Scanning tunneling microscopy study of epitaxial Fe₃GeTe₂ monolayers on Bi₂Te₃

Abstract Introducing magnetism to the surface state of topological insulators, such as Bi₂Te₃, can lead to a variety of interesting phenomena. We use scanning tunneling microscopy (STM) to study a single quintuple layer (QL) of the van der Waals magnet Fe₃GeTe₂ (FGT) that is grown on Bi₂Te₃ via molecular beam epitaxy. STM topographic images show that the FGT grows as free-standing islands on Bi₂Te₃ and outwards from Bi₂Te₃ steps. Atomic resolution imaging shows triangular lattices of 390 ± 10 pm for FGT and 430 ± 10 pm for Bi₂Te₃, consistent with the respective bulk crystals. A moiré pattern is observed on FGT regions with a periodicity of 4.3 ± 0.4 nm that can be attributed solely to this lattice mismatch and thus indicates zero rotational misalignment. While most of the surface is covered by a single QL of the FGT, there are small double QL regions, as well as regions with distinct chemical terminations due to an incomplete QL. The most common partial QL surface termination is the FeGe layer, in which the top two atomic layers are missing. This termination has a distinctive electronic structure and a (\sqrt{3}x\sqrt{3})R{30}^∘ reconstruction overlaid on the moiré pattern in STM images. Magnetic circular dichroism measurements confirm these thin FGT films are ferromagnetic with T _C ∼190 K.