Electron Spin Resonance in Single-Walled Carbon Nanotubes

We have performed electron spin resonance (ESR) measurements on various types of single-walled carbon nanotube (SWNT) samples. As catalyst particles were chemically removed from the SWNTs, the linewidth of the conduction electron spin resonance (CESR) signal became smaller, reaching 43 G at 5 K for acid-purified HiPco SWNTs. For every type of SWNT tested, a ferromagnetic resonance (FMR) signal was observed; we show that this is attributed to catalyst particles. The $g$-factor for the CESR signal was slightly shifted from the free electron value; as purity increased, the resonance moved closer to $g$ = 2.003. The conduction electron signal increased as the temperature was decreased, indicating that we are observing both Pauli and Curie paramagnetism. No spin gap for either laser-oven acid-purified or HiPco acid-purified SWNTs was seen when the temperature was taken from 295 K to 5 K. In addition, the FMR signal decreased as the temperature was decreased. SWNTs suspended in aqueous surfactant solutions were also measured. As a function of purity, the FMR signal was substantially decreased. We show that as a function of metal catalyst content, the ESR lineshape of SWNTs changes significantly.