From rust to riches

4th February 2021

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Could rust be the secret to next-gen computing technology?

  •  Artist’s impression of the connection of rust with cosmic strings

    Artist’s impression of the connection of rust with cosmic strings created by R. Shetty, K. Jani and Hariom Jani 

Current silicon-based computing technology is incredibly energy-inefficient and Wadham Fellow, Professor Paolo Radaelli from Oxford’s Department of Physics, working with Diamond Light Source, has been leading research into silicon alternatives. His group’s surprising findings are published in Nature today.

This article, from the Oxford Science Blog share's Paolo's findings.

By 2030, information and communications technology (ICT) is projected to gobble up over 20% of the global electricity production. After the end of the pandemic, climate change is likely to resume its position as the number one challenge for humanity and finding ways decarbonise tech is an obvious target for energy savings.

So just how energy-inefficient are computers? Well, the human brain uses approximately a millionth of the energy required by a computer to perform the same operation; to put that in context, the human brain uses about 12 watts of power, while an equivalent computer would use 12 megawatts – the output of a small power station. However, because of its immense success and ubiquitous nature, it is very difficult to replace silicon technology.

The promise of oxide electronics

Researchers have been working for a long time on alternative technologies that might afford greater efficiency. Oxides of common metals such as iron and copper are natural targets for this research – not least as oxides are already a technology staple and are present in silicon-based computers so there is a high chance of compatibility between the two technologies. There is one obvious drawback: oxides are great to store information but not so much to move information around– and the latter is required for computation. One property of oxides however is that many are magnetic and a number of ideas have emerged recently on how it might be possible to move around magnetic ‘bits’, both in oxides and in other magnets, with very little energy being required.

Tiny ‘cosmic strings’

The kinds of bits we are talking about here must be tiny – ten nanometres (ten billionths of a metre, about 20 times the diameter of an atom) is the typical target figure – and must be robust even when ‘shaken and stirred’. This is very challenging, because the risk of them being simply dissipated away is very high when the bit is small. One possible solution came from the most unlikely of directions: a curious parallel between solid-state physics and cosmology. In fact, the inspiration for this project was set in the form of a challenge: can we replicate cosmic strings in a magnet?

So what are cosmic strings – and do they even exist? Cosmic strings are supposed to be filaments in space, much thinner than an atom but potentially as long as the distance between stars. But do they exist? Certain cosmological theories predict that they could have formed instants after the Big Bang as the universe was cooling rapidly; interestingly, once formed, cosmic strings would be stable and do not ‘evaporate’, so astronomers may be able to discover them in the future.

But what do cosmic strings have to do with computers? The relevance comes from the fact that the mathematical description of cosmic strings is rather simple, and the same kind of mathematical conditions that favour the formations of strings may be found in many other physical systems, including magnets. It is the beauty of physics: mathematical equations describing the ‘macrocosm’ at parsec scales may also work in the microcosm at nanometre scales. With the challenge set, all that remained to do was to find a suitable magnet. Once again, the candidate turned out to be most unlikely: common rust.

Rust to riches

Iron oxide (chemical formula Fe2O3) is a main constituent of rust. Each iron atom acts as a tiny compass, but this particular form of Fe2O3 is not magnetic in the ordinary sense of attracting and being attracted by other magnets: it is an antiferromagnet, so that half of the Fe compasses point ‘north’ and the other half ‘south’.

Two years ago, working at the Diamond Light Source on samples produced at University of Wisconsin, Madison, our Oxford group had already discovered the magnetic equivalent of cosmic strings in Fe2O3, and imaged them using a powerful X-ray microscope. In essence, these tiny objects known as ‘merons’ are magnetic whirls, where the compass needle rotates (NESW or NWSE) as one moves from one atom to the next in a nanometre-scale loop. With hindsight, finding magnetic merons was a huge stroke of luck, since we know that they are very difficult to stabilise in the conditions used for that first experiment. For the paper published today, we extended our collaboration to the National University of Singapore and managed to find the key to create and destroy magnetic merons at will, exploiting the mathematical equivalent of the ‘Big Bang cooling’.

Back to the future

This is undoubtedly fascinating basic research, but how realistic are the prospects of using ‘rust’ in super-efficient computers? We are optimistic: though very simple in architecture, the Fe2O3-based device where merons and bimerons were found already contains all the ingredients to manipulate these tiny bits quickly and efficiently – by flowing a tiny electrical current in an extremely thin metallic ‘overcoat’. Indeed, controlling and observing the movement of merons and bimerons in real time is the goal of a future X-ray microscopy experiment, currently in the planning phase.

When moving from basic to applied research, cost and compatibility considerations are also of paramount importance. Iron oxide itself is extremely abundant and cheap, but the fabrication techniques employed by colleagues at Singapore and Madison are rather complex and require atomic-scale control. Here again, we are optimistic: very recently, they demonstrated that is possible to ‘peel off’ a thin layer of oxide from its growth medium and stick it almost anywhere, its properties being largely unaffected. Next steps? Design and fabrication of proof-of-principle devices based on ‘cosmic strings’ to follow in short order…