Graphene is steadily evolving into a modern-day wonder material. Comprising just a single atomic layer of carbon atoms arranged in a hexagonal pattern, it plays host to a myriad of unusual structural, electronic and magnetic properties. An investigation of this fascinating material by Shyamal Kumar Saha and colleagues at the Indian Association for the Cultivation of Science now indicates that it might also be possible to use graphene to make ‘spin-valve’ devices,1 which could find applications in computer hard disks.

A conventional spin valve comprises alternating layers of ferromagnetic and nonmagnetic materials. When the internal magnetic fields of the magnetic layers are parallel to one another, the electrical resistance of the device is small. When they run antiparallel, the resistance is much higher. This is known as giant magnetoresistance; a phenomenon that earned its discoverers — Albert Fert and Peter Grünberg — the 2007 Nobel Prize in Physics.

Fig. 1: (Left) Transmission electron microscopy image of graphene nanoribbons fragmented into ultrafine 'quantum sheets'. (right) High-resolution lattice images of the quantum sheets.© 2010 Wiley-VCH

Previous theoretical studies have indicated that the zigzag edge of a graphene sheet has ferromagnetic properties. What is more, opposite edges normally have antiparallel magnetization, but this can be switched so that they are parallel by simply applying an external magnetic field. This intrinsic arrangement lends itself very well to spin-valve applications because the area between the edges is nonmagnetic. Such an approach is particularly advantageous as it would mean a device could be created from a single material, rather than a combination of different compounds.

Saha and his co-workers used mechanical vibrations to create ribbons of graphene, which they then fragmented into ultrafine sheets just 2–5 nm in size — structures that the researchers refer to as ‘quantum sheets’. The sheets’ thinness increases the edge-to-area ratio and thereby increases the antiferromagnetic coupling between the edges.

The ribbons displayed the characteristic hysteresis loops of ferromagnetism, and the researchers found that the resistance across the sample dropped by 35% when subject to an external magnetic field — a clear indication of a spin-valve effect. “We propose that these graphene quantum sheets could be used as a type of spin transistor,” says Saha. “The next step is to place a sheet between two nanoelectrical contacts to investigate spin-polarized current through the device.”