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

Abstract

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