HIGH PERFORMANCE COMPUTING WITH SPINTRONICS

 

Electron ‘spin’ could hold the key to managing the world’s growing data demands without consuming huge amounts of energy. Now, researchers have shown that energy-efficient superconductors can power devices designed to achieve this. What once seemed an impossible marriage of superconductivity and spin may be about to transform high performance computing.

One shouldn’t lose sight of what we are doing here. We aren’t just trying to do something better; we are offering something entirely different and new.

Jason Robinson

In the early days of the computer, calculators were room-sized and public demand was low. Now, it’s the reverse. Digital technology has become smaller and faster, and our dependence on it has grown.

We are almost desensitised to a stream of facts about the startling rate at which this is occurring. In 2016, IBM found that humans now create 2.5 quintillion bytes of data daily. From the start of this decade to its end, the world’s data will increase 50 times over.

The basic building blocks of electronic devices, such as the transistor, work by moving packets of charge around a circuit. A single unit of charge is an electron, and its movement is governed by semiconductors, commonly made from silicon. But technology based on these principles is now reaching a point where it cannot get much smaller or faster. A paradigm shift is due.

“There have been many failed attempts to oust silicon from its predominance,” reflects Professor Mark Blamire, Head of Materials Science at Cambridge. “Something has to be done because the technology can’t be scaled to smaller sizes for very much longer. It’s already a major source of power consumption. There’s no obvious competitor, so in a sense the opportunity is there.”

Blamire and his colleague Dr Jason Robinson are leading several major programmes investigating one such competitor, known as superconducting spintronics.

The launch of a UK-based programme last year provoked excitement within the scientific community. “Cambridge Uni spins up green and beefy supercomputer project,” announced British tech site The Register, for example. One reason in particular is because superconducting spintronics might address the eye-watering energy consumption of the huge server farms that handle internet traffic. Data centres account for 3% of the world’s electricity supply and about 2% of greenhouse gas emissions.

The project combines two phenomena: superconductivity and spin. Superconductivity refers to the fact that at low temperatures some materials carry a charge with zero resistance. Unlike, for example, copper wires, which lose energy as heat, superconductors are therefore extremely energy efficient.

‘Spin’ is the expression for electrons’ intrinsic source of magnetism. Originally it was thought that this existed because electrons were indeed spinning, which turned out to be wrong, but the name stuck, and it is still used to describe the property in particles that makes them behave a bit like tiny bar magnets. Like a magnet, this property makes the electrons point a certain way; the spin state is therefore referred to as ‘up’ or ‘down’.

Researchers have been using the magnetic moments of electrons to store and read data since the 1980s. At their most basic, spintronic devices use the up/down states instead of the 0 and 1 in conventional computer logic.

Spintronics could also transform the way in which computers process information. The researchers envisage that instead of the devices moving packets of charge around, they will transmit information using the relative spin of a series of electrons, known as a ‘pure spin current’, and sense these using magnetic elements within a circuit.

By eliminating the movement of charge, any such device would need less power and be less prone to overheating – removing some of the most significant obstacles to further improving computer efficiency. Spintronics could therefore give us faster, energy-efficient computers, capable of performing more complex operations than at present.

To generate large enough spin currents for memory and logic devices, significant charge is required as an input, and the power requirements of this currently outweigh many of the benefits. Using a superconductor to provide that charge, given its energy efficiency, would present a solution. But the magnetic materials used to control spin within spintronic devices also interfere with superconductivity.

This problem was thought insurmountable until, in 2010, Robinson discovered how to combine superconductors and spintronics so that they can work together in complete synergy. His team added an intervening magnetic layer (a material called holmium). By using this interface, they were able to preserve the delicate balance of electron pairing that’s needed to achieve superconductivity, but still managed to create a bias within the overall spin of the electrons.

This, explains Robinson, “created a marriage that opens up the emerging field of superconducting spintronics.” Over the next five years, he and Blamire developed the field, and last year were awarded a major grant from the Engineering and Physical Sciences Research Council: “To lead the world in understanding the coupling of magnetism and superconductivity to enable future low energy computing technologies.” Robinson has since been awarded a second grant with Professor Yoshi Maeno, from the University of Kyoto, to broaden materials research on superconducting spintronics.

Although still at an experimental stage, the project – which includes collaborators from Imperial College London, University College London and Royal Holloway London – is tackling questions such as how to generate and control the flow of spin in a superconducting system. And its scope is already expanding. “We have found more ways of achieving what we are trying to do than we originally dreamed up,” Robinson says.

One example involves making potentially innovative use of superconductivity itself. In ‘conventional’ spintronics, spin is manipulated through the interactions between magnetic materials within the device. But Blamire has found that when a superconductor is placed between two ferromagnets, its intrinsic energy depends on the orientation of those magnetic layers. “Turning that on its head, if you can manipulate the superconducting state, you can control the orientation of the magnetic layers, and therefore the spin,” he says.

Meanwhile, Robinson has led a study that for the first time enabled graphene, a material already recognised for its potential to revolutionise the electronics industry, to superconduct. This raises the possibility of using this extraordinary material, and other two-dimensional materials like it, in superconducting spintronics.

Although approaches like this are still being tested, Blamire says that by 2021 the team will have developed sample logic and memory devices that fuse superconductivity and spin. These proof-of-concept models could, perhaps, be incorporated into a new type of computer processor. “It would be a huge step to get from there to a device that could be competitive,” he admits. “It’s not necessarily difficult, but it would require considerable investment.”

The project is set up to enable industrial collaboration in the years to come. A key partner is the Hitachi Lab in Cambridge, while the project’s advisory board also features representatives from the Cambridge-based semiconductor firm ARM, and HYPRES, a digital superconductor company in the USA.

Robinson points out that the UK – and Cambridge in particular – has historical strengths in research into superconductivity and spintronics, but adds that a “grand challenge” has long been needed to focus academic investigation on a meaningful partnership with industry.

Leading low-energy computing into a post-semiconductor age is certainly grand. Silicon’s domination, after all, stretches from its eponymous valley in California, to a fen in Cambridge, a gulf in the Philippines and an island in Japan.

Can the unlikely – not to say still primitive – marriage of spintronics and superconductivity really replace an electronic empire on which the sun never sets? “I suspect people had similar questions at the dawn of the semiconductor,” Robinson observes. “One shouldn’t lose sight of what we are doing here. We aren’t just trying to do something better; we are offering something entirely different and new.”

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