Mn/Bi site mixing and complex magnetic structure in MnBi₂Te₄

Intrinsic magnetic topological insulators (TIs) provide a fertile playground to pursuit exotic quantum states of matter from the nontrivial band topology, such as the quantum anomalous Hall insulators and axion insulators. Recently, the van der Waals magnet MnBi₂Te₄, with the stacking of septuple-layers, is the first instance of an intrinsic antiferromagnetic TI, which attracts much interest. However, MnBi₂Te₄ and its related compounds tend to have a significant amount of structural imperfections, notably the Mn/Bi site mixing. In this work, we have performed a detailed study on the chemical and magnetic structures of MnBi₂Te₄ via single-crystal neutron diffraction at beamline CORELLI, SNS. In contrast to the previous reports of a simple A-type antiferromagentic structure, we found that the Mn ions on both Mn and Bi sites are magnetically ordered at low temperatures, forming a ferrimagnetic structure. This work calls for further studies on the ferrimagnetic structure's role in the band topology and the surface state of this novel material.