Various topological spin textures and their dynamics in helimagnets*

The ability to manipulate topological spin textures such as skyrmions is a key to the development of electronic devices that are very low in power consumption¹. Recently, hexagonal skyrmion lattice (hex-SkL) have been discovered in several systems, such as chiral/polar^2-3 magnets and ferromagnets with uniaxial anisotropy⁴. In addition to the hex-SkL, we have observed the first for a square lattice of merons and antimerons (sq-ML)—topologically distinguish with skyrmions in a thin plate of the helimagnet Co₈Zn₉Mn₃⁵. By finely varying the magnetic field applied perpendicularly to the plate, the transformation between the sq-ML and hex-SkL has been obtained. We found that the skyrmions were very robust, lasting even as we lowered the temperature of the plate, but the merons and antimerons were much more sensitive, and relaxed into spin helices as the temperature fell.
The transition between a hex-SkL and non-topological spin textures, helical or conical structure, have been also demonstrated by the in-situ Lorentz transformation electron microscopy observations with current excitation or field cooling in a helimagnet FeGe⁶. The skyrmion dynamics, such as the collective transformation of sparsely-populated skyrmions to microcrystals of skyrmions, and the current-driven skyrmion motion have been observed.
This work has been done in collaboration with Profs. Yoshinori Tokura, Naoto Nagaosa, Taka-hisa Arima, Yusuke Tokunaga and Fumitaka Kagawa, and with Drs. Wataru Koshibae, Yasujiro Taguchi, Daisuke Morikawa, Naoya Kanazawa, Tomoyuki Yokouchi and Kiyou Shibata.
[1] N. Nagaosa and Y. Tokura, Nat. Nanotechnol. 8, 899 (2013).
[2] X.Z. Yu, et al., Nature 465, 901 (2010).
[3] I. Kézsmárki, et al., Nat. Mater. 14, 1116 (2015).
[4] X.Z. Yu, et al., PNAS 109, 8856 (2012).
[5] X.Z. Yu, et al., Nature 564, 95 (2018).
[6] X.Z. Yu, et al., Nat. Phys. 14, 832 (2018).