Metallic bundles of single-wall carbon nanotubes probed by electron spin resonance

C59N magnetic fullerenes inside single-wall carbon nanotubes (SWCNTs) are used to probe the density of states (DOS) on the host tubes using electron spin resonance (ESR). The C59N radicals are separated by C-60 fullerenes to prevent dimerization and C59N-C-60 heterodimers are formed at low temperatures. The electron spin-lattice relaxation time, T-1, of the heterodimers is deduced from the homogeneous ESR line-width. The analysis is supported by saturation ESR studies. The inverse of the heterodimer T-1 follows a linear behavior in the 20-300 K temperature range, the so-called Korringa law, evidencing a metallic DOS on all tubes in a bundle.