Magnetic properties of ultrafine Co particles

Fine particles of Co in the size range of 50-350 Å by the vapor deposition technique have been prepared. The dependence of magnetization on size and temperature has been studied. A maximum coercivity of 1500 Oe was obtained for a particle size of 350 Å. Magnetic and structural measurements indicate a shell-type structure with an FCC (face centered cubic) Co core surrounded by a Co-oxide shell. The effect of exchange interactions between the antiferromagnetic Co-oxide shell and the ferromagnetic Co core on the magnetic behavior of the powders has been investigated. The core/shell structure is responsible for the large coercivities present at cryogenic temperatures and the nonsaturation effects, which increase with decreasing particle size and with higher oxidation. The disappearance of the shift at 150 K marks the loss of exchange interaction between the Co-core and its CoO oxide coating. Thus the 150 K temperature could either be the blocking temperature or the Neel temperature of CoO crystallites present in the oxide shell