Preferential antiferromagnetic coupling of vacancies in graphene on SiO_2: Electron spin resonance and scanning tunneling spectroscopy

摘要

Monolayer graphene grown by chemical vapor deposition and transferred toSiO_2 is used to introduce vacancies by Ar^+ ion bombardment at a kineticenergy of 50 eV. The density of defects visible in scanning tunnelingmicroscopy (STM) is considerably lower than the ion fluence implying that mostof the defects are single vacancies. The vacancies are characterized byscanning tunneling spectroscopy (STS) on graphene and HOPG exhibiting a peakclose to the Fermi level. The peak persists after air exposure up to 180 min,albeit getting broader. After air exposure for less than 60 min, electron spinresonance (ESR) at 9.6 GHz is performed. For an ion flux of 10/nm^2, we find asignal corresponding to a g-factor of 2.001-2.003 and a spin density of 1-2spins/nm^2. The ESR signal consists of a mixture of a Gaussian and a Lorentzianof equal weight exhibiting a width down to 0.17 mT, which, however, depends ondetails of the sample preparation. The g-factor anisotropy is about 0.02%.Temperature dependent measurements reveal antiferromagnetic correlations with aCurie-Weiss temperature of -10 K. Albeit the electrical conductivity ofgraphene is significantly reduced by ion bombardment, the spin resonanceinduced change in conductivity is below 10^{-5}.