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June 11th, 2012

Skyrmions—magnetic bubbles that may be useful places to store data

Published on June 11th, 2012 | By: Eileen De Guire

Magnetic eddies in silicon manganese, so-called skyrmions, form a regular grid. Moving these skyrmions needs 100,000 times smaller currents than existing technologies. But, inquiring minds will ask, what is a skyrmion? Credit: TUM.

Part of what I enjoy about my job is coming across cool words that come with cool science, like skyrmion and helimagnet.

Wikipedia—the “Easy Button” of information—has this to say about skyrmions.

The skyrmion is a hypothetical particle related to baryons. It was described by Tony Skyrme and consists of a quantum superposition of baryons and resonance states. Skyrmions are homotopically non-trivial classical solutions of a nonlinear sigma model with a non-trivial target manifold topology—hence, they are topological solitons. … Skyrmions have been reported, but not conclusively proven, to be in Bose-Einstein condensates, superconductors, thin magnetic films and also chiral nematic liquid crystals.

That muddles things more than it clarifies for me, but the abstract of a new paper in PNAS sheds some light.

According to the abstract of the paper (below), a skyrmion is, topologically speaking, a magnetic bubble, or cylindrical domain, in a ferromagnetic film. They show up in “noncentrosmmetric helimagnets.” Skyrmions were studied in the 1970s for data storage, and I remember seeing a video of the bubbles doing a lemming-like march off the end of their world (a chip of some sort) at a seminar given by an IBM scientist when I was in graduate school. The skyrmions studied in this paper sound almost gourmet: “magnetic oxide–M-type hexaferrite with a hint of scandium.”

Skyrmions occur in metals, too. In February, researchers at the Technische Universitaet Muenchen discovered a “grid of magnetic eddies”—skyrmions—in silicon manganese. The press release says that the TUM team was able to show that “even the tiniest of currents are sufficient to move the magnetic eddies,” which opens up the possibility of manipulating them in, for example, hard drives.

The PNAS paper is “Magnetic stripes and skyrmions with helicity reversals,” by Xiuzhen Yu, Maxim Mostovoy, Yusuke Tokunaga, Weizhu Zhang, Koji Kimoto, Yoshio Matsui, Yoshio Kaneko, Naoto Nagaosa, and Yoshinori Tokura; PNAS; May 21, 2012; doi: 10.1073/pnas.1118496109.

Here is the abstract:

It was recently realized that topological spin textures do not merely have mathematical beauty but can also give rise to unique functionalities of magnetic materials. An example is the skyrmion-a nano-sized bundle of noncoplanar spins-that by virtue of its nontrivial topology acts as a flux of magnetic field on spin-polarized electrons. Lorentz transmission electron microscopy recently emerged as a powerful tool for direct visualization of skyrmions in noncentrosymmetric helimagnets. Topologically, skyrmions are equivalent to magnetic bubbles (cylindrical domains) in ferromagnetic thin films, which were extensively explored in the 1970s for data storage applications. In this study we use Lorentz microscopy to image magnetic domain patterns in the prototypical magnetic oxide-M-type hexaferrite with a hint of scandium. Surprisingly, we find that the magnetic bubbles and stripes in the hexaferrite have a much more complex structure than the skyrmions and spirals in helimagnets, which we associate with the new degree of freedom-helicity (or vector spin chirality) describing the direction of spin rotation across the domain walls. We observe numerous random reversals of helicity in the stripe domain state. Random helicity of cylindrical domain walls coexists with the positional order of magnetic bubbles in a triangular lattice. Most unexpectedly, we observe regular helicity reversals inside skyrmions with an unusual multiple-ring structure.

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