Hopfions at the Breaking Point
In the past few years, physicists have created long-predicted quasiparticles called hopfions—3D, localized, knot-like arrangements of a magnetic material’s spin texture. A hopfion can be described in terms of its Hopf number H, which counts how many loops of spins are interlinked in the knot. Researchers have proposed using hopfions in spintronic computers, where H would encode information. This is because a hopfion is a topologically protected state, meaning H stays the same under many deformation conditions. In new computational work, Shoya Kasai of the University of Tokyo and colleagues have developed a method for splitting high-H hopfions into multiple lower-H hopfions, an operation that would be useful for such spintronic information-storage devices [1, 2].
The team modeled a two-layer structure in which hopfions were hosted by a magnetic material adjacent to a heavy metal. An electric current flowing along the heavy-metal layer generated a perpendicular spin-polarized current via the spin Hall effect. This spin-polarized current leaked into the magnetic layer and exerted a torque on the magnetic moments, pulling different parts of the magnetic texture in opposite directions and thereby stretching the hopfions. The researchers found that, once the torque exceeded a threshold, it could overcome a hopfion’s topological protection and tear a higher-H hopfion into multiple lower-H hopfions. An H = 4 hopfion, for example, could split into four H = 1 hopfions or two H = 2 hopfions depending on the strength of the spin-orbit torque.
Before hopfion-based computational devices can be built, Kasai says, much work remains to develop simpler ways of creating, manipulating, and detecting these quasiparticles.
–Sophia Chen
Sophia Chen is a freelance science writer based in Columbus, Ohio.
References
- S. Kasai et al., “Controlling knot topology in magnetic hopfions via spin-orbit torque,” APS Open Sci. 1, L000007 (2026).
- S. Kasai et al., “Nonequilibrium dynamics of magnetic hopfions driven by spin-orbit torque,” Phys. Rev. B 113, 134445 (2026).