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Magnetic Anisotropies of Single Walled Carbon Nanotubes (SWNTs)

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Abstract

We use optical anisotropy to study the magnetism of SWNTs in suspension. Alignment of nanotubes in a magnetic field is used to infer their intrinsic and extrinsic magnetic properties. First, a purely absorptive strategy uses a combination of Raman scattering and polarized absorbance to perform high-resolution studies of SWNT alignment in suspensions.1 Magnetic alignment studies demonstrate that a fraction of acid purified single wall carbon nanotubes (SWNTs) exhibit both inear-orbital (diamagnetic/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 than previously available. We find that after standard acid purification, approximately 15% of SWNTs exhibit FM anisotropy, most probably associated with residual catalyst particles tethered to the SWNTs. Magnetic gradient fractionation is used to reduce this quantity, but some residual catalyst remains. In contrast, we find that DNA-wrapped SWNTs available from DuPont show essentially no FM impurity moment.2 The latter studies rely on anisotropic absorbance and photoluminescence to improve selectivity to specific tube species. We are thus able to compare diamagnetic responses for different wrapping vectors (m,n) and to compare these with analytic predictions3 and more recent ab-initio calculations.4 Work in the Kikkawa lab performed by graduate students Daniel E. Milkie, Omar N. Torrens and Han Y. Ban. HiPCO and laser oven work done in collaboration with Mohammad Islam and Arjun Yodh at the University of Pennsylvania Department of Physics. DNA-wrapped tube work done in collaboration with Ming Zheng, G. Bibiana Onoa and Timothy Gierke at Du Pont Central Research and Development. We thank ONR/DARPA and NSF for grant support.

© 2006 Optical Society of America

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