Berlin 2015 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 16: Graphene: Theory (HL with O/TT)

O 16.8: Vortrag

Montag, 16. März 2015, 16:45–17:00, ER 164

Antiferromagnetic coupling of vacancies in graphene on SiO₂ — •Stephan Zimmermann¹, Sven Just², Marco Pratzer², Markus Morgenstern², Vladislav Kataev¹, and Bernd Büchner¹ — ¹IFW Dresden, Institute of Solid State and Materials Research, 01069 Dresden, Germany — ²II. Institute of Physics B and JARA-FIT, RWTH Aachen, 52074 Aachen, Germany

Monolayer graphene grown by chemical vapor deposition and transferred to SiO₂ 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 is considerably lower than the ion fluence, implying that most of the defects are single vacancies as expected from the low ion energy. The vacancies are characterized by scanning tunneling spectroscopy on graphene and highly oriented pyrolytic graphite (HOPG). A peak close to the Dirac point is found within the local density of states of the vacancies similar to the peak found previously for vacancies on HOPG. The peak persists after air exposure up to 180 min, such that electron spin resonance (ESR) at 9.6 GHz can probe the vacancies exhibiting such a peak. After an ion flux of 10/nm², we find an ESR signal corresponding to a g factor of 2.001-2.003 and a spin density of 1-2 spins/nm². The peak width is as small as 0.17 mT indicating exchange narrowing. Consistently, the temperature-dependent measurements reveal antiferromagnetic correlations with a Curie-Weiss temperature of -10 K. Thus, the vacancies preferentially couple antiferromagnetically, ruling out a ferromagnetic graphene monolayer at ion induced spin densities of 1-2 nm².