Micromagnetic Insights into History-Dependent Domain Behavior in MBE-grown Magnet/Topological Insulator Heterostructure Fe₃GeTe₂/Bi₂Te₃

Heterostructures of 2D van der Waals magnets and topological insulators are promising platforms for spin‑torque switching and the quantum anomalous Hall effect. Among them, Fe₃GeTe₂/Bi₂Te₃ thin films grown by molecular beam epitaxy stand out for their high spin-orbit torque efficiency and robust interfacial coupling. Using scanning transmission X-ray microscopy at the MAXYMUS beamline of the BESSY II synchrotron, we observe speckled magnetic domains previously unreported from exfoliated single crystal Fe₃GeTe₂. The domain structures are stable across a wide range of thicknesses and temperatures. To investigate their origin, we performed micromagnetic simulations with varying Dzyaloshinskii–Moriya interaction (DMI), exchange stiffness, and perpendicular anisotropy. Comparing the experiment, we identify the role of DMI in modulating magnetic interactions by analyzing domain sizes and shapes. As DMI dominates, the domain evolves from uniform reversal to fragmented speckled patterns and to stripe-like textures. Finally, we present a phase diagram of domain structures as functions of magnetic field, DMI, exchange stiffness, and anisotropy.