EXCHANGE-ANISOTROPY IN OXIDE PASSIVATED CO FINE PARTICLES

Oxide passivated Co particles have been prepared by vapor deposition in the size range of 50-350 angstrom. A strong exchange anisotropy was observed due to the core-shell structure of the Co particles, where the core consists of metallic Co and the shell of Co oxides. The exchange anisotropy of the particles was studied using shifted hysteresis loops after the sample was field cooled (FC). The shift in the FC loop exhibited a maximum at a particle size of 80 angstrom (shift=10.7 kOe) and is related to the amount of surface oxidation. The shift in the loops disappeared around 150 K in all the samples irrespective of their particle size, and this is attributed to the superparamagnetic behavior of the antiferromagnetic oxide shell.