The word \u201cfractals\u201d might inspire images of psychedelic colors spiraling into infinity in a computer animation. An invisible, but powerful and useful, version of this phenomenon exists in the realm of dynamic magnetic fractal networks.<\/p>\n
Dustin Gilbert, assistant professor in the Department of Materials Science and Engineering, and colleagues have published new findings in the behavior of these networks\u2014observations that could advance neuromorphic computing capabilities.<\/p>\n
Their research is detailed in their article \u201cSkyrmion-Excited Spin-Wave Fractal Networks,\u201d the cover story for the August 17, 2023, issue of\u00a0Advanced Materials<\/em>.<\/p>\n \u201cMost magnetic materials\u2014like in refrigerator magnets\u2014are just comprised of domains where the magnetic spins all orient parallel,\u201d said Gilbert. \u201cAlmost 15 years ago, a German research group discovered these special magnets where the spins make loops\u2014like a nanoscale magnetic lasso. These are called skyrmions.\u201d<\/p>\n Named for legendary particle physicist Tony Skyrme, a skyrmion\u2019s magnetic swirl gives it a non-trivial topology. As a result of this topology, the skyrmion has particle-like properties\u2014they are hard to create or destroy, they can move and even bounce off of each other. The skyrmion also has dynamic modes\u2014they can wiggle, shake, stretch, whirl, and breath.<\/p>\n As the skyrmions \u201cjump and jive,\u201d they are creating magnetic spin waves with a very narrow wavelength. The interactions of these waves form an unexpected fractal structure.<\/p>\n \u201cJust like a person dancing in a pool of water, they generate waves which ripple outward,\u201d said Gilbert. \u201cMany people dancing make many waves, which normally would seem like a turbulent, chaotic sea. We measured these waves and showed that they have a well-defined structure and collectively form a fractal which changes trillions of times per second.\u201d<\/p>\n Fractals are important and interesting because they are inherently tied to a \u201cchaos effect\u201d\u2014small changes in initial conditions lead to big changes in the fractal network.<\/p>\n \u201cWhere we want to go with this is that if you have a skyrmion lattice and you illuminate it with spin waves, the way the waves make its way through this fractal-generating structure is going to depend very intimately on its construction,\u201d said Gilbert. \u201cSo, if you could write individual skyrmions, it can effectively process incoming spin waves into something on the backside\u2014and it\u2019s programmable. It\u2019s a neuromorphic architecture.\u201d<\/p>\n The Advanced Materials cover illustration depicts a visual representation of this process, with the skyrmions floating on top of a turbulent blue sea illustrative of the chaotic structure generated by the spin wave fractal.<\/p>\n \u201cThose waves interfere just like if you throw a handful of pebbles into a pond,\u201d said Gilbert. \u201cYou get a choppy, turbulent mess. But it\u2019s not just any simple mess, it\u2019s actually a fractal. We have an experiment now showing that the spin waves generated by skyrmions aren\u2019t just a mess of waves, they have inherent structure of their very own. By, essentially, controlling those stones that we \u2018throw in,\u2019 you get very different patterns, and that\u2019s what we\u2019re driving towards.\u201d<\/p>\n The discovery was made in part by neutron scattering experiments at the Oak Ridge National Laboratory (ORNL) High Flux Isotope Reactor and at the National Institute of Standards and Technology (NIST) Center for Neutron Research. Neutrons are magnetic and pass through materials easily, making them ideal probes for studying materials with complex magnetic behavior such as skyrmions and other quantum phenomena.<\/p>\n Gilbert\u2019s co-authors for the new article are Nan Tang, Namila Liyanage, and Liz Quigley, students in his research group; Alex Grutter and Julie Borchers from National Institute of Standards and Technology (NIST), Lisa Debeer-Schmidt and Mike Fitzsimmons from Oak Ridge National Laboratory; and Eric Fullerton, Sheena Patel, and Sergio Montoya from the University of California, San Diego.<\/p>\n The team\u2019s next step is to build a working model using the skyrmion behavior.<\/p>\n \u201cIf we can develop thinking computers, that, of course, is extraordinarily important,\u201d said Gilbert. \u201cSo, we will propose to make a miniaturized, spin wave neuromorphic architecture.\u201d<\/p>\n He also hopes that the ripples from this UT Knoxville discovery inspire researchers to explore uses for a spiraling range of future applications.<\/p>\n
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