Double success for PhD student with ‘spin relaxation’

University of Groningen PhD student Siddhartha Omar is having a good week. He has had two papers accepted by the journal Physical Review: Rapid Communications. And he is excited about both. They present new techniques and knowledge about ‘spin relaxation’.

Omar left India in 2014 to follow the Top Master’s programme in Nanoscience in Groningen. He did a research project in the lab of Prof. Bart van Wees (Physics of Nanodevices), this year’s winner of the Spinoza Prize, the most prestigious science award in the Netherlands. And he stayed here for his PhD.

Van Wees works on making spintronics a reality. While normal electronics relies on the charge carried by electrons, spintronics relies on another of their properties: a magnetic one called spin, which can take the values ‘up’ or ‘down’. It’s a quantum mechanical effect, but to put it simply, electron spin has a magnetic direction which can point up or down. This property can be used to store, transport and manipulate information.

Relaxation

‘The idea of spintronics has been around for a while, but semiconductors, which are the building blocks of microelectronics, proved unsuitable for this type of electronics’, explains Omar. Then it was discovered that graphene, the one-dimensional form of carbon, was an excellent conductor of not only electron currents, but also spin currents.

There is one snag: when one type of spin (up or down) is injected, it will ‘relax’ after a while and become a mixture of up and down. ‘Theorists predicted that the spin relaxation time in graphene would be very long, but experimental physicists found that relaxation occurred faster than predicted.’ That is a problem, for you can only use spin to transport information when it is stable. ‘The theme of my PhD project is to find out how spin relaxation occurs in graphene, and what affects it.’

The first of Omar’s two papers stems from his time as a Master’s student and describes a novel way to measure spin relaxation in graphene. ‘The normal way is to measure the average spin current, but we extracted the noise, the variation in spin transport’, he explains. This variation is caused by processes inside the graphene, and thus tells you what is happening there. Omar concluded that impurities in graphene contribute to spin relaxation. ‘This had already been described by theorists, but we are the first to prove it in an experiment.’ A bonus is that the noise can also be used to measure spin currents, ‘so it is an extra probe for measuring spin transport.’

Stacking

In the second paper, Omar made a device to show how a semiconductor material (tungsten disulphide, WS₂) would affect the spin current inside graphene. ‘If you stack different materials on a layer of graphene, you can influence its properties. Last year, our lab showed that graphene becomes magnetic when you put it on a magnetic material.’

Using a thin layer of WS₂, Omar succeeded in ‘absorbing’ spins from a graphene channel or injecting spin into the channel, depending on the current in the semiconductor. ‘This means we have a kind of switch to stop and start the spin flow’, says Omar. And rather than an on/off switch, it is more like a dimmer switch. ‘Changing the spin current makes this device behave like a transistor, in which the electron current can be switched using a gate current.’ How exactly the tungsten disulphide ‘absorbs’ the spins from the channel is unknown. ‘We need to do more work to understand this.’

Together, the papers describe several innovations in spintronics research. ‘We’ve shown that impurities play an important part in spin relaxation in graphene, we’ve built a kind of spin-transistor using the WS₂ switch and we’ve shown that the spin current can be controlled’, is how Omar puts it. ‘All this is important for creating a logic device that operates on spins.’

He is now in the final year of his PhD project, and more articles are in the pipeline. ‘The next paper will be even more interesting’, he says with a broad smile. To be continued…

References:

1. S. Omar, M.H.D. Guimarães, A. Kaverzin, B.J. van Wees, and I.J. Vera-Marun, Spin relaxation 1/f noise in graphene, Phys. Rev. B 95, 081403(R). DOI 10.1103/PhysRevB.95.081403

2. S. Omar and B.J. van Wees, ‘Graphene-WS₂ heterostructures for tunable spin injection and spin transport’, arXiv:1610.07196