Topological Defects in Helical Magnets
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Helical magnets which violated space inversion symmetry have rather peculiar topological defects. In isotropic helical magnets with exchange and Dzyaloshinskii–Moriya interactions, there are only three types of linear defects: ±π and 2π-disclinations. Weak crystal anysotropy suppresses linear defects on large scale. Instead, planar defects appear: domain walls that separate domains with different preferential directions of helical wavevectors. The appearance of such domain walls in the bulk helical magnets and some of their properties were predicted in the work . In a recent work by an international team of experimenters and theorists , the existence of new types of domain walls on crystal faces of helical magnet FeGe was discovered. They have many features predicted by theory , but display also unexpected properties, one of them is the possibility of arbitrary angle between helical wavevectors. Depending on this angle, the domain walls observed in  can be divided in two classes: smooth and zig-zag. This article contains a mini-review of the existing theory and experiment. It also contains new results that explain why in a system with continuous orientation of helical wavevectors domain walls are possible. We discuss why and at what conditions smooth and zig-zag domain walls appear, analyze spin textures associated with helical domain walls, and find the dependence of their width on the angle between helical wavevectors.
This work was supported by the University of Cologne Center of Excellence QM2 and by William R. Thurman’58 Chair in Physics, Texas A&M University. We are thankful to D. Meier for very useful discussion of experimental procedure and sending us the original version of Fig. 5. Our thanks due to him and Y. Tokura for kind permission to use experimental figures from article  and to Chen Sun for his courteous help in preparation of Fig. 2.
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