Preparation, characterization and thermal evolution of oxygen passivated nanocrystalline cobalt

Nanocrystalline cobalt powders have been prepared by the inert gas evaporat ion method. After preparation the materials were passivated by pure oxygen and air exposure. In the present paper we describe the application of diffe rent techniques like transmission electron microscopy (TEM), X-ray diffract ion( XRD), X-ray photoelectron spectroscopy XPS), X-ray absorption spectros copy XAS) and electron energy loss spectroscopy(EELS) to achieve an exhaust ive chemical and structural characterization of the nanocomposite material in its original state (passivated Co powder) and after thermal treatments i n vacuum. After passivation the cobalt fee nanocrystals (typical sizes 2-20 nm) are covered by an amorphous oxide passivation layer with a short range structural order similar to the cubic (fcc) CoO phase. Upon heating, cryst allisation of the oxide layer is observed together with the formation of th e Co3O4 spinel phase. In situ XPS experiments allow the determination of th e relative amounts of oxide and metal as a function of the oxygen dose. The present study is relevant due to the correlation between microstructure an d the unusual magnetic properties of this type of material.