Subsurface Ni nanolayers from selective Mn surface oxidation and diffusion in NiMn alloys

We demonstrate by spatially resolved chemical and structural analysis in transmission electron microscopy that the selective oxidation of Ni-Mn alloys produces a distinct MnO/Ni-rich/NiMn trilayer structure, in which the Ni-rich layer contains locally pure Ni regions and a compositionally graded transition region toward the NiMn substrate. Off-axis electron holography directly visualizes magnetic induction in the subsurface Ni-rich layer, which exhibits ferromagnetic behavior and a Curie temperature close to that of bulk Ni. These findings reveal that selective oxidation can serve as a controlled, bottom-up strategy to synthesize nanoscale ferromagnetic layers for advanced device applications.