Magnetic heterogeneity and alignment of single wall carbon nanotubes

Magnetic alignment studies demonstrate that a fraction of acid purified single wall carbon nanotubes (SWNTs) exhibit both linear-orbital (diamagnetic and/or paramagnetic) and ferromagnetic (FM) anisotropies. The latter are unexpected, as prior studies have assumed that remaining FM impurities are encapsulated and play no role in SWNT alignment. The data imply a FM easy axis aligned with the nanotube, permit direct estimates of the FM moment size, and provide a more accurate measure of the SWNT magnetic anisotropy. In light of recent observations of FM proximity effects in nanotubes, our observation of attached or intrinsic FM anisotropy in these systems is an important consideration for future studies of carbon nanotube magnetism.

Figure 1

(a) A schematic of our magnetic fractionation method. (b) dc magnetization of SWNT buckypaper obtained using chemical purification only (unfractionated) and chemical purification followed by magnetic fractionation (fractionated). A linear diamagnetism has been subtracted.

Figure 2

Polarized absorbance for unfractionated HiPCO and laser-oven SWNTs at $B=0\mathrm{T}$ and $B=7\mathrm{T}$.

Figure 3

The nematic order parameter, $S$, vs magnetic field for (a) HiPCO and (b) laser-oven SWNTs. Unfractionated (solid) and fractionated (long dashes) samples have the same SWNT density. The dotted and short dashed lines indicate mixed model fits to the data. The inset compares $S\left(B\right)$ for high (solid triangles) and low (solid line) gradients, $\nabla B∕B=0.21{\mathrm{cm}}^{-1}$ and $\nabla B∕B<0.006{\mathrm{cm}}^{-1}$, respectively. The inset axis labels are the same as in (b). $S$ is unscaled in (a) as described in the text.

Figure 4

$S\left(B\right)$ for $\vec{\mu }$-driven and $\Delta \chi$-driven alignment for unfractionated HiPCO. Rapid saturation of $\vec{\mu }$-driven SWNTs is a robust result of our modeling, despite large scaling uncertainties in $\Delta \chi$ and $S$ for HiPCO that arise from unknown $\Delta \sigma$. The inset polar plots show the corresponding distribution functions, $f_{\Delta \chi +\vec{\mu }}\left(\theta \right)$ and $f_{\Delta \chi }\left(\theta \right)$, respectively, at $B=7\mathrm{T}$.