Estimation of Magnetic Susceptibility Anisotropy of Carbon Nanotubes Using Magnetophotoluminescence

We have carried out a magnetophotoluminescence excitation spectroscopy study on micelle-suspended single-walled carbon nanotubes in high magnetic fields. By analyzing field-dependent spectral changes, we determined the degree of magnetic alignment of the observed semiconducting nanotubes at 45 T. This, together with an independently measured length distribution of the nanotubes, allowed us to estimate the magnitude of the magnetic susceptibility anisotropy | - ┴ to be 1.4 × 105 emu/mol for 1-nm-diameter semiconducting nanotubes. Single-walled carbon nanotubes (SWNTs), either metallic or semiconducting, are predicted to possess novel magnetic properties. 1-3 For example, while a field applied parallel to the tube axis modifies the band structure through the Aharonov-Bohm (AB) phase, leading to a logarithmically divergent paramagnetic susceptibility for metallic tubes, a perpendicular field is predicted to induce lattice instability and distortion. At low magnetic fields ( , 0, where is the magnetic flux threading the tube and 0 ) h/e is the magnetic flux quantum), semiconducting SWNTs are predicted to be diamagnetic both in the tube axis direction and in the perpendicular direction, with the diamagnetic susceptibility in the perpendicular direction ┴ being more negative than the susceptibility in the axial direction | while metallic SWNTs are predicted to be paramagnetic in the tube axis direction and diamagnetic in the perpendicular direction. This suggests that both semiconducting and metallic SWNTs should align in a magnetic field. Early experimental results probing diamagnetic anisotropy performed on multiwalled carbon nanotubes (MWNTs) 4-6 showed that j ┴j j |j; Kotosonov 8 argues that the previous work