Sketch of a graphene/WSe2 spin-field-effect transistor. The spins will reverse sign at zero backgate voltage (Vbg) when they propagate through the channel. Precession decreases when the Vbg does not reach zero and spins don't reverse signs. Credit: Ingla-Aynes et al.
Research in spintronics aims to co-ordinately manipulate electron spins at room temperatures using electric current. This is a crucial goal. This would allow for the development of many devices, including spin-field-effect transistors.
Engineers and physicists using conventional materials have only seen coherent spin precession at low temperatures and in the ballistic regime. Two-dimensional (2D) materials have unique properties that could allow for new control knobs to manage spin procession.
Recent spin precession experiments at room temperature were performed by researchers at CIC nanoGUNE-BRTA in Spain, and University of Regensburg in Germany. This was done in the absence of a magnetic fields in bilayer graphene. Their paper was published in Physical Review Letters. They used 2D materials to create a spin-field-effect transistor.
"In our group there is a long tradition in studying spin transport within multiple materials, like simple metals," Josep InglaAynes and Franz Herling, Jaroslav Fbian, Luis E. Hueso, Felix Casanova, who were the researchers behind the study, shared with Phys.org via email. "Our primary goal is to understand how spin of electrons can carry information, and how this degree can help us create new functionalities in devices."
Graphene is one of the most spin-relaxing materials. However, manipulating spins on graphene is difficult and can only be done using external magnetic fields. This is not ideal for practical applications.
Ingla-Ayns has been working with his colleagues to examine how heterostructures made from different 2D materials (also known as van der Waals heterostructures) perform in spintronics. Van der Waals heterostructures are graphene-based 2D material with layers that have not been chemically bonded.
The researchers said that they have been particularly interested in structures in which a material that has weak spin-orbit co-ordination (such as graphene), is stacked with a material having a strong spin orbit coupling (such WSe 2) and observed experimentally how the spin-orbit transferling into graphene is actually done by proximity. Technically, it is possible to imprint an efficient spin-orbit coupling on graphene (that acts like a magnetic field) which can reverse spins. This is possible by creating strong interactions between layers.
Ingla-Ayns used two materials, but not one. His colleagues combined them to create a mixture of two different material with significant properties. The graphene is the first, with its weak spin-orbit coupling but long spin relaxation length. The second material is WSe 2. It has strong, anisotropic spin orbit coupling.
Researchers stated that they prepared bilayer graphene/WSe2 van der Waals heterostructures by using a dry polymer based stacking technique. To promote the proximity between layers, we annealed the samples at 400 degrees Celsius. We used ferromagnetic electrodes to measure spin transport. These electrodes, when combined with magnetic fields allow us to measure the in-plane as well as out-of-plane travels of spins across the graphene/WSe2 channel.
Ingla-Ayns, along with his colleagues, were able control spin transport times in their material by applying an in plane electric field to it and a backgate potential to them. This enabled electrical control of spin precession at ambient temperature without the need for an external magnetic field.
The researchers stated that this was a long-standing goal of the community, which has explored many materials and failed to find one. This finding is important for spintronics because it resembles the Datta-Das spin transistor which has been a goal of spintronics ever since its inception in 1990.
The paper presented the first room-temperature spin field effect transistor using their spin precession strategy. Their work could lead to the practical implementation and optimization of spin-based logic that is more efficient.
The researchers stated that their study has a fundamental consequence. It provides valuable information about how spin transport is affected in van der Waals heterostructures made of graphene. "In our next studies we plan to examine multiple combinations of 2D materials that will provide new physical effects associated with the spin degree of freedom."
Continue reading Scientists create electronic 2-D spin transistors
More information: Electrical control of Valley-Zeeman spin-orbit-coupling-induced spin precession at room temperature. Physical Review Letters (2021). Journal information: Physical Review Letters Electrical control of Valley-Zeeman spin-orbit-coupling-induced spin precession at room temperature.(2021). DOI: 10.1103/PhysRevLett.127.047202
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