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

Abstract

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.The magnetism of carbon-based materials (i.e. graphene, graphite, carbon nanotubes, and fullerenes) has recently been the subject of intense research.1 These materials exhibit exotic magnetism such as ferromagnetism above room temperature, which is believed to be induced by defects in the graphitic network. Our understanding of the intrinsic magnetism in these materials without such intentionally introduced defects, however, has also been very limited due to magnetic impurities remaining in samples. In the present work, we investigated single-wall carbon nanotubes (SWCNTs), which are rolled-up tubes of graphene sheets that exhibit unusually anisotropic electrical and magnetic properties. 2-6In the present paper, we focus our attention on the magnetism of SWCNTs, because despite theoretical predictions of novel magnetic features, experimental studies of SWCNT magnetism have been very limited.The magnetism of SWCNTs is expected to be dominated by their orbital magnetic susceptibility, which is 2 orders of magnitude higher than the spin magnetic susceptibility. The magnetic susceptibility is strongly anisotropic: it shows a large diamagnetic response in a magnetic field perpendicular to the tube axis (χ ⊥ ). In contrast, the magnetic susceptibility in a magnetic field parallel to the tube axis (χ ) depends on whether the SWCNT is metallic or semiconducting. Approximately one-third of SWCNTs are metallic and the rest are semiconducting, depending on the chirality. The cylindrical shape of the SWCNT leads to an Aharonov-Bohm effect when a magnetic field is introduced parallel to the tube axis, 4-6 resulting in a paramagnetic (diamagnetic) χ for metallic (semiconducting) SWCNTs. Actually, the predicted large magnetic susceptibility anisotropy ∆χ = χ − χ ⊥ was estimated indirectly by magneto-optical experiments of aligned SWCNT sample in high magnetic fields.7-10 Furthermore, the magnetic susceptibility for SWCNTs depends linearly on the nanotube diameter d, and that there is universal scaling in scaled magnetic susceptibility χ/d as a function of scaled temperature k B T /∆ 0 , where ∆ 0 is the characteristic energy, and corresponds to the bandgap for a semiconducting SWCNT. 6Magnetic susceptibility measurements of SWCNTs, however, are extre...

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