NMR evidence for an antisite-induced magnetic moment on Bi in a topological insulator heterostructure MnBi$_2$Te$_4$/(Bi$_2$Te$_3$)$_n$

Abstract

MnBi$_2$Te$_4$ (MBT) is the first intrinsic magnetic topological insulator, combining a topologically protected surface metallic state and intrinsic magnetic order. A structural compatibility with the nonmagnetic Bi$_2$Te$_3$ (BT) parent compound gives a possibility to create MBT/BT heterostructures and manipulate their magnetic state in view of optimizing the Quantum Anomalous Hall Effect (QAHE). In this work an extensive Nuclear Magnetic Resonance (NMR) study, supported by the bulk magnetization measurements has been performed at 4.2 K on a self-organized single crystal MnBi$_2$Te$_4$/(Bi$_2$Te$_3$)$_n$ heterostructure, obtained from the Mn$_{0.81}$Bi$_{2.06}$Te$_{4.13}$ melt. $^{55}$Mn and $^{209}$Bi NMR signals have been recorded as a function of the out-of-plane magnetic field up to 6 T, covering a spin-flop transition (SFT) from the antiferromagnetic (AFM) to the canted antiferromagnetic (CAFM) configuration of the Mn layers. The canting angle at different external field values has been estimated based on NMR data. Presence of the AFM-coupled Mn antisites has been evidenced and shown to induce an antiparallel magnetic moment on Bi atoms within the host Bi layer. Detection of the induced magnetic moment on bismuth which contributes a new ferromagnetic (FM) component is of utmost importance for understanding the magnetic interactions in the MBT/BT system. These findings have potentially important implications for engineering the QAHE devices.