Synthesis and magnetic behavior of self-assembled Co nanorods and nanoballs

e-cobalt nanoparticles with different shapes and sizes, e.g., rod-shaped Co (2×25 nm) and ball-shaped Co (diameter 4–9 nm), were synthesized by the rapid pyrolysis of octacarbonyldicobalt in o-dichlorobenzene solution in the presence of the surfactant, oleic acid. The size, size distribution, and the shape of the nanoparticles were influenced by changing the preparation conditions, such as the molar ratio of the surfactant (oleic acid) to reactant (cobalt carbonyl). The effects of the preparation conditions and annealing temperature on the shape and magnetic properties of cobalt nanoparticles have been investigated. It was found that as-synthesized rod-shaped cobalt nanoparticles (2×25 nm) show paramagnetic properties, but as-synthesized ball-shaped cobalt nanoparticles exhibit superparamagnetic behavior. After postannealing at 773 K for 30 min, the rod-shaped Co nanoparticles formed hcp structure and show ferromagnetic properties.