WHY THIS MATTERS IN BRIEF
Computer circuits that don’t use electricity or generate heat will help us create a wide range of new computers and devices.
Researchers at MIT in the US have developed the first in what they hope to be a revolutionary new type of electrical computer circuit that, counter intuitively, and unlike any other computer today on the planet, doesn’t use any electricity to work. Instead it uses a funky phenomenon called Spintronics which I’ll discuss in a moment, and it’s thought that the development will help open the way to helping us create practical so called Magnetic-based Electronic devices that have the potential to compute far more efficiently than today’s electronic systems – just imagine, for example, being able to run a computing farm or a computer datacentre without needing to use or rely on electricity and you’ve got the vision right there.
Needless to say today’s classical computers use massive amounts of electricity for their computing and data storage needs and generate a lot of wasted heat, so in search of more efficient and sustainable alternatives there are now several researcher groups around the world that have started developing magnetic-based “Spintronic” devices, which use little to no electricity and generate practically no heat.
Spintronic devices leverage the “Spin wave” which is a quantum property of electrons in magnetic materials that have a lattice structure and the approach involves modulating the spin wave properties to produce some measurable output that can be correlated to computation. Until now, modulating spin waves has required injected electrical currents using bulky components that can cause signal noise and effectively negate any inherent performance gains.
The researchers developed a circuit architecture that uses only a nanometer-wide domain wall in layered nanofilms of magnetic material to modulate a passing spin wave, without any extra components or electrical current. In turn, the spin wave can be tuned to control the location of the wall, as needed. This provides precise control of two changing spin wave states, which correspond to the 1s and 0s used in classical computing. A paper describing the circuit design was published today in Science.
In the future, pairs of spin waves could be fed into the circuit through dual channels, modulated for different properties, and combined to generate some measurable quantum interference — similar to how photon wave interference is used for quantum computing. Researchers hypothesise that such interference-based spintronic devices, like quantum computers, could execute highly complex tasks that conventional computers struggle with.
“People are beginning to look for computing beyond silicon. Wave computing is a promising alternative,” says Luqiao Liu, the professor who led the project. “By using this narrow domain wall, we can modulate the spin wave and create these two separate states, without any real energy costs. We just rely on spin waves and intrinsic magnetic material.”
Spin waves are ripples of energy with small wavelengths. Chunks of the spin wave, which are essentially the collective spin of many electrons, are called magnons. While magnons are not true particles, like individual electrons, they can be measured similarly for computing applications.
In their work, the researchers used a customised “magnetic domain wall,” a nanometer-sized barrier between two neighbouring magnetic structures. They then layered a pattern of Cobalt-Nickel nanofilms — each a few atoms thick — with certain desirable magnetic properties that could handle a high volume of spin waves. Then they placed the wall in the middle of a magnetic material with a special lattice structure, and incorporated the system into a computing circuit.
On one side of the circuit, the researchers excited constant spin waves in the material. As the wave passes through the wall, its magnons immediately spin in the opposite direction: Magnons in the first region spin north, while those in the second region — past the wall — spin south. This causes the dramatic shift in the wave’s phase (angle) and slight decrease in magnitude (power).
In experiments, the researchers placed a separate antenna on the opposite side of the circuit, that detects and transmits an output signal. Results indicated that, at its output state, the phase of the input wave flipped 180 degrees. The wave’s magnitude — measured from highest to lowest peak — had also decreased by a significant amount.
Then, the researchers discovered a mutual interaction between spin wave and domain wall that enabled them to efficiently toggle between two states. Without the domain wall, the circuit would be uniformly magnetized; with the domain wall, the circuit has a split, modulated wave.
By controlling the spin wave, they found they could control the position of the domain wall. This relies on a phenomenon called, “Spin-transfer torque,” which is when spinning electrons essentially jolt a magnetic material to flip its magnetic orientation.
In the researchers’ work, they boosted the power of injected spin waves to induce a certain spin of the magnons. This actually draws the wall toward the boosted wave source. In doing so, the wall gets jammed under the antenna — effectively making it unable to modulate waves and ensuring uniform magnetisation in this state.
Using a special magnetic microscope, they showed that this method causes a micrometer-size shift in the wall, which is enough to position it anywhere along the material block. Notably, the mechanism of magnon spin-transfer torque was proposed, but not demonstrated, a few years ago.
“There was good reason to think this would happen,” Liu says. “But our experiments prove what will actually occur under these conditions.”
The whole circuit is like a water pipe, Liu says. The valve (domain wall) controls how the water (spin wave) flows through the pipe (material). “But you can also imagine making water pressure so high, it breaks the valve off and pushes it downstream,” Liu says. “If we apply a strong enough spin wave, we can move the position of domain wall — except it moves slightly upstream, not downstream.”
Ultimately such innovations could enable practical wave-based computing for specific tasks, such as the signal-processing technique, called “fast Fourier transform,” and next the researchers hope to build a working wave circuit that can execute basic computations. Among other things, they have to optimise materials, reduce potential signal noise, and further study how fast they can switch between states by moving around the domain wall.
“That’s next on our to-do list,” Liu says.