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

TL;DR

This study investigates vacancies in graphene on SiO_2, revealing their magnetic properties and antiferromagnetic interactions through electron spin resonance and scanning tunneling spectroscopy, despite minimal impact on electrical conductivity.

Contribution

It provides new insights into the magnetic coupling of vacancies in graphene and demonstrates their characterization using ESR and STM techniques.

Findings

Vacancies exhibit a peak near the Fermi level in STS.

ESR detects a g-factor of 2.001-2.003 indicating localized spins.

Antiferromagnetic correlations with a Curie-Weiss temperature of -10 K were observed.

Abstract

Monolayer graphene grown by chemical vapor deposition and transferred to SiO_2 is used to introduce vacancies by Ar^+ ion bombardment at a kinetic energy of 50 eV. The density of defects visible in scanning tunneling microscopy (STM) is considerably lower than the ion fluence implying that most of the defects are single vacancies. The vacancies are characterized by scanning tunneling spectroscopy (STS) on graphene and HOPG exhibiting a peak close 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 spin resonance (ESR) at 9.6 GHz is performed. For an ion flux of 10/nm^2, we find a signal corresponding to a g-factor of 2.001-2.003 and a spin density of 1-2 spins/nm^2. The ESR signal consists of a mixture of a Gaussian and a Lorentzian of equal weight exhibiting a width down to 0.17 mT, which, however, depends on details of the sample preparation. The g-factor anisotropy is about 0.02%. Temperature dependent measurements reveal antiferromagnetic correlations with a Curie-Weiss temperature of -10 K. Albeit the electrical conductivity of graphene is significantly reduced by ion bombardment, the spin resonance induced change in conductivity is below 10^{-5}.