Synthesis, Magnetic Properties, and Electronic Structure of Magnetic Topological Insulator MnBi₂Se₄

The intrinsic magnetic topological insulators MnBi₂Te₄ and MnBi₂Se₄ support novel topological states related to symmetry breaking by magnetic order. Unlike MnBi₂Te₄, the study of MnBi₂Se₄ has been inhibited by the lack of bulk crystals, as the van der Waals (vdW) crystal is not the thermodynamic equilibrium phase. Here, we report the layer-by-layer synthesis of vdW MnBi₂Se₄ crystals using nonequilibrium molecular beam epitaxy. Atomic-resolution scanning transmission electron microscopy and scanning tunneling microscopy identify a well-ordered vdW crystal with septuple-layer base units. The magnetic properties agree with the predicted layered antiferromagnetic ordering but disagree with its predicted out-of-plane orientation. Instead, our samples exhibit an easy-plane anisotropy, which is explained by including dipole-dipole interactions. Angle-resolved photoemission spectroscopy reveals the gapless Dirac-like surface state, which demonstrates that MnBi₂Se₄ is a topological insulator above the magnetic-ordering temperature. These studies show that MnBi₂Se₄ is a promising candidate for exploring rich topological phases of layered antiferromagnetic topological insulators.