We have used a microemulsion technique to synthesize metallic cobalt particles in the size range 18 to 44 Å diameter. The particles are spherical, not aggregated due to their surfactant coating, and free of oxide. Magnetic properties such as total moment per particle, blocking temperature, and hysteresis all show reasonable size dependencies. The effects of small size are seen in: (1) the anisotropy constant increased markedly as particle size decreased, and (2) the total magnetic moment per atom in the Co particles was enhanced with decreasing particle size by as much as 30% over the bulk value. Magnetization versus applied field curves indicate the particles are heterogeneous with two magnetic phases, possibly a core-shell structure. The core phase has a large total moment and the shell phase is superparamagnetic with an effective moment of 7.5±1${\mathrm{\mu }}_{\mathit{B}}$ for all sizes. We propose that the shell phase is responsible for the enhanced anisotropy and magnetization.

• Received 10 November 1994

DOI:https://doi.org/10.1103/PhysRevB.51.11527