Observation of the intrinsic magnetic susceptibility of highly purified single-wall carbon nanotubes

We report the observation of the intrinsic magnetic susceptibility of highly purified SWCNT samples prepared by a combination of acid treatment and density gradient ultracentrifugation (DGU). We observed that the diamagnetic susceptibility of SWCNTs increases linearly with increasing nanotube diameter. We found that the magnetic susceptibility divided by the diameter is a universal function of the scaled temperature. Furthermore, the estimated magnetic susceptibilities of pure semiconducting and pure metallic SWCNT samples suggest that they respond differently to changes in carrier density, which is consistent with theory. These findings provide experimental verification of the theoretically predicted diameter, temperature, and metallicity dependence of the magnetic susceptibility.

Figure 1

Field dependence of the magnetization for SWCNT samples with different diameters at 300 K. Linear diamagnetic responses gave a dominant contribution at high fields, while a contribution from ferromagnetic impurities was observed at low fields. The absolute value of the diamagnetic susceptibility, which is seen as a negative slope, increases with increasing nanotube diameter.

Figure 2

Temperature dependence of the magnetic susceptibility. The solid symbols represent raw experimental data consisting of intrinsic diamagnetism and paramagnetic impurities. The open symbols represent the estimated diamagnetic susceptibility for the SWCNT, which was obtained by subtracting a Curie term from the raw data. The solid lines represent theoretical calculations with $|E_{\mathrm{F}}|=0.2{\mathrm{\Delta }}_0$, which agree well with the experimental data (see text). Note that the typical error in reading calculated data (solid line) from Ref. [6] was less than 10%.

Figure 3

Diameter dependence of the magnetic susceptibility. Note that the magnetic susceptibility at 100 K was plotted for the 2.65-nm-diameter SWCNT sample, for which the magnetic susceptibility leveled off at low temperatures. Solid, dashed, and dotted lines represent theoretical calculations of randomly oriented semiconducting SWCNTs, metallic SWCNTs, and their mixtures with a ratio of semiconducting to metallic tubes of 2:1 and $|E_{\mathrm{F}}|=0.2{\mathrm{\Delta }}_0$ [6]. The dot-dash line represents mixtures with $E_{\mathrm{F}}=0$ [6]. The observed diamagnetic susceptibility of purified SWCNTs increased linearly with increasing nanotube diameter, and was consistent with theoretical calculations for mixed SWCNTs with $|E_{\mathrm{F}}|=0.2{\mathrm{\Delta }}_0$.