Friday, April 22, 2011

Flowing electrons magnetize graphene

Physicists in the UK have discovered another useful property of graphene – the material can be magnetized by simply passing a current of electrons through it. The effect could prove useful in creating spintronic or quantum-information devices that use the spin of the electron.
Graphene is a honeycomb-like 2D sheet of carbon just one atom thick. It acts as a semi-metal and is often touted as a "wonder material" with the potential to make extremely small electronic devices of the future.
This latest work was done by Andre Geim and Konstantin Novoselov at the University of Manchester in the UK, who shared the 2010 Nobel Prize for Physics for creating the first sheets of graphene. The research also involved scientists in the US, Russia, Japan and the Netherlands.

Spin currents

The researchers made their discovery by passing an electrical current along a piece of graphene in the presence of a small magnetic field. They found that spin-up and spin-down currents are produced in opposite directions, perpendicular to the direction of the electrical current. The effect is to magnetize the graphene sheet (see figure). The effect is important because it offers physicists a way of controlling spin using electrical current.
The researchers studied more than 20 devices, with two types of graphene – graphene grown on an oxidized silicon wafer and another system where crystals of hexagonal boron nitride were placed between the graphene and the silicon wafer.
While this is not the first time a form of graphene has been magnetized, it is the first time that net magnetization has been created in graphene using spin currents. The research also suggests that spins can be generated, even if graphene has no magnetic moment.
"The central [result] is that they can create large spin currents, which allow them to separate spatially the up and the down spins," says Markus Mueller, at the Abdus Salam International Centre for Theoretical Physics in Italy. Mueller believes that the experiment offers a way to produce simple and robust spin-current sources, which could have many applications.

Imbalance at the Dirac point

Mueller explains that the effect is related to an unusual property of graphene – a Dirac or "neutrality" point where the valence and conduction bands meet. Particles above the Dirac point and holes below the Dirac point react in opposite ways to a magnetic field. The result is an imbalance explains Mueller. "You have more 'up' spins so that their Fermi surface lies in the particle-like region; and fewer 'down' spins, which at their Fermi level are hole-like. That's all you need to create a strong spin current."
Another peculiarity of graphene is that even a very small concentration of charge carriers will hold the magnetization. This is unlike normal substances in which opposite spins can be induced, but a large number of charge carriers are required to maintain the magnetization. If the concentration of charge carriers is decreased, most materials begin to act as insulators and the magnetization is lost. But as Geim explains, in graphene "the dominant phenomenon [of magnetization] still occurs... it even increases as the concentration of charge carriers is reduced, as it is inversely related and this is a salient feature of graphene".

"Non-local quantum effect"

Another surprising finding is that the spins maintain their orientation for relatively long distances in the graphene – a property that is very desirable for spintronics and quantum-information applications. Antonio Castro Neto of Boston University believes that this "non-local quantum effect" is also related to the Dirac point. Writing in Science, he explains that "close to the Dirac point the charge of the electron behaves incoherently (and hence, classically) but its spin behaves coherently (and thus quantum mechanically)".
Francisco Guinea at the Instituto de Ciencia de Materiales de Madrid says the results are very important for spintronic applications, especially as spin currents can be used to retrieve information stored in magnetic devices. Mueller agrees: "It seems this is a quite interesting way to transfer information to spatially different locations via voltage signals, which are easy to process and detect."
The research is reported in Science 332 328 .

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