Insight into Topological Magnetic States in van der Waals Ferromagnets Enabled with Cryo Electron Microscopy

Emergent quantum phenomena, such as topological spin states, often occur at cryogenic temperatures, so that the development of advanced cryogenic capability to image topological spin textures in quantum materials is important. Here, we have leveraged cryo-Lorentz transmission electron microscopy (LTEM) to directly visualize and understand local spin textures and microstructure of ferromagnetic van der Waals (vdW) materials with high-resolution and correlate their behavior. 2D magnetic vdW materials consist of atomic layers separated by vdW gaps, resulting in strong intra-plane interaction but weak inter-plane interaction [1]. This unique crystal structure and strong intrinsic spin interactions give rise to fascinating phenomena such as rich topological spin textures, including Bloch bubbles and Néel skyrmions and merons. We have investigated the behavior of topological spin textures in the vdW transition metal tri-chalcogenides (TMTC), Cr₂Ge₂Te₆ (CGT) and Cr₂Si₂Te₆ (CST). Both crystals have the same rhombohedral structure with the space group R, resulting in a Te-Cr/Si-Cr-Cr/Si-Te sandwich structure. The Curie temperatures of CGT and CST crystals are 65 K and 33 K, respectively. Although both CGT and CST possess perpendicular magnetic anisotropy, CST has a stronger magnetocrystalline anisotropy than that of CGT. By using cryo LTEM we observed Bloch bubbles in CGT and the effect of local strain on stripe and bubble domains [2]. CST was found to host Néel skyrmions . We will demonstrate how interplay between microstructure and spin structure results in pinning of skyrmions at structural defects in CST. This work demonstrates that TMTCs can be a promising material platform to realize novel topological magnetic states by controlling the chalcogen.

[1] K. S. Burch, D. Mandrus, J.G. Park, Nature, 563 47 (2018).
[2] A. R. C. McCray, Y. Li, E. Qian et al., Adv. Funct. Mater. 33, 2214203 (2023)
[3] This work is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Science and Engineering Division. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.