The magnetic properties of single-wall carbon nanohorns (SWNH) were studied by electron spin resonance (ESR) and static magnetic susceptibility measurements. The SWNHs were ESR active with linewidth (ΔH) of ∼6 G in vacuo at room temperature. ΔH was susceptible to the partial pressure of O2 and became 53 G at 1 atmospheric pressure of O2, while the integrated ESR intensity was independent on O2 pressure and behaved as Curie-like, suggesting an intrinsic ESR origin with localized electron spin character. The diamagnetic susceptibility for SWNHs indicated a value smaller than that of randomly oriented graphite by an order of magnitude, but showing a magnitude comparable to those of C60 and C70. It is suggested that the large diamagnetism expecting for sp2 networked carbon materials will be canceled by the Van Vleck constant paramagnetism.
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