We have studied the adsorption of both single Fe atoms and Fe chains in carbon nanotubes by the first-principles methods. We find that a single Fe atom bonds stably with carbon nanotubes, while Fe chains do not bond stably with carbon nanotubes except for the small size nanotubes. The configurations for the $d$ electrons in the single Fe atoms and in the Fe chains are very different. We propose a physical mechanism and show that the transfer energy of spin-up electrons in a half-filled $d$ orbital to the spin-down electrons in another half-filled $d$ orbital determines the bonding ability of single Fe atoms and Fe chains with carbon nanotubes. The results explain the experiment that Fe nanowires are easy to move inside the nanotubes.

• Received 1 June 2007

DOI:https://doi.org/10.1103/PhysRevB.76.035407