Skyrmions - magnetic flowers blooming on spiral magnets

[image1_illustration of a skyrmion]  https://phys.org/news/2017-05-synchronized-skyrmion.html

As I was surfing the internet, I found an interesting image[image 4] - a hedgehog configuration of a skyrmion. I had been wondering about whether a one pole magnet can exist, warping a magnet's shape. The image seemed as if magnetic force pointed outward from the sphere. 

Skyrmions

The concept of a skyrmion was proposed by Tony Skyrme as a model of the nucleon. Skyrmions are also found in spiral magnets when an external magnetic field is applied. Bubbles of skyrmion form on the surface of the magnet, and the direction of the magnetic field changes as it approaches the center of the skyrmion. 

 

[image2_skyrmion lattice on a spiral magnet] https://www.nanowerk.com/nanotechnology-news/newsid=47102.php

There are three types of skyrmions; Néel, Bloch, and Anti skyrmions. In a Néel skyrmion, the magnetic fields change direction like a water fountain. In Bloch skyrmions, they change direction gradually, circling around the center of the skyrmion. Anti skyrmions look the opposite of Néel skyrmions. 

                               [image3_Neel, Bloch and Anti skyrmions]                                   https://link.springer.com/article/10.1007/s10909-019-02202-w

Hedgehog Configuration & Extreme Projection

[image4_extreme projection of a hedgehog configuration] https://www.researchgate.net/figure/a-A-non-chiral-skyrmion-configuration-bottom-is-obtained-by-unfolding-a-hedgehog_fig4_263036321

An image similar to the one above was the reason I started researching about skyrmions. I had though that the sphere at the top - a hedgehog configuration of a skyrmion, as a new type of magnet. In fact, the hedgehog configuration of a skyrmion is just an mapping of the skyrmion on a sphere. Extreme projection of the hedgehog configuration leads to a skyrmion.

 

What's noticeable in the image above is that the skyrmion is mapped in a single layer on a sphere. This means that the winding number* is 1. A winding number is a topological one that cannot be changed easily. The winding number(N=1) of a skyrmion does not chance easily, too. N=0 means the absence of a skyrmion, and N=1 means that a skyrmion exists. Due to topological protection(that a topological number does not change easily), a skyrmion cannot easily disappear and therefore is stable.

*winding number: the winding number of a closed curve in the plane arround a given point is an integer representing the total number of times that curve travels counterclockwise around the point - Wikipedia

[image5_winding number] https://en.wikipedia.org/wiki/Winding_number

Uses

Storage

As mentioned above, a skyrmion isn't destroyed or changed easily. Because of this, skyrmions couldbe used in methods of storage. On tapes, small areas differently polarized are used to store and carry information. If skyrmions are used instead of these magnetic areas, information density will become higher because the diameter of a skyrmion is so small. Also, information wont be damaged easily due to topological protection.

 

The biggest problem with using skyrmions as a storage device is the skyrmion hall effect. On an electric current, skyrmions don't move in a straight line, making it hard to control their movements. Luckily, scientists are working on this problem by using various types of magnets. 

[image6_skyrmion hall effect]      https://www.nature.com/articles/nphys4030

Reservoir Computing

[image7_reservoir computing]        https://www.nature.com/articles/srep22381

In reservoir computing, where complicated data is displayed in higher dimensions enabling linear calculations to be swiftly done, skyrmions can be used. In such cases, the resevoir layer will consist of several skyrmions.