Abstract Submitted to the    NANOTUBE'04 Conference:

The next generation of materials for magnetic recording media will require i) the design of regular arrays of ferromagnetic nanoparticles with well controlled morphology and behaviour and ii) to physically separate these particles, either by vacuum or by a nonmagnetic material to fully discriminate the bits of information. Other requirements are the chemical stability, the obtention of a definite direction for magnetization, the mecanical stiffness… The growth of carbon nanotubes by a catalytic CVD process requires the presence of nanoscaled transition metals particles (Fe, Co, Ni), which are ferromagnetic. These particles are encapsulated after growth on the top of the nanotubes. Moreover in appropriate deposition conditions they took a very anisotropic shape, and the particle fullfills the nanotube which is a nonmagnetic material. Various other carbon nanostructures can be grown (nanocones, carbon nanofibers, …) depending on the experimental CVD and the metallic dispersion parameters. The magnetic properties of these arrays of Co nanoparticles encapsulated into carbon were investigated by SQUID and MFM. Different magnetic behaviour were evidenced - Superparamagnetic behaviour for small particles (5 à 8 nm) encapsulated into non oriented nanotubes - Strong magnetic anisotropy in the plan of the substrate for nanoparticles (~30 nm) encapsulated at the top of nanocones, due to an exchange coupling between a metallic core and a thin antiferromagnetic CoO external layer. - Strong magnetic anisotropy perpendicular to the plan of the substrate of metallic nanowires (diameter~25 nm and aspect ratio from 1/4 to 1/10), induced by the cork-like shape of the nanoparticles. In this case the coercitive field (750 Oe), the magnetic anisotropy combined with high density (1010particles/cm2) and the weak dipolar interactions evidenced by MFM are very attractive for dense storage media.