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A*STAR research centres on spintronics

Posted: 06 Feb 2013 ?? ?Print Version ?Bookmark and Share

Keywords:spintronics? electron gas? spin polarisation?

A team of researchers from A*STAR Data Storage Institute has revealed the details of a theoretical research focusing on spintronics, a form of signal processing used in traditional electronics but takes advantage of a property of electrons known as spin. Spin is often visualized as an arrow about which the electron rotates, much like a top spinning around its axis. Generating a stream of electrons in which these 'arrows' are all parallela so-called spin-polarized currentis the foundation upon which spintronics is based. Imperfections in a material, however, can easily destroy polarisation. Simply applying an oscillating voltage across the device could help to maintain a spin-polarized current even in the presence of impurities, the team added.

Seng Ghee Tan and his colleagues considered a 2D electron gas: a system in which the electrons can move only in one plane. When a spin-polarized current flows through such a material, the spins interact with the electron's motion through an effect known as Rashba spin-orbit coupling. This makes the spins start to 'wobble' or precess: at first they point upwards but then point downwards, and this reduces the total spin polarisation to zero. "We want to prolong the life span of a spin current in the channel by controlling the strength of the Rashba coupling," noted Tan. To this end, he and his team investigated a device, known as a spin-current rectifier, that lets a spin current flow with one particular polarisationupwards only, for example.

The researchers developed a simple mathematical equation that predicts the behaviour of the spin current as an alternating voltage is applied across the device. Their model shows that when the frequency of the voltage is zero, the spin polarisation goes back and forth as expected. "However, by increasing the frequency, we see an increasingly asymmetrical pattern of oscillation in favour of positive polarisation," said Tan. "We call this a gradual process of rectification."

Their approach can even suppress precessional motion entirely. When the external modulation frequency is much faster than the natural precessional frequency of the spins, known as the Larmor frequency, the spins have no time to change direction so remain pointing upwards. Consequently, the system maintains a spin-polarized current.

Once spin currents can be sustained, spintronics will have all the potential of electronics with the additional advantage of an extra degree of control. The spin-current rectifier investigated by Tan and his co-workers could therefore become a vital component in this future technology.





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