Role of Magnetic Defects in Tuning Ground States of Magnetic Topological Insulators

Magnetic defects play an important, but poorly understood, role in magnetic topological insulators (TIs). For example, topological surface transport and bulk magnetic properties are controlled by magnetic defects in Bi2Se3‐based dilute ferromagnetic (FM) TIs and MnBi2Te4 (MBT)‐based antiferromagnetic (AFM) TIs. Despite its nascent ferromagnetism, the inelastic neutron scattering data show that a fraction of the Mn defects in Sb2Te3 form strong AFM dimer singlets within a quintuple block. The AFM superexchange coupling occurs via Mn–Te–Mn linear bonds and is identical to the AFM coupling between antisite defects and the FM Mn layer in MBT, establishing common interactions in the two materials classes. It is also found that the FM correlations in (Sb1−xMnx)2Te3 are likely driven by magnetic defects in adjacent quintuple blocks across the van der Waals gap. In addition to providing answers to long‐standing questions about the evolution of FM order in dilute TI, these results also show that the evolution of global magnetic order from AFM to FM in Sb‐substituted MBT is controlled by defect engineering of the intrablock and interblock coupling. Inelastic neutron scattering measurements reveal the formation of linear Mn–Te–Mn antiferromagnetic dimers in Mn‐doped Sb2Te3, following the Goodenough–Kanamori superexchange rule. Although the system is magnetically dilute, the experiments identify ferromagnetic dimers and correlations in the interblock across the van der Waals gap. These interactions explain the role of defects in tuning ground‐states in magnetic topological insulators.