Abstract
Activated carbon fibers (ACFs), comprising a three-dimensional disordered network of metallic nanographite domains, have huge specific surface areas due to the presence of nanosized pores (nanopores) surrounded by nanographite domains. Around the edge of the nanographene sheets, there are localized spins originating from the nonbonding -electron state (edge state). We investigate the effect of molecular adsorption on the magnetotransport of ACFs using oxygen, nitrogen, argon, and helium as the guest species. The magnetoresistance, which has a large positive value at the liquid helium temperature, is considerably reduced upon the adsorption of magnetic oxygen molecules in spite of the insensitivity to nonmagnetic molecular species. This change in the magnetoresistance induced by the oxygen molecule uptake can be explained in terms of the antiferromagnetic internal field of oxygen molecules affecting the edge-state spin. The change in the magnetoresistance yields the strength of the exchange interactions in the temperature range from , suggesting the presence of strong interactions between the spin of the oxygen molecule and edge-state spin. A theoretical analysis effectively explains the interaction mechanism on the basis of the interaction between the electric dipole of the edge state and the quadrupole of the adsorbed oxygen molecule. This finding, which demonstrates an important role of exchange interactions between the graphitic electrons and the spin of the adsorbed oxygen molecules, suggests that the adsorbed oxygen molecules are weakly bonded to the graphitic planes through charge transfer.
1 More- Received 21 August 2006
DOI:https://doi.org/10.1103/PhysRevB.76.035442
©2007 American Physical Society