This is an illustration of anisotropic spin transport in a bilayer graphene flake between injector and detector electrodes. The out-of-plane spins are well transmitted, whereas the in-plane spins decay quickly. Image: Talieh Ghiasi/Van Wees Lab/University of Groningen.Physicists at the University of Groningen in the Netherlands, in collaboration with a theoretical physics group from the University of Regensburg in Germany, have built an optimized bilayer graphene device that displays both long spin lifetimes and electrically controllable spin-lifetime anisotropy. It has the potential for practical applications such as spin-based logic devices. The physicists describe the new graphene device in a paper in Physical Review Letters.
Advances in computing over the past decades have mainly been driven by the steady miniaturization of the elements on a computer chip. This miniaturization has now reached scales below 100 atoms and is approaching its fundamental limit. Since the increasing variety of computing applications also makes higher demands in terms of performance and energy efficiency, new computing concepts are required that can provide enhanced functionalities.
In this context, researchers are studying the potential of using electron spins to transport and store information. Spin is a quantum mechanical property of electrons that gives them a magnetic moment, which could be used to transfer or store information. The field of spin-based electronics (spintronics) has already made its way into the hard drives of computers, and also promises to revolutionize the processing units.
A current focus of spintronics research is on optimizing materials for the transport and control of spins. Graphene is an excellent conductor of electron spins, but it is hard to control spins in this material because of their weak interaction with the carbon atoms (the spin-orbit coupling). In previous work, researchers in the Physics of Nanodevices group at the University of Groningen, led by Bart van Wees, placed graphene in close proximity to a transition metal dichalcogenide, a layered material with a high intrinsic spin-orbit coupling strength. They found this caused the high spin-orbit coupling strength to be transferred to graphene via a short-range interaction at the interface, making it possible to control the spin currents, but it was achieved at the cost of reduced spin life.
In the new study, the researchers managed to control spin currents in a graphene bilayer. “This was actually predicted in a theoretical paper in 2012, but the technology to measure the effect accurately only became available recently,” explains Christian Leutenantsmeyer, a PhD student in the Van Wees group and first author of the paper.
The 2012 paper predicted anisotropic spin transport in graphene bilayers, as a consequence of spin-orbit coupling. Anisotropic spin transport describes the situation in which spins pointing either in or out of the graphene plane are conducted with different efficiencies. This was indeed observed in the devices produced by Leutenantsmeyer and his colleagues.
The researchers found they could control the spin current using spin-lifetime anisotropy, since in-plane spins decay much faster than out-of-plane spins, and were thus able to polarize spin currents. “We found that the strength anisotropy is comparable to graphene/transition metal dichalcogenide devices, but we observed a 100 times larger spin lifetime,” said Leutenantsmeyer. “We therefore achieved both efficient spin transport and efficient control of spins.”
This work provides insight into the fundamental properties of spin-orbit coupling in bilayer graphene. “And furthermore, our findings open up new avenues for the efficient electrical control of spins in high-quality graphene, a milestone for graphene,” Leutenantsmeyer added.
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.