Ferromagnetism in graphitic forms of carbon, like nanotubes and graphene, has been observed several times, yet the microscopic details of the magnetic ordering, including its origin, are still unclear. Many theoretical studies on graphene have predicted edge states, due to nonbonding electrons, that give rise to ferromagnetism. Nanographite, on the other hand, has been predicted to exhibit more complex properties, like spin-glass behavior.

Now, C. N. R Rao and colleagues in India1 have tackled this issue through an extensive study of the magnetic properties of three different types of graphene samples, obtained respectively by thermal exfoliation of graphitic oxide, conversion of nanodiamond, and arc evaporation of graphite under hydrogen. All of their samples exhibited magnetic hysteresis loops indicative of ferromagnetism up to room temperature, and the possible influence of magnetic impurities was excluded as a result of careful structure analysis.

Fig. 1: Divergence of magnetization (M) with decreasing temperature (T) between field-cooled (FC) and zero-field-cooled (ZFC) graphene.

A closer look at the magnetic properties showed that the magnetic configuration at the microscopic level may not be comprised of a simple alignment of magnetic moments. With decreasing temperature, the magnetization changed differently depending on whether an external magnetic field was applied (Fig. 1). This divergence in behavior at low temperatures below around 150 K is often present in frustrated magnetic systems, and is a signature of the coexistence of both ferromagnetic and antiferromagnetic interactions.

To elucidate the origin of the magnetic properties of graphene, the team also studied the effect of the adsorption of molecules that could accept or donate electrons. In both cases, by increasing the concentration of such molecules, the magnetization decreased dramatically, strongly suggesting that the magnetism in the graphene samples is intrinsic.

For Rao, the results have the merit of revealing the coexistence of edge effects that give rise to ferromagnetic ordering and antiferromagnetism derived from the six-membered network of the graphite. Overall, the study sheds new light on the magnetic properties of this widely investigated material.