Room Temperature Topological Spin Textures in a van der Waals Ferromagnet Revealed by Multi-modal Lorentz Electron Microscopy

Two-dimensional van der Waals (2D vdW) magnets offer a promising platform for exploring magnetic and topological phases, owing to their unique layered structure and properties sensitive to stacking order [1,2]. Among the vdW materials for studying 2D magnetism, the Fe_NGeTe₂ (FGT, N=3-5) system is exceptional due to its tunability of magnetic properties with chemical doping and the existence of ferromagnetism above room temperature.



Here, we explore the chemically driven structural and magnetic phase transitions in (Fe_1-xCo_x)₅GeTe₂ (FCGT) using a combination of atomic resolution imaging and Lorentz four-dimensional scanning transmission electron microscopy (4D-STEM). Upon Co-doping, we find that the FCGT undergoes both structural and magnetic phase transitions from an antiferromagnetic, AA-stacking (x=0.46) to a ferromagnetic, polar AA’-stacking (x=0.50). Further, room temperature Néel-type skyrmions emerge in the AA’ phase as revealed by Lorentz 4D-STEM [3]. In summary, our work paves the way for studying structural and magnetic phase transition in vdW magnetic materials.



References:

[1] K. F. Mak, J. Shan, and D. Ralph, Nature Review Physics 1, 641 (2019).

[2] K. S. Burch, D. Mandrus, and J.-G. Park, Nature 563, 47 (2018).

[3] H. Zhang et al., Science Advances 8, eabm7103 (2022).