The discovery gives a magnetic twist to neuromorphic computing

The word “fractals” might inspire images of psychedelic colors spiraling into infinity in a computer animation. An invisible, yet powerful and useful version of this phenomenon exists in the realm of dynamic magnetic fractal networks.

Dustin Gilbert, assistant professor in the Department of Materials Science and Engineering, and colleagues have published new findings about the behavior of these network observations that could advance neuromorphic computing capabilities.

Their research is detailed in their article “Skyrmion-Excited Spin-Wave Fractal Networks,” cover story for the Aug. 17, 2023 issue of Advanced material.

“Most magnetic materials, like refrigerator magnets, consist only of domains where the magnetic spins are all oriented parallel,” Gilbert said. “Nearly 15 years ago, a German research team discovered these special magnets whose twists form loops similar to a nanoscale magnetic lasso. These are called skyrmions.”

Named after legendary particle physicist Tony Skyrme, a skyrmion’s magnetic vortex gives it a non-trivial topology. As a result of this topology, skyrmion has particle-like properties, difficult to create or destroy, they can move and even bounce off each other. The skyrmion also has dynamic modes: it can wiggle, shake, stretch, spin and breathe.

As skyrmions “jump and move”, they create magnetic spin waves with a very narrow wavelength. The interactions of these waves form an unexpected fractal structure.

“Just like a person dancing in a pool of water, they generate waves that ripple outward,” Gilbert said. “Many people dancing create many waves, which would normally look like a turbulent and chaotic sea. We have measured these waves and shown that they have a well-defined structure and collectively form a fractal that changes trillions of times per second.”

Fractals are important and interesting because they are intrinsically linked to a “chaos effect”: small changes in the initial conditions lead to large changes in the fractal network.

“The point we want to get at is that if you have a skyrmion lattice and you light it with rotational waves, how the waves make their way through this fractal-generating structure will very intimately depend on its construction,” he said. Gilbert. . “So if we could write individual skyrmions, they could actually process the spin waves coming into something on the back and it’s programmable. It’s a neuromorphic architecture.”

THE Advanced material the cover illustration shows a visual representation of this process, with the skyrmions floating on a turbulent blue sea illustrating the chaotic structure generated by the spin wave fractal.

“Those waves interfere just like throwing a handful of pebbles into a pond,” Gilbert said. “You get an unstable, turbulent mess. But it’s not just a mess, it’s actually a fractal. We now have an experiment that shows that the spin waves generated by skyrmions aren’t just a mess of waves, they have an intrinsic structure of their own structure.” very personal. Essentially, by controlling those stones that we “throw in,” we get very different patterns, and that’s where we’re going.”

The discovery was made in part by neutron scattering experiments at the Oak Ridge National Laboratory (ORNL) High Isotope Flux Reactor and at the National Institute of Standards and Technology (NIST) Center for Neutron Research. Neutrons are magnetic and pass easily through materials, making them ideal probes for studying materials with complex magnetic behaviors such as skyrmions and other quantum phenomena.

Gilbert’s coauthors for the new paper are Nan Tang, Namila Liyanage, and Liz Quigley, students in his research group; Alex Grutter and Julie Borchers of the National Institute of Standards and Technology (NIST); Lisa DeBeer-Schmidt and Mike Fitzsimmons of Oak Ridge National Laboratory; and Eric Fullerton, Sheena Patel and Sergio Montoya of the University of California, San Diego.

The team’s next step will be to build a working model using the behavior of skyrmions.

“If we can develop thinking computers, that, of course, is tremendously important,” Gilbert said. ‘Then, we will propose to make a miniaturized spin-wave neuromorphic architecture.’ He also hopes the ripples from this UT Knoxville discovery will inspire researchers to explore uses for a dizzying array of future applications.