Exploring Spin-Structure Correlation in van der Waals Ferromagnet Fe_5-xGeTe₂ Using (4D-)STEM

Van der Waals (VDW) ferromagnet Fe_5-xGeTe₂has attracted great research interest in recent years as it hosts high and tunable Curie temperatures, topological spin states, and thickness-dependent magnetism down to monolayer, in favor of the next generation spintronic devices. However, the mechanism enabling such rich magnetic behaviors in a single system remains elusive. It has been hypothesized that the complex magnetic structures in Fe_5-xGeTe₂are linked to local structural order and disorder induced by the Fe deficiency within an average structure of R-3m. To investigate the correlation between local structure and chemistry, we systematically characterized Fe_5-xGeTe₂ using a combination of (4D)-scanning transmission electron microscopy (STEM).<br/><br/>From our atomic resolution STEM imaging, we show that the Fe_5-xGeTe₂form split-site ordering, coexisting with disordered intralayer structure when viewing along [1-10]. The split-site ordering breaks the inversion symmetry within each layer, and forms √3×√3 superlattice in ab-plane. Stacking faults can be observed in our STEM images as well, suggesting further symmetry breaking combining the split-site ordering and stacking faults. Furthermore, our atomic-resolution core-loss electron energy loss spectroscopy (EELS) shows a strong correlation between local Fe concentration and intralayer ordering and disordering, where disordered layers systematically have lower Fe concentration than ordered layers. We further performed large-scale domain mapping using scanning electron nanodiffraction (SEND), a 4D-STEM technique. We observed phase segregation, where disordered layers forms micron-scale domains within the ordered-layer dominant matrix.<br/><br/>To clarify how the spin is affected by structural ordering in Fe_5-xGeTe₂, we used Lorentz 4D-STEM to measure the induction field in our sample, as well as its response to external field, temperature and tilting. In the ab-plane, we show Fe_5-xGeTe₂ hosts stripe domains when cooled below Curie temperature without external field. Increasing the external field drives the magnetic stripe-bubble transition until saturation. When field-cooled, a mixed type-I and type-II skyrmionics bubbles can be observed, and we show that the type of the magnetic bubbles can be controlled by in-plane field strength using tilting. Along axis, on the other hand, ferromagnetic domains form mainly in the ordered-layer dominant matrix, while the domains terminate at disordered domains. Our Lorentz 4D-STEM result indicates that the Fe_5-xGeTe₂ shows the characteristics of a centrosymmetric, anisotropic magnet. Within the ordered domains, the induction field is not perturbed by stacking faults or coexisting ordered and disordered layers, suggesting a long-range spin interaction across the VdW layers. However, the micron-scale disordered and ordered domains have distinct magnetic behavior, which suggests that the mesoscopic averaged structure of Fe_5-xGeTe₂ could still play an essential role in determining the magnetic behavior of Fe_5-xGeTe_2.<br/><br/>Altogether, by combining (4D-) STEM and EELS, we reveal the correlation between local structure, chemistry and magnetic properties in a VdW ferromagnet Fe_5-xGeTe₂. We can clarify that the structural ordering of Fe_5-xGeTe₂can be driven by local Fe concentration, and the magnetic structure is impacted by micron-scale ordered/disordered domains. Our findings could greatly advance the understanding of the complex spin ordering in Fe_5-xGeTe₂ and could lay the foundation for precise tuning of the magnetic properties of Fe_5-xGeTe₂ by chemistry and structural engineering.