Anomalous spin relaxation in graphene nanostructures on the high temperature annealed surface of hydrogenated diamond nanoparticles.

The electronic and magnetic structures of diamond nanoparticles with a hydrogenated surface are investigated as a function of annealing temperature under vacuum annealing up to 800-1000 °C. Near edge X-ray absorption fine structure (NEXAFS) spectra together with elemental analysis show successive creation of defect-induced nonbonding surface states at the expense of surface-hydrogen atoms as the annealing temperature is increased above 800 °C. Magnetization and ESR spectra confirm the increase in the concentration of localized spins assigned to the nonbonding surface states upon the increase of the annealing temperature. Around 800 °C, surface defects collectively created upon the annealing result in the formation of graphene nano-islands which possess magnetic nonbonding edge states of π-electron origin. Interestingly, extremely slow spin relaxation is observed in the magnetization of the edge state spins at low temperatures. The relaxation time is well explained in terms of a lognormal distribution of magnetic anisotropy energies instead of the classical Néel relaxation mechanism with a unique magnetic anisotropy energy, in addition to the contribution of the quantum mechanical tunnelling mechanism. The spin-orbit interaction enhanced by the electrostatic potential gradient created at the interface between the core diamond particle and surface graphene nano-islands is responsible for the slow spin relaxation.

Magnetic edge-states in nanographene, HNO3-doped nanographene and its residue compounds of nanographene-based nanoporous carbon.

We investigated the magnetic and electronic properties of nanographene and its charge transfer effect, using near edge X-ray absorption fine structure (NEXAFS), magnetic susceptibility and ESR measurements, and elemental analysis, with the employment of nanoporous carbon, which consists of a three dimensional disordered network of loosely stacked nanographene sheets, in relation to the host-guest interaction with HNO3 as the electron-accepting guest. The adsorption of electron acceptor HNO3 decreases the intensity of the edge state peak in NEXAFS as a result of the charge-transfer-induced Fermi energy downshift, in agreement with the decrease in the edge-state spin concentration, and it also induces the structural expansion, which makes the inter-nanographene sheet distance elongated, resulting in weakening of the inter-nanographene-sheet antiferromagnetic interaction as evidenced by the decrease in the Weiss temperature. In addition, the decomposition of HNO3, which takes place with the electron-rich edge state as an oxidation catalyst, results in the creation of oxygen/nitrogen-containing functional groups bonded to the periphery of the nanographene sheets. Heat-treatment of the HNO3-ACFs under evacuation desorbs the HNO3 molecules completely, though a part of the oxygen/nitrogen-containing species remains strongly bonded to the edge even at a high temperature of ∼800 °C, according to NEXAFS and elemental analysis results. These remaining species participate in the charge transfer, modifying the electronic structure as observed with the decrease in the orbital susceptibility and the strengthening of the inter-nanographene-sheet antiferromagnetic interaction.