Magnetic Interactions and Defects in Magnetic Topological Insulators

The coupling of magnetic moments to topological fermions is the primary factor controlling the emergence of unique quantum topological phases such as the quantum anomalous Hall and axion insulators. Here, we report seminal findings in both dilute and intrinsic magnetic topological insulator (TI) systems by using neutron scattering to characterize their magnetic order and key magnetic interactions. These efforts have revealed unanticipated and important roles of electron correlations, magnetic frustration, van der Waals (vdW) metamagnetism, reduced dimensionality, and magnetic defects.<br/><br/>MnBi₂Te₄ (MBT) is the first example of an intrinsic antiferromagnetic (AF)-TI compound. MBT is an A-type AF consisting of ferromagnetic (FM) triangular Mn layers with perpendicular moments whose direction alternates layer-to-layer. Long-range interactions within the FM layer include frustrated AF next-nearest neighbor intralayer coupling. The interlayer interactions and single-ion anisotropy are consistent with vdW metamagnetism leading to a spin-flop transition in applied magnetic fields. In MnBi₄Te₇, we find a strongly reduced spin gap compared to MBT, consistent with weaker coupling between septuple blocks.<br/><br/>In the closely related compound MnSb₂Te₄ (MST), the spin gap and spin-flop field collapse, indicating that magnetic anisotropy and interblock exchange are strongly modified by the substitution of Bi for Sb. In addition to the general increase in the magnetic energy scale compared to MBT, we also observe a sharp resonant magnetic excitation in MST that originates from AF coupling between Mn/Sb antisite defects and the main Mn layer. These antisite defects are prevalent in MST and lead to defect-driven ferrimagnetism.<br/><br/>The intrinsic magnetic antisite disorder found in MBT or MST generates Bi or Sb layers hosting low concentrations of Mn ions, similar to that found in dilute magnetic TI such as (Bi_1-<i>x</i>Mn<i>_x</i>)₂Te₃. In dilute magnetic TIs, the origin of the long-range FM coupling is disputed. The investigation of dilute magnetic TIs with neutrons provides unique information about single-ion, dimer, and collective excitations that may not be easily accessible in dense magnetic lattices. Our current studies of (Sb_1-<i>x</i>Mn<i>_x</i>)₂Te₃ and (Bi_1-<i>x</i>Mn<i>_x</i>)₂Te₃ address important questions regarding the nature of magnetic coupling and the role of magnetic disorder with implications for MBT and MST. For example, our data on (Sb_0.97Mn_0.03)₂Te₃ show evidence for AF dimers within the quintuple block. These are the same interactions that drive ferrimagnetism in MST. We also find evidence for FM fluctuations originating from coupling between Mn ions across the vdW gap. This interaction is associated with the ability of particular defect concentrations to control the global FM or AF magnetic state in MST.<br/><br/>Work at Ames Laboratory and ORNL is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. DE-AC02-07CH11358. This research used resources at the Spallation Neutron Source, a DOE Office of Science UserFacility operated by the Oak Ridge National Laboratory.