Abstract
Magnetism of graphene can be created by atomic defects, either hydrogen adsorption or single-carbon vacancy formation, owing to the unpaired $\ensuremath{\pi}$ electrons around the defects. Here we explore, based on rigorous first principles calculations, the possibility of voltage manipulation of two such types of $\ensuremath{\pi}$ magnetism in graphene via a scanning tunneling microscope tip. We find a remarkably different behavior. For the hydrogen, the magnetic moment can be switched on and off with voltage-induced doping, whereas, for the carbon vacancy, the spin splitting of the $\ensuremath{\pi}$ bands persists, almost independent of the extent of doping, due to the coupling between the $\ensuremath{\pi}$ and the $\ensuremath{\sigma}$ bonds. Furthermore, the local atomic structures near the vacancy can be reversibly manipulated by a coordination mechanism between an intermediate tip-defect distance and a moderate tip voltage, consequently leading to the reversal of spin polarization of the $\ensuremath{\pi}$ bands. Voltage control of the local magnetic states may open a new avenue for potential applications in spintronics.
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