The ability to electrically control the magnetic properties of materials would add momentum to the development of spintronic devices. The search for such electrical control is the major motivation for research on materials exhibiting the magnetoelectric (ME) effect, by which magnetization is induced by an electric field, or conversely, electrical polarization is induced by application of a magnetic field.

The ME effect is typically found in transition metals or other materials with localized electronic d orbitals. Now, however, Wanlin Guo from the Nanjing University of Aeronautics and Astronautics in China and colleagues1 predict that magnetization in graphene nanoribbons — long, narrow strips of atomically thin graphite — could be controlled by the application of a voltage.

“Carbon-based magnets possess long spin relaxation and decoherence time because the spin–orbit coupling is weak and the small nucleus spin of carbon atoms results in very small hyperfine interaction,” says Guo. “So, realizing electrically controlled magnetism in the emerging graphene-based magnets is desirable.”

Fig. 1: Schematic illustration of graphene nanoribbons on a silicon substrate. Arrows show the direction of charge transfer.

The team used ab initio calculations to study the magnetization of a bilayer graphene nanoribbon with zigzag edges on a silicon substrate (Fig. 1). The atoms of the bottom layer form sp3 bonds and do not exhibit any unique magnetic behavior. However, the edges of the top layer exhibit specific spin configurations: at one edge, conduction band states have spin-up orientation and valence band states have spin-down orientation; at the other, the spin orientations are reversed.

The calculations show that when a voltage is applied to the gate at the bottom of the silicon substrate, the magnetization of the nanoribbon varies linearly. The researchers attribute this to the transfer of charge to and from the nanoribbon, which changes the balance between conduction and valence electrons leading to a variation in the occupied spin states at the ribbon's edges.

“Our results reveal a new ME mechanism for electrically controlled magnetism in metal-free magnets, for which previously reported ME principles do not work.” says Guo. “To the best of our knowledge, this is the first time linear ME effects in carbon—silicon systems have been revealed.” The results are especially promising as they demonstrate the compatibility of graphene with silicon-based technology for the development of spintronic devices.