  Graphene has attracted a great interest in material science due to its novel
electronic structrues. Recently, magnetism discovered in graphene based systems
opens the possibility of their spintronics application. This paper provides a
comprehensive review on the magnetic behaviors and electronic structures of
graphene systems, including 2-dimensional graphene, 1-dimensional graphene
nanoribbons, and 0-dimensional graphene nanoclusters. Theoretical research
suggests that such metal-free magnetism mainly comes from the localized states
or edges states. By applying external electric field, or by chemical
modification, we can turn the zigzag nanoribbon systems to half metal, thus
obtain a perfect spin filter.
