This image shows the multi-layered device used in the study. WS2: tungsten disulfide; hBN: boron nitride layer. Image: S. Omar, University of Groningen.
This image shows the multi-layered device used in the study. WS2: tungsten disulfide; hBN: boron nitride layer. Image: S. Omar, University of Groningen.

In order to make transistors that operate using the spin of electrons, rather than their charge, it is necessary to find a way of switching spin currents on and off. Furthermore, the lifetime of the spins should at least be equal to the time taken for these electrons to travel through a circuit.

Scientists at the University of Groningen in the Netherlands have now created a device that meets both of these requirements, based on a double layer of graphene on top of a layer of tungsten disulfide. They report this advance in a paper in Physical Review B.

Graphene, a two-dimensional (2D) form of carbon, is an excellent conductor of electron spins. However, it is difficult to manipulate spin currents in this material. Spin is a quantum mechanical property of electrons, which makes them behave like tiny magnets. The Physics of Nanodevices group at the University of Groningen, led by Bart van Wees, has been working on this problem. They had previously shown that it is possible to control spin currents in graphene if it is placed on top of a layer of tungsten disulfide (another 2D material).

“However, this approach reduces the lifetime of the spins,” says Siddhartha Omar, a postdoc in the Van Wees group. Tungsten is a metal and its atoms influence the electrons passing through the graphene, dissipating the spin currents. This led Omar to try using a double layer of graphene on the tungsten disulfide, based on the theory that electrons passing through the upper layer should 'feel' less of the metal atoms' influence.

Omar also used another new technique, in which two different types of spin current are passed through the graphene. Spin is a magnetic moment that has a given direction. In normal materials, spins are not aligned. However, the magnetic moment of spin currents – like that of magnets – has a preferred alignment. Relative to the material through which the electrons are passing, their spins can either have an in-plane orientation or an out-of-plane orientation.

“We found that, as the electrons pass through the outer graphene layer, the in-plane spins are dissipated very quickly – in mere picoseconds,” explains Omar. “However, the lifetime of the out-of-plane spins is about 100 times longer.” This means that, even in the presence of tungsten disulfide, one component of spin currents (spins with an out-of-plane orientation) can travel far enough to be used in devices such as transistors.

The energy level of the spin currents observed by Omar caused them to pass through the upper layer of graphene. This energy level can be boosted by applying an electric field, pushing the spin currents into the lower layer. “Down there, the spins will feel the full effect of the metal atoms and the spin currents will quickly dissipate,” Omar says. This ability to switch the spin current off using an electric field is important, as it could be used to 'gate' transistors based on this technology.

“Unfortunately, certain technical limitations of the substrate on which we built these devices prevent us from creating electric fields that are strong enough to produce this gating effect,” says Omar. “However, we have shown that it is possible to send spin currents through a heterostructure made of graphene and tungsten disulphide. That is an important step towards the creation of a spin transistor.”

This story is adapted from material from the University of Groningen, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.