Room Temperature Néel-Type Skyrmions 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 crystallographic symmetries sensitive to stacking order. Real-space topological spin textures, such as magnetic skyrmions consisting of swirling spin configurations, are often stabilized by an antisymmetric Dzyaloshinksii-Moriya interaction (DMI) present in materials with broken inversion symmetry. 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, energy dispersive x-ray spectroscopy (EDS) mapping, and Lorentz four-dimensional scanning transmission electron microscopy (4D-STEM) along with an electron microscopy pixel array detector (EMPAD). Upon Co-doping, we find that the FCGT undergoes both structural and magnetic phase transitions from an antiferromagnetic, centrosymmetric AA-stacking (x=0.46) to a ferromagnetic, polar AA’-stacking (x=0.50). This structural phase transition is accompanied by a change in Fe and Co ordering as revealed by atomic resolution STEM-EDS. More interestingly, room temperature Néel-type skyrmions emerge in the AA’ phase as revealed by Lorentz 4D-STEM. In summary, our work paves the way for studying structural and magnetic phase transition in vdW magnetic materials.

Work supported by the AFOSR Hybrid Materials MURI, award # FA9550-18-1-0480 and USC Viterbi start-up. Facilities supported by the National Science Foundation (DMR-1429155, DMR- 2039380, DMR-1719875)