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.<br/><br/>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.<br/><br/>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)