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Title: Stable free-standing hopfions in bulk chiral magnets
Plain: A hopfion is a three-dimensional knotted twist of spins in which the lines of equal spin direction form linked loops. In bulk crystals hopfions have so far been seen only attached to skyrmion strings, and a free one has not been observed.
Precise: In a bulk or thick-film chiral magnet (FeGe, other B20 compounds, Co-Zn-Mn alloys) or a frustrated magnet, stabilize an isolated hopfion with Hopf index $Q_{H}=1$ not linked to skyrmion strings or sample surfaces, and image it in three dimensions. An answer is the system, the stability window in H and T, and a tomographic reconstruction showing $Q_{H}=1$.
Settled by: Electron or x-ray magnetic vector tomography resolving the closed, linked preimage loops of a single hopfion away from surfaces and skyrmion strings.
Kind: well-posed
Answer type: identification
References: (empty)
Literature status: open
Status note: Zheng et al. (Nature 2023) imaged hopfion rings in FeGe bound to skyrmion strings.
Posed since: (empty)