Magnetic and Structural Response Tuned by Coexisting Mn Concentration‐Dependent Phases in MnBi ₂ Te ₄ Thin Film Grown on GaAs(001) by Molecular Beam Epitaxy

Abstract MnBi ₂ Te ₄ belongs to a set of van der Waals (vdW) materials and a new class of quantum materials that manifest a combination of topological insulator properties with magnetic ordering. Despite having already been grown on some substrates by molecular beam epitaxy, the systematic growth on GaAs is poorly explored. In this work, structural and magnetic properties are systematically investigated by varying the substrate temperature and manganese concentration. Atomic force microscopy, high‐resolution X‐ray diffraction, and scanning transmission electron microscopy are used to investigate the structural properties, while the superconducting quantum interference device is used to probe the magnetic properties of the MnBi ₂ Te ₄ thin films. The experimental results of X‐ray scattering are in agreement with theoretical modeling using dynamical theory of X‐ray diffraction (XRD). Despite the structure having a majority signature expected for MnBi ₂ Te ₄ , their magnetic properties are consistent with the coexistence of magnetic phase orders ferromagnetically and antiferromagnetically. It is shown that these phases emerge from local imbalance in the MnBi ₂ Te ₄ stoichiometry, which is a characteristic of this structure, especially in thin films. The findings provide valuable insights into the development of the magnetic topological insulators with magnetic phases tuned by Mn concentration.