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What Are Skyrmions?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

In 1962, Tony Skyrme developed a hypothetical object in which the vectors of a magnetic field are twisted and knotted in such a way that they result in a spin-effect or in a radioactive pattern inside a shell depending on the desired outcome, resulting in a 3D object that acts like a particle. The topology, or the math used to describe the shape and properties of the object, is considered non-trivial, aka difficult to describe. The key is that the surrounding magnetic field is still uniform and that only this smallest possible area has been affected. It was named a skyrmion after him and for years they were just a useful tool in finding properties of subatomic particle interactions but no evidence for their actual existence was found at the time. But as the years progressed, signs of their existence were found (Masterson, Wong)

Creating a skyrmion.

Creating a skyrmion.

From Theory to Confirmation

In 2018, scientists from Amherst College and The Aalto University in Finland made a skyrmion using an “ultra-cold quantum gas.” Conditions were right for a Bose-Einstein condensate to form, a kind of coherence atoms reach that make the system act as one. From here, they selectively changed the spin of some atoms so they pointed in an applied magnetic field. When electric fields were then activated in opposite directions, no charge was present and the atoms with the altered spin started to move about and form a knot of orbiting particles, an “interlocking rings system” – a skyrmion – which is about 700-2000 nanometers in size. The magnetic field lines in them begin to link in a closed causality, becoming linked in complex ways and the particles on those orbits spin in a spiraling pattern along their orbit. And interestingly, it seems to operate much like ball lightning does. Is there a possible connection or just happenstance? It would be hard to imagine such a quantum process in a room temperature, macroscopic level environment but maybe some parallels could exist (Masterson, Lee, Rafi, Wang).

Skyrmions need magnetic fields to operate so naturally magnetic would be ideal places to spot them. Scientists have observed spin textures that match the patterns associated with skyrmions, depending on the topology of the situation. Scientists from MLZ studied Fe­1-xCoxSi (x=0.5), a helimagnet, to see “topological stability and phase conversion” of skyrmions collapsing as the material transitions back to a helimagnet. That is because the magnets contain skyrmion lattices, which are crystal in nature and are therefore rather regular. The team used magnetic force microscopy we well as small-angle neutron scattering in their efforts to map out the decay of the skyrmions in the lattice. Using these details, they were able to witness the lattice form in the magnet as fields were reduced, capturing detailed images that can assist in the decay models scientists are running (Milde).

The skyrmion spectrum.

The skyrmion spectrum.

Potential Memory Storage

That crazy knotting effect of skyrmions wouldn’t seem to have any applications, but then you may not have met some creative scientists. One such idea is memory storage, which is really just the manipulation of set magnetic values in electronics. With skyrmions, only a small amount of current would be needed to accelerate the particle, making it a low-power option. But if skyrmions were to be used in this fashion, we would need them to exist in close quarters to each other. If each one was oriented a little differently that would reduce the chances of them interacting with each other, enabling contrasting fields to keep each at bay. Xuebing Zhao and team took a look at skyrmion clusters inside FeGe nanodisks “using Lorentz transmission electron microscopy,” to see how they operated. The cluster which formed at low temperature (near 100 K) was a group of three that got closer together as the overall magnetic field increased. Eventually, the magnetic field was so great that two of the skyrmions canceled each other out and the final one was unable to sustain itself and so collapsed. The situation did change with higher temperatures (near 220 K), with 6 appearing instead. Then as the magnetic field was increased, it became 5 as the center skyrmion disappeared (leaving a pentagon). Further increased whittle down the number to 4 (a square), 3 (a triangle), 2 (a double bell) and then 1. Interestingly, the lone skyrmions were not pinned to the center of the former cluster, possibly because of defects in the material. Based on the readings, an H-T phase diagram comparing field strength to temperature for these magnetic objects was found, similar in principle to a matter phase change diagram (Zhao, Kieselev).

Another possible orientation for memory storage is skyrmion bags, which can best be described as nestling-skyrmion-dolls. We can have groupings of skyrmions which in concert act like individual ones, creating a new topology for us to work with. Work by David Foster and team showed the different configurations were possible so long as the right manipulation of fields as well as sufficiently energy was present to place the skyrmions into other ones by expanding some while moving others (Foster).

Sounds crazy, I know, but isn’t that the way of the best scientific ideas?

Works Cited

Foster, David et. al. “Composite Skyrmion bags in two-dimensional materials.” arXiv:1806.0257v1.

Kieselev, N.S. et al. “Chiral skyrmions in thin magnetic films: new objects for magnetic storage technologies?” arXiv:1102.276v1.

Lee, Wonjae et al. “Synthetic electromagnetic knot in a three dimensional skyrmion.” Sci. Adv. Mar. 2018.

Masterson, Andrew. “Ball lightning on a quantum scale.” Cosmosmagazine.com. Cosmos, 06 Mar. 2018. Web. 10 Jan. 2019.

Milde, P. et al. “Topological unwinding of a Skyrmion lattice by magnetic monopoles.” Mlz-garching.de. MLZ. Web. 10 Jan. 2019.

Rafi, Letzer. “The ‘Skyrmion’ May Have Solved the Mystery of Ball Lightening.” Livescience.com. Purch Ltd., 06 Mar. 2018. Web. 10 Jan. 2019.

Wang, X.S. “A theory on skyrmion size.” Nature.com. Springer Nature, 04 Jul. 2018. Web. 11 Jan. 2019.

Wong, S.M.H. “What exactly is a Skyrmion?” arXiv:hep-ph/0202250v2.

Zhao, Xuebing et al. “Direct imaging of magnetic field-driven transitions of skyrmion cluster states in FeGe nanodisks.” Pnas.org. National Academy of Sciences of the United States of America, 05 Apr. 2016. Web. 10 Jan. 2019.

© 2019 Leonard Kelley

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