Abstract
Nanosized graphite (nanographite) is unique carbon material with localized spins originating from the nonbonding $\ensuremath{\pi}$-electron state (edge-state) on their edge sites instead of the diamagnetic properties of bulk-sized graphite. Activated carbon fibers (ACFs) consist of a three-dimensional disordered network of nanographite metallic domains and have large specific surface areas due to the presence of nanosized pores (nanopores). The electron transport in ACFs can be explained by the Coulomb-gap-type variable hopping between nanographite domains, and it exhibits a large positive magnetoresistance at low temperatures. This magnetoresistance decreases significantly upon magnetic oxygen adsorption, in spite of its insensitivity to nonmagnetic molecular species such as nitrogen, argon, and helium. This result suggests the presence of a strong interaction between the oxygen molecule spin and edge-state spin. The strong effect of oxygen adsorption on magnetotransport is theoretically explained in terms of the interaction between the electric dipole moment of the edge-state $\ensuremath{\pi}$-electron and electric quadrupole moment of the adsorbed oxygen molecule. Theoretical calculation reproduces the experimentally reported strength of the exchange interaction between the oxygen molecule spin and edge-state spin, in addition to the behavior of the magnetoresistance.
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