  The pristine graphene is strongly diamagnetic. However, graphene with single
carbon atom defects could exhibit paramagnetism with local magnetic moments ~
1.5 per vacancy1-6. Theoretically, both the electrons and electrons of graphene
contribute to the magnetic moment of the defects, and the pi magnetism is
characterizing of two spin-split DOS (density-of-states) peaks close to the
Dirac point1,6. Since its prediction, many experiments attempt to study this pi
magnetism in graphene, whereas, only a notable resonance peak has been observed
around the atomic defects6-9, leaving the pi magnetism experimentally so
elusive. Here, we report direct experimental evidence of the pi magnetism by
using scanning tunnelling microscope. We demonstrate that the localized state
of the atomic defects is split into two DOS peaks with energy separations of
several tens meV and the two spin-polarized states degenerate into a profound
peak at positions with distance of ~ 1 nm away from the monovacancy. Strong
magnetic fields further increase the energy separations of the two
spin-polarized peaks and lead to a Zeeman-like splitting. The effective
g-factors geff around the atomic defect is measured to be about 40. Such a
giant enhancement of the g-factor is attributed to the strong spin polarization
of electron density and large electron-electron interactions near the atomic
vacancy.
