The ex situ preparation of chemically synthesized cobalt nanoparticles leads unavoidably to particles coated with organic surfactants and a thin CoO shell. Argon ion $\left({\mathrm{Ar}}^{+}\right)$ etching is a method often used to remove the surfactant and the oxide shell. Ex situ prepared nanoparticles of $9.7\mathrm{nm}$ diameters were investigated as a function of the sputtering time, by means of soft x-ray absorption spectroscopy at the C and O K edges and at the Co ${\mathrm{L}}_{2,3}$ edges. Low-energy $\left(500\mathrm{eV}\right)$ ${\mathrm{Ar}}^{+}$ ion etching was effective in removing the organic coating layer of the nanoparticles, while much higher energy and longer sputtering times were required to affect the CoO shell. On the other hand, only a short time using low-energy hydrogen ion $\left({\mathrm{H}}^{+}\right)$ etching was necessary to completely reduce the oxide shell. Magnetic circular dichroism of soft x-rays was used to measure the magnetic properties of the etched samples. We found that the orbital to spin magnetic moment ratio, ${\mu }_l∕\mu _s{}^{\mathrm{eff}}$, increases as a function of the ${\mathrm{Ar}}^{+}$ etching time. Such an increase may be ascribed to the incomplete removal of the CoO shell as well as to surface strain, increased roughness, and an effective reduction of the particle size induced by the high-energy ion etching. A much smaller effect was observed for the low-energy $\left(500\mathrm{eV}\right)$ and short time ${\mathrm{H}}^{+}$ etched samples, where the magnetic moment ratio after the reducing procedure ${\mu }_l∕\mu _s{}^{\mathrm{eff}}=0.12$ is comparable to values obtained for bulk cobalt.

• Received 1 December 2004

DOI:https://doi.org/10.1103/PhysRevB.72.014448