Pep Ingla-Aynés, a PhD student in the nanodevices group headed by Bart van Wees at the University of Groningen, has obtained the longest spin relaxation length ever achieved. With a length of 24 micrometers he doubled the previous record distance, set by the same group.
And no, this is not some obscure Guinness Book of Records stuff. It is hard science, aiming to produce a new form of electronics called ‘spintronics’, which makes use of an intrinsic quantum property of electrons. Apart from a negative charge, they also carry spin, which could be viewed as (but as with so many quantum phenomena isn’t really) the rotation of the electron. Electron spin can have two values: either up, or down.
Spin can be used to store or transport information, just like charge. ‘But spin transport produces much less heat, so it’s more energy efficient’, explains Ingla-Aynés. There are other advantages to spintronics, but there are also some problems that need to be solved before spin-based logic circuits become a reality.
‘The spin dissipates due to interactions between electrons and atoms’, says Ingla-Aynés. This limits the distance over which spin transport is possible. Lighter atoms interact less with atoms, so carbon is a good material to use for spintronics. Graphene – a special, two-dimensional form of carbon – is a very good spin conductor, so scientists have been looking at ways to optimize spin transport in this material.
The key, says Ingla-Aynés, is purity. Impurities ruin spin transport. But there are other factors as well. ‘The number of layers of graphene is important, and previous experiments suggested bilayers could be more efficient.’ So Ingla-Aynés and his colleagues tried an approach using graphene bilayers. ‘Earlier work from a PhD student in our group showed that when you deposit the graphene on a layer of boron nitride, it increases the speed of spin transport.’ This meant the spin could travel a greater distance, but the relaxation time (the time taken for the spin to dissipate) was still the same.
In the new experiment, a bilayer of graphene was sandwiched between two layers of boron nitride. Spin transport was again fast, but the relaxation time was also longer. The result: a massive increase in the distance the spin could travel, from 12 to 24 micrometers at 4 degrees Kelvin. At room temperature, a still very respectable 13 micrometers of spin transport was achieved.
The previous record also originated from the Van Wees lab, and Ingla-Aynés was co-author of the paper describing it. Then competition was close, with another lab clocking 10 micrometers. Now, Ingla-Aynés is the first author of the paper with the new record, which appeared recently in Physics Review Letters B. The secret of this success? ‘We have a unique method to make stacks of graphene and other materials, which produces very clean samples. That means better spin transport. I’ve been trained by Marcos Guimares, the first author of the paper describing the previous record, and he is second author on this new paper.’
Ingla-Aynés is now working on a different issue. ‘We are trying to direct spin transport using an electric field. The transport we have measured so far is just diffusion, so it is not really directed. Our next job is to make the spin travel in a particular direction.’
Text: Science LinX
J. Ingla-Aynés1, M. H. D. Guimaraes1,2, R. J. Meijerink1, P. J. Zomer1, and B. J. van Wees1: 24-μm spin relaxation length in boron nitride encapsulated bilayer graphene, Physical Review B, DOI 10.1103/PhysRevB.92.201410
1Physics of Nanodevices, Zernike Institute for Advanced Materials, University of Groningen, The Netherlands
2Kavli Institute at Cornell, Cornell University, Ithaca, New York 14853, USA
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