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 ${\mu }_L/{\mu }_S^{\mathrm{eff}}=0.24\mathrm{}\pm 0.06$ determined by XMCD is 300% enhanced in comparison to the ratio obtained from a $g$-factor analysis which yields $g=2.150\mathrm{}\pm 0.015$ corresponding to ${\mu }_L/{\mu }_S^{\mathrm{eff}}=0.075\mathrm{}\pm 0.008$ (bulk fcc Co ${\mu }_L/{\mu }_S^{\mathrm{eff}}=0.08).\mathrm{}$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.

• Received 14 April 2003

DOI:https://doi.org/10.1103/PhysRevB.68.064424