Antiferromagnetic behavior in single-wall carbon nanotubes

The electronic properties of single-wall carbon nanotubes (SWNTs) have been studied using dc magnetization and electron spin resonance (ESR). The dc magnetization displays a weak diamagnetic susceptibility of $\approx {10}^{-7}\mathrm{emu}∕\mathrm{g}$. ESR measurements reveal a narrow resonance line of low intensity and metallic line shape. The spin susceptibility shows a major Pauli contribution at high temperatures $(T>150\mathrm{K})$ and an appreciable Curie component at lower temperatures. A marked drop of the spin susceptibility is observed at $T<14\mathrm{K}$, pointing to the opening of a spin gap at low temperatures. The underlying mechanism is discussed in terms of an electronic instability of the SWNTs or the occurrence of defect-mediated spin magnetism.

Figure 1

Magnetization loops of SWNTs at different temperatures. The upper inset shows the temperature dependence of the dc magnetization at 500 and $1000\mathrm{Oe}$. The lower inset shows the field dependence of the differential susceptibility $\partial M∕\partial H$ at $10\mathrm{K}$.

Figure 2

(Color online) ESR spectra of the SWNTs at various temperatures and magnetic field scans. Arrows in the larger field scans (left panels) point out the narrow ESR line superimposed on the FMR of the catalyst particles. Thin dashed lines in the small scan spectra (right panels) show the best fit metallic line shapes at 5 and $14\mathrm{K}$ after subtraction of the ferromagnetic background.

Figure 3

Temperature dependence of the spin susceptibility ${\chi }_s$ and ${\chi }_s$ $T$ for the SWNTs. Solid and dashed lines depict the decomposition of ${\chi }_s\left(T\right)$ to a constant Pauli term ${\chi }_0$ and a Curie ${\chi }_C=C∕T$ contribution. The inset shows in detail the low-temperature variation of ${\chi }_s\left(T\right)$.

Figure 4

Temperature dependence of the linewidth $\Delta H$, $g$ factor, and the asymmetry parameter $\alpha$ for the narrow ESR line of the SWNTs.