Imaging of Magnetic Domain Behavior in van der Waals Ferromagnets

Magnetic van der Waals (vdW) materials are increasingly studied due to the discovery of ordered ferromagnetism down to monolayer thickness as well as them hosting chiral and topologically protected magnetic domains. The realization of such states in van der Waals ferromagnets opens further opportunities to explore and control their behavior using interfacial interactions such as strain or interfacing with dissimilar materials to create novel functionality in heterostructures. In this work, we will present the magnetic domain behavior of two iso-structural compounds – Cr2Si2Te6 (CST) and Cr2Ge2Te6 (CGT). They both belong to the R–3 space group, with Cr forming honeycomb network and octahedrally coordinated with Te atoms. CST shows a Curie temperature of 30 K while CGT has a Curie temperature of 70 K. We have performed an in-situ study and directly imaged the magnetic domain structure under varied temperature and external magnetic fields using cryo-Lorentz transmission electron microscopy (LTEM). They both show easy-axis of magnetization along the c-axis of the crystal. In CGT, we observe the formation of Bloch-type domains and bubble lattices with mixed chirality. Despite a lack of chiral energy terms in CGT, we have also observed topologically protected lattices containing homochiral bubbles. By contrast, in CST, we observe formation of Néel-type stripe domains and skyrmions. Both these materials also possess a strong magneto-elastic coupling. We will discuss the implications of strain and magnetostriction on the alignment of magnetic domains and skyrmion/bubble lattices in these materials.