show peak-height change and energy shift of Co-L_2,3 and Ni-L_2,3 edges driven by the oxygen vacancies. These results demonstrate a new pathway of tailoring oxygen vacancies via He-ion irradiation, useful for designing new functionalities in other complex oxide thin-films.