Ratio of orbital-to-spin magnetic moment in Co core-shell nanoparticles

Arrays of monodisperse colloidal 11.4-nm Co nanoparticles were investigated by multifrequency ferromagnetic resonance and x-ray magnetic circular dichroism (XMCD). The ratio of orbital-to-spin magnetic moment μL/μSᵉᶠᶠ=0.24±0.06 determined by XMCD is 300% enhanced in comparison to the ratio obtained from a g-factor analysis which yields g=2.150±0.015 corresponding to μL/μSᵉᶠᶠ=0.075±0.008 (bulk fcc Co μL/μSᵉᶠᶠ=0.08).We show that the difference can be explained by the presence of uncompensated Co magnetic moments at the interface to a 2--2.5-nm CoO shell surrounding a metallic fcc-like Co core. This magnetic analysis is confirmed by high-resolution transmission electron microscopy.