Being able to control magnetism in materials using only an applied voltage will be important for spintronics – a technology that exploits the spin of an electron as well as its charge. The spin can either be "up" or "down", and this property could be used to store and process information in spintronics devices. Such circuits could be smaller and more efficient than conventional electronic circuits – which rely on just switching charge – because switching spins from up to down might be done using very little energy.

For such devices to be practical, however, physicists need to work out a way to flip spins by applying electric fields, rather than magnetic fields. This is because creating magnetic fields requires much more energy. Materials in which electron spin can be controlled using electric fields alone are known as a type of magnetoelectric, but they are few and far between. The most popular are the bismuth ferrites, but they are expensive and difficult to make.

Applied voltage generates strong spin-charge coupling

Now, Elton Santos of Stanford is saying that an external voltage can not 
only be used to control the magnetic properties of multilayer graphene functionalized with aryl radicals, but that it can also generate a strong spin-charge coupling that 
induces a magnetoelectric effect in this purely organic system. Graphene in its natural state is a so-called “zero-gap” semiconductor (since it does possess a bandgap between its valence and conduction bands).

Santos says that the magnetoelectric effect can also be tuned with the number of graphene layers and that it increases as the number of layers increases. He calculated that the magnetoelectric coefficients used to characterize the size of the coupling between the applied voltage and the magnetization lie between 6.49 and 200.0 × 10–14
G cm2/V. These values compare well to those found in thin films 
based on 3d transition metals (such as Fe, Co and
Ni), perovskite interfaces and half-metals.

Defect states have different spin polarizations

“The main physics behind this effect is localized around the material’s Fermi level – that is, the energy level that determines which levels are occupied or partially occupied and those that are unoccupied by electrons,” explains Santos. “As the applied field polarizes the graphene layers, charge transfer occurs, which changes how the defect states pinned at the Fermi level are occupied. This causes the defect states to have a different spin polarization (up or down).”

The effect also produces a, much sought-after, half-metallic state in graphene, he told “Indeed, inducing a half-metallic state in graphene in this way is much more reliable than previously employed techniques involving “edge” states that strongly depend on the quality of the edges and borders in the carbon material.”

Independent experiments confirm predictions

According to Santos, potential applications for magnetoelectric graphene include energy-efficient data-storage devices, such as electric memories and magnetic-electric field sensors.

“It is also worth mentioning that the predictions made in this work have recently and independently been observed in laboratory experiments by a group of researchers at the University of California at Berkeley, UC Riverside, Georgia Tech and Florida International University,” he added. “These experiments are detailed in the same issue of ACS Nano in which our results are published and we are now all working together to further explore the magnetoelectric effect in graphene with a view to making novel spintronics devices.”