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The nested carbon nanotubes discovered by Ijima and co-workers as a by-product of fullerene production extended the dimension and geometries of fullerenes into the domain of nanoparticles. These nanotubes are fiberlike structures consisting of concentric graphite sheets nested along the axis extending as long as several micrometers, and diameters of these nanotubes are limited to less than 100 nm. In this paper we report preparation, structural and magnetic properties of cobalt-doped carbon nanotubes. We observed a tubular structure made of coaxial graphite sheets. The inside, of these carbon nanotubes, was filled with cobalt particles. The magnetic properties of the Co-doped nanotubes were measured using a SQUID magnetometer. The undoped (pure) carbon nanotubes exhibit diamagnetic behavior. The Co-doped nanotubes exhibit superparamagnetic behavior with saturation of magnetization at about 0.5 T and coercive field of 750 G.

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