The research published in Nature Communications
, outlines how the electron spins were achieved from the interplay between the blilayer of graphene and boron nitride, used in the device. The result is the increase of the spin signal a hundredfold, which renders it large enough to be used in real life applications.
Spin injection and detection
''Spin'' is a term used to describe the magnetic property of electrons, which can be detected as a magnetic field with either up and down orientations. The science of spintronics seeks to exploit this phenomenon, with one of the most common applications being for the storage, transportation and manipulation of information. Yet exploiting electron spin in a device requires control over the ratio of electrons with a spin up or down, known as spin polarisation. However, this has been notoriously difficult to achieve, with the ratio of ups and downs remaining small.
The research presented in the paper and drawing on work from the EU-funded GRAPHENECORE1 project (itself part of the EU''s 10 year, Graphene Flagship started in 2013), is based on ongoing investigation into spin behaviour in different materials. As the research lead Professor Bart van Wees of University of Groningen says, ‘Spin polarisation can be achieved by sending the electrons through a ferromagnetic material.’ This creates an excess of one type of spin. The study looked specifically at spin injection - getting electrons with polarized spins into a device - and detection.
The team were able to demonstrate that they could make the injection and detection of spin electrons into graphene more efficient by using a sandwich of materials. At the core was a one atom thick layer of graphene resting on a boron nitride insulator layer, itself on top of a silicon semiconductor. Above the graphene was a very thin layer of boron nitride, a few atoms thick, to protect the electrons in the graphene.
‘Graphene is a very good material for spin transport, but it does not allow one to manipulate the spins,’ the professor explains. ‘To inject spins into the graphene, one has to make them pass from a ferromagnet through a boron nitride insulator by quantum tunnelling. We found that using a two-atom layer of boron nitride resulted in a very strong spin polarisation of up to 70 percent, 10 times what we usually get.’ With a similar tenfold increase in spin detection observed, overall the signal increased by a factor of 100.
Unexpected results and potential applications
An unexpected result for the devices produced was that voltage could increase polarisation, running counter to the prevailing wisdom that only ferromagnetic material could polarises spin. It seems that spin polarisation is generated from the quantum tunnelling used to inject spins into the graphene of the research devices.
These findings open up many possibilities. As the professor speculates, ‘We can now inject spins into graphene and measure them easily after they travel some distance. One application would be as a detector for magnetic fields, which will affect the spin signal.’ Another option could be to build a spin logic gate or a spin transistor.
One of the key focus areas of the Graphene Flagship is to develop techniques to scale-up the production of high-quality graphene, which will also integrate the results of its various initiatives to maximise the workflow and quality of the end-products.
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