Magnetic susceptibility of buckytubes
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A. Patashinski and J. B. Ketterson Department of Physics and Astronomy and Materials Research Center, Northwestern University, Evanston, Illinois 60208-3108 (Received 16 August 1993; accepted 9 February 1994)
The measured room temperature magnetic susceptibility of a bulk sample of buckytubes (buckybundle) is -10.75 X 10~6 emu/g for the magnetic field parallel to the buckybundle axis, which is approximately 1.1 times the perpendicular value and 30 times larger than that of C6o- The experimental results are discussed in terms of a graphite-like electronic structure model.
I. INTRODUCTION In the first paper reporting the observation of C6o, Kroto et al. suggested that the molecule was aromaticlike with its inner and outer surfaces covered with a "sea" of Tr-electrons.1 Accordingly, C60 should exhibit a large diamagnetic susceptibility associated with a TTelectron ring current. This suggestion appeared to be supported by NMR chemical shift measurements.2-3 Theoretical calculations of the magnetic susceptibility of C6o performed by various groups were not consistent; some authors predicted a vanishingly small diamagnetism4"6 while others predicted strong diamagnetism (as in an aromatic system).7'8 Susceptibility measurements on Ceo and C70 show that the value for C70 is about twice that of C6o, but that both are very small.5'9 It is then of interest to extend the measurements to larger fullerenelike structures. With the availability of bulk samples of carbon nanotubes (bundles of buckytubes), a systematic study of the magnetic properties becomes possible. A cylindrical bulk deposit, with a diameter of 10 mm consisting of an inner core and an outer cladding, is formed on the graphite cathode during the arc process. The outer cladding, a gray-shell, is mainly composed of amorphous carbon and buckydoughnuts.10 The inner core with a diameter of 8 mm consists of an array of rather evenly spaced, parallel, and closely packed bundles approximately 50 fim in diameter, as shown in Fig. 1. High resolution electron microscopy reveals that the buckybundles in our best sample are comprised of buckytubes running parallel to one another.11 A bucky tube in each bundle can be pictured as a rolled-up graphitic sheet with a diameter ranging from 8 A to 300 A and a length of a few microns. The tube is capped by surfaces involving six pentagons (per layer) on each end.12'13 The purity of the bulk sample was examined by energy dispersive x-ray analysis. No elements heavier than 1578
http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 6, Jun 1994
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1 0.0/' m
(a)
(b)
FIG. 1. A bulk sample of buckybundles (bundles of buckytubes) measured for a magnetic field either (a) perpendicular or (b) parallel to the bundle axis.
carbon were observed. The C60 powder was extracted from the soot formed in the same chamber employed for production of the buckytubes. The purity of the C60 powder used in this study is better than 99% as examined by high-performance liquid chromatography. II. EXPERIMENTAL Magnetic suscept
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