  We present a detailed atomistic picture of the oxidation mechanism of Co
nanoparticles and its impact on magnetism by experimentally following the
evolution of the structure, chemical composition, and magnetism of individual,
gas-phase grown Co nanoparticles during controlled oxidation. The early stage
oxidation occurs in a twostep process characterized by (i) the initial
formation of small CoO crystallites randomly distributed across the
nanoparticle surface, until their coalescence leads to structural completion of
the oxide shell and passivation of the metallic core; (ii) progressive
conversion of the CoO shell to Co3O4, accompanied by void formation due to the
nanoscale Kirkendall effect. The Co nanoparticles remain highly reactive toward
oxygen during phase (i), demonstrating the absence of a pressure gap whereby a
low reactivity at low pressures is postulated. Our results provide an important
benchmark for an improved understanding of the magnetism of oxidized cobalt
nanoparticles, with potential implications on their performance in catalytic
reactions.
