Nature Phys. http://dx.doi.org/10.1038/nphys2183 (2012)

Relative to its optical or electrical properties, the magnetic properties of graphene are not well understood or controlled. They are nevertheless potentially important, for example, for building spintronic devices. Experimental work has suggested that point defects in graphene's lattice carry magnetic moment, and that these moments can be ordered and can lead to ferromagnetism. However, these conclusions remain controversial, in part because of inconsistencies between different studies, and the possibility of alternative interpretations. Irina Grigorieva and colleagues at the University of Manchester, the University of Helsinki and Aalto University have now directly confirmed that point defects in graphene carry spin-1/2 magnetic moments, but have found no evidence for defect-induced ferromagnetism.

The researchers used SQUID (superconducting quantum interference device) magnetometry to measure the magnetic moment of a collection of non-interacting graphene sheets, a relatively clean and well characterized system. Defects introduced either through fluorine adatoms, or through vacancies caused by ion irradiation, caused the samples to be paramagnetic. The total magnetic moment induced for each vacancy defect was larger than that for each adatom, because the fluorine atoms migrated to form clusters whose interiors contributed zero magnetic moment.

The results provide direct confirmation of the magnetic nature of defects in graphene. Importantly, no defect-induced ferromagnetism was observed for any defect density or sample temperature down to 2 K, suggesting that previous observations of ferromagnetism in graphene, graphite and carbon films did not result from defects of the kind studied here.