Understanding spin-texture behavior in van der Waals ferromagnets using Cryo Lorentz Electron Microscopy

Magnetic van der Waals (vdW) materials have unique crystal structures, which consists of layered atoms separated by vdW gaps, leading to strong intra-plane interaction and weak inter-plane vdW interaction. This unique structure and strong intrinsic spin interactions give rise exotic physical phenomena, such as rich magnetic spin textures, namely, stripe domains, skyrmions and merons. It is essential to understand the behavior of spin textures in vdW materials in response to external stimuli such as magnetic field, temperature and strain. Lorentz transmission electron microscopy (LTEM) allows for imaging the local spin textures and microstructure at high resolution in such materials and related heterostructures.

In this talk, we will present the spin texture imaging of two types of vdW ferromagnets: (1) metallic Fe₃GeTe₂ (FGT) and (2) semiconducting Cr₂Ge₂Te₆ (CGT). FGT is an itinerant ferromagnet in the 2D limit with strong perpendicular anisotropy and with the Curie temperature of around 230 K. We will show the creation and the control of magnetic spin textures in the vdW FGT as a function of temperature and magnetic field by using in-situ cryo LTEM. We also gain insight into the order and interaction of the collective skyrmion lattice, showing short-range order and thermally hysteretic behavior. In addition, we will also report the formation of skyrmions in a field-free condition in an oxidized FGT (O-FGT) heterostructure due to the interfacial interaction between FGT and O-FGT. CGT is another vdW ferromagnet with the Curie temperature of 65 K, which however has weaker uniaxial anisotropy compared to Fe₃GeTe₂. We will show formation of Bloch-type stripes and bubbles in the CGT. CGT is a known magnetostrictive material, and we will demonstrate the effect of local strain on stripe and bubble domains, which provides another approach to controlling the order of spin texture in such materials.