Skyrmions in van der Waals magnets

Cryogenic and in situ Lorentz TEM of topological spin textures in 2D magnets.

projects

Papers: Thermal hysteresis and ordering behavior of magnetic skyrmion lattices, Nano Letters (2022) · Direct observation of magnetic bubble lattices and magnetoelastic effects in van der Waals Cr₂Ge₂Te₆, Advanced Functional Materials (2023)

Layered van der Waals (vdW) ferromagnets like Fe₃GeTe₂ (FGT) and Cr₂Ge₂Te₆ (CGT) host magnetic skyrmions and bubbles, which are topologically protected spin textures. As vdW materials, they can be exfoliated, stacked, and strained in ways bulk magnets cannot. Most of them order only below room temperature, so during my Ph.D. at Northwestern and Argonne I used cryogenic (LN₂ and LHe) and in situ Lorentz TEM to study how these textures form, order, and respond to changing temperature, field, and strain.

Néel skyrmion lattices in Fe₃GeTe₂

FGT hosts Néel skyrmions, which only produce Lorentz contrast when the sample is tilted. Because skyrmion size in FGT depends strongly on temperature, field-cooling forces the lattice to adapt as it forms. Combining LTEM with CNN-based skyrmion detection, we tracked lattice order and skyrmion size across field-cooling and field-heating cycles, which show a pronounced thermal hysteresis. Combining these statistics with an analytical model of FGT’s magnetic energy landscape explains how field and temperature control the lattice.

Néel skyrmions in Fe₃GeTe₂. (a) In-focus image of the flake. (b–d) Tilted, out-of-focus LTEM images during field-cooling, showing the skyrmion lattice forming. (e, f) Magnified region and its reconstructed in-plane magnetic induction.

Bubble lattices and magnetostriction in Cr₂Ge₂Te₆

In CGT, field-cooling produces lattices of Bloch-type magnetic bubbles, which can be either homochiral or of mixed chirality after identical cooling procedures in the same region of the flake. CGT’s large thermal expansion on cooling also let us directly observe magnetoelastic coupling: strain in the flake aligns the magnetic stripe domains.

Homochiral and mixed-chirality bubble lattices meeting at an extended defect in CGT, formed in a single field-cooling run. (a) TIE-reconstructed phase image showing right-handed (bright) and left-handed (dark) bubbles. (b, c) Integrated induction maps of the orange and green boxes in (a): homochiral above the defect, mixed chirality below it.

I also contributed imaging to collaborations on twisted stacking domains in CrI₃ (Nature Communications, 2024) and spin textures in an insulating vdW ferromagnet (Advanced Materials, 2024).