The quasi-two-dimensional magnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Se}}_4$, stabilized via nonequilibrium molecular beam epitaxy, is investigated by resonant soft x-ray scattering. Kiessig fringes are observed, confirming a high sample quality and a thin film thickness of 10 septuple layers $(\sim 13 \mathrm{nm})$. An antiferromagnetic Bragg peak is observed at the structurally forbidden reflection, whose magnetic nature is validated by studying its temperature, energy, and polarization dependence. Through a detailed analysis, an A-type antiferromagetic order with in-plane moments is implied. This alternative spin structure in ${\mathrm{MnBi}}_2{\mathrm{Se}}_4$, in contrast to the Ising antiferromagnetic states in other magnetic topological insulators, might be relevant for hosting new topological states.

• Received 17 August 2023
• Accepted 24 April 2024

DOI:https://doi.org/10.1103/PhysRevB.109.184418