The magnetic properties of carbon nanohorns were investigated by electron spin resonance. Two decoupled electronic systems were found. The first one, which has a spin susceptibility linearly varying with temperature, was tentatively attributed to two-dimensional graphenelike sheets. The second contribution, showing a Curie-like behavior with a non-negligible Pauli part, was ascribed to the interior of the structures of coagulated nanohorns. Surprisingly, this second line shows a strong suppression of susceptibility at 17 K. Due to a concomitant line broadening and g shift below 17 K, this transition was assigned to antiferromagnetic pairing of localized spins. The low spin concentration is implying that a kind of magnetism, mediated by the free spin carriers, has to be invoked to explain the phenomenon.

• Received 8 June 2000

DOI:https://doi.org/10.1103/PhysRevB.62.17115