  We report on the preparation of ferromagnetic cobalt nanospheres with
antiferromagnetic oxide capping layer and its implication for the variation in
magnetic property. The hcp cobalt nanospheres were prepared by thermal
decomposition of cobalt carbonyl in the presence of organic surfactants. The
spherical nanoparticles thus prepared were oxidized to grow antiferromagnetic
layers of varying composition and thickness on top of cobalt spheres. High
resolution transmission electron microscopy confirmed growth of Co3O4 in one
case and CoO in another case. Strong exchange anisotropy and enhanced coercive
field was observed due to the core-shell structure in Co-CoO system. On the
other hand only a marginal improvement was seen in Co-Co3O4 system. A low
temperature paramagnetic behavior was also observed that is interpreted in the
framework of crystal defects in the oxide shell.
