A crucial goal of spintronics research is to coherently manipulate electron spins at room temperature using electrical current. This is particularly valuable as it would enable the development of numerous devices, including spin field-effect transistors. In experiments using conventional materials, engineers and physicists have so far only observed coherent spin precession in the ballistic regime and at very low temperatures. Two-dimensional (2D materials), however, have unique characteristics that could provide new control knobs to manipulate spin procession.
Researchers at CIC nanoGUNE BRTA in Spain and the University of Regensburg in Germany have recently demonstrated spin precession at room temperature in the absence of a magnetic field in bilayer graphene. In their paper, published in Physical Review Letters, they used 2D materials to realize a spin field-effect transistor.
Josep Ingla-Aynes, Franz Herling, Jaroslav Fabian, Luis E. Hueso, and Felix Casanova, the researchers who carried out the study said, in our group, there is a long tradition of studying spin transport in multiple materials, such as simple metals, for instance, Our main goal is to understand how the spin of the electron can carry information and how this degree of freedom can help to create devices with new functionalities.
Graphene is among the materials with the greatest spin relaxation lengths. Nonetheless, manipulating spins as they travel on graphene can be very challenging and has so far only been achieved using external magnetic fields, which is far from ideal for practical applications.
Recently, Ingla-Aynés and his colleagues have been examining how heterostructures based on different 2D materials, also known as van der Waals heterostructures, perform in spintronics. Van der Waals heterostructures, are a class of graphene-based 2D materials with layers that are not chemically bonded.
The researchers said, we have particularly been exploring structures where a material with weak spin-orbit coupling (such as graphene) is stacked with a material with a strong spin-orbit coupling (such as WSe2) and observing experimentally how this spin-orbit coupling is actually transferred into the graphene by proximity. More technically, by achieving a strong interaction between the layers, it is possible to imprint such an efficient spin-orbit coupling on the graphene (that acts as an effective magnetic field) that can reverse the spins without the need for applying a magnetic field and this is what we wanted to do.
Instead of using a single material, Ingla-Aynés and his colleagues used a combination of two materials with different significant properties. The first of these materials is graphene, which has a weak spin-orbit coupling and long spin relaxation length. The second is WSe2, which has a strong and anisotropic spin-orbit coupling.
The researchers said, we prepared bilayer graphene/WSe2 van der Waals heterostructures using a dry polymer-based stacking technique. Then, to promote proximity between the layers, we annealed our samples above 400 degrees Celsius. To measure spin transport, we used ferromagnetic electrodes that, combined with magnetic fields, allow us to measure in-plane and out-of-plane spins that travel across the graphene/WSe2 channel.
Ingla-Aynés and his colleagues were able to control the spin transport times in the material they used by applying an in-plane electric field and a back gate voltage to them. This ultimately enabled the electrical control of spin precession at room temperature, without the need to apply an external magnetic field.
The researchers said, this has been sought by the community for decades and exploring many different materials, yet no one was successful, until now. This finding has implications for the applicability of spintronics, as our device operates like the long-sought-after Datta-Das spin transistor, which has been one of the goals of spintronics since it was first proposed in 1990.
In their paper, the researchers presented the first spin field-effect transistor at room temperature using the spin precession strategy they developed. In the future, their work could pave the way toward the practical implementation of energy-efficient spin-based logic.
The researchers said, our study also has a fundamental consequence, as it provides valuable information on how to spin transport is affected by the spin-orbit interactions in graphene-based van der Waals heterostructures. In our next studies, we plan to study multiple other combinations of 2D materials which will provide new physical effects related to the spin degree of freedom.
Journal Information: Electrical control of Valley-Zeeman spin-orbit-coupling-induced spin precession at room temperature. Physical Review Letters(2021). DOI: 10.1103/PhysRevLett.127.047202