  We report the synthesis of cobalt nanoparticles with different shapes and
sizes by rapid pyrolysis of cobalt carbonyl in the presence of various
surfactants. The size and shape of the nanoparticles were influenced by
reaction conditions, such as type of the surfactant, molar ratio of surfactant
to precursor, reflux temperature and reaction time. The shapes that we have
achieved include spherical, nearly spherical, disc and cube. The presence of
linear amine yielded nanodiscs and they spontaneously self-assembled into long
ribbons. The effect of shape anisotropy on magnetic nanoparticles has been
investigated. Spherical nanoparticles of diameter 14.5 nm show strong
ferromagnetic behavior at low temperature and superparamagnetism at room
temperature. On the other hand the cubic nanoparticles of 45 nm sides showed
negligible coercive field at T = 10 K and ferromagnetism that persisted above T
= 300 K. The cobalt nanospheres were oxidized to grow cobalt oxide shell of
varying thickness to study exchange bias effect. A pronounced exchange bias and
a strong temperature dependant magnetization were observed in oxidized cobalt
nanospheres.
