Since it was first isolated four years ago, condensed matter physicists have explored the properties of graphene. The high charge mobility and excellent mechanical properties also make this two-dimensional layer of carbon a promising candidate for future electronic device applications. But the magnetic properties of this sheet of carbon atoms are still not fully understood.

Now Yongsheng Chen and colleagues in China1 have made the surprising discovery that layers synthesized by annealing of graphene oxide exhibit ferromagnetism at room temperature.

Fig. 1: Ferromagnetic graphene was produced by using graphite from graphite oxide.

There have been several observations of ferromagnetism in carbon structures in the past, usually at low temperature. The most likely explanation for previous reports has been the presence of defects in the lattice—such as vacancies—that are linked to unpaired spins, producing the magnetic properties. Ferromagnetism, however, is a collective state and it is not yet clear how these spins interact to give rise to a macroscopic phenomenon. Moreover, it is usually difficult to prevent the incorporation of magnetic impurities.

Chen and co-authors studied three samples obtained by annealing graphene oxide sheets at 400°C, 600°C and 800 °C. A clear magnetic hysteresis loop—signature of ferromagnetism—was observed in both the 400°C and 600 °C samples, but not in unannealed material or the sample heated to 800 °C.

The graphene oxide was assumed not to have any unpaired spin by the group. Annealing however, removed atomic groups attached to the surface resulting in an ensemble of coupled spins that gave rise to ferromagnetism. But it is not clear why the sample annealed at 800°C was not ferromagnetic. On the other hand, as the starting material for this 800°C sample was the same the others, the team excluded the presence of magnetic impurities.

For Chen, the importance of these results is that the ferromagnetism is observed in a material that is stable, inexpensive and can be produced via a solution process. “In addition to a more systematic study of the ferromagnetism, it would be very important to explore its potential applications in spintronic, memory units, magnetoresistance and other multi-channel (magnetic/electrical) devices,” says Chen.