Physicochemical reduction of CoO to metallic Co by non-destructive, low-energy hydrogen-ion irradiation

Abstract

Low-energy hydrogen-ion irradiation provides a non-destructive, precise route to tailor materials by modifying bulk and interfacial structures, enabling the conversion of paramagnetic oxides to ferromagnetic metals with minimal damage. We apply this approach to CoO/Pd multilayers, achieving reduction to Co/Pd while elucidating the mechanism. Deuterium is employed to isolate hydrogen-specific effects. The saturation magnetization increases with acceleration energy, indicating a progressive CoO → Co transformation driven by oxygen-vacancy-mediated out-diffusion. Depth-resolved chemical profiling, compared with simulations of defect production, reveals an energy-dependent crossover: at lower energies, dissociation of OH species supplies oxygen that diffuses out; at higher energies, direct oxygen removal dominates. X-ray reflectivity shows that smoother, more uniform interfaces promote oxygen out-diffusion and thereby accelerate reduction. Together, these results establish sub-keV hydrogen-ion irradiation as a controllable, non-destructive tool for nanoscale physicochemical phase control and for coupled tuning of bulk and interface states. Beyond the CoO/Pd system, the ability to program magnetic properties within a single heterostructure by energy modulation highlights opportunities for spintronic thin films and device-relevant surface engineering.