Unveiling the impact of anti-site defects in magnetic transitions of few-layer MnBi2Te4 by operando heating

As the first experimentally discovered intrinsic magnetic topological insulator, MnBi2Te4 has attracted widespread attentions, providing a unique platform for the exploration of topological quantum phases, such as quantum anomalous Hall effect and axion insulator state. Despite the increasing number of potential factors affecting samples being identified, obtaining the high-quality device performance with desired topological quantum phases remains a challenge. In this work, by comparing the reflective magnetic circular dichroism (RMCD) of crystals with different defect densities that are characterized by atomically resolved scanning tunneling microscopy, we demonstrate that anti-site defects play an essential role in achieving ideal magnetic states. By measuring RMCD hysteresis loops with operando heating, we find that MnBi2Te4 few-layer samples are highly susceptible to thermal impact, even at temperature as low as 45°C. The magnetic behavior of heating-treated samples is akin to that of samples fabricated into devices, revealing the thermal impact on devices as well. Starting from few-layers with ideal layer-dependent magnetic order, thermal heating leads to the convergence of magnetization and transition fields between odd- and even-layers. The observed heating-induced magnetic evolution can serve as a valuable reference for assessing the sample quality or the density of anti-site defects. Our findings not only point out the long-standing hidden factor that arose controversies in MnBi2Te4, but also pave the way for controllably engineering the topological quantum phenomena.

💡 Research Summary

This paper investigates how anti‑site defects influence the magnetic transitions and layer‑dependent behavior of few‑layer MnBi₂Te₄, a prototypical intrinsic magnetic topological insulator. The authors compare two batches of crystals: Type‑A, grown with an optimized flux method, and Type‑B, grown by a conventional flux method. Scanning tunneling microscopy (STM) reveals that Type‑A crystals have a markedly lower density of Mn‑Bi anti‑site defects (~1.9 %) than Type‑B (~7 %). Both types also contain Bi‑Te anti‑site defects, but at much lower concentrations.

Reflective magnetic circular dichroism (RMCD) measurements on freshly exfoliated flakes show that low‑defect Type‑A samples exhibit the theoretically expected A‑type antiferromagnetic (AFM) behavior: odd‑layer flakes display a sharp spin‑flip transition, while even‑layer flakes only show a spin‑flop transition at higher fields. In contrast, high‑defect Type‑B flakes show spin‑flip hysteresis loops in both odd and even layers, indicating that Mn atoms occupying Bi sites introduce local moments antiparallel to the bulk Mn sublattice, thereby breaking the ideal layer‑parity magnetic order.

To connect magnetic properties with electronic transport, the authors fabricate devices from low‑defect flakes using a stencil‑mask Au evaporation process that avoids polymer contamination. Simultaneous transport (Rₓₓ, R_yₓ) and RMCD measurements on a 5‑septuple‑layer (5 SL) device reveal quantized Hall resistance (±h/e²) and near‑zero longitudinal resistance at gate voltages near the charge‑neutral point, confirming a Chern‑insulating (quantum anomalous Hall) state with Chern number ±1. Both spin‑flip (≈±0.5 T) and spin‑flop (≈±3 T) transitions appear in the transport data and are mirrored exactly in the RMCD hysteresis, demonstrating that RMCD provides a direct probe of magnetization independent of carrier density. Optical microscopy combined with RMCD imaging shows a single magnetic domain across the device, contrasting with bulk reports of multi‑domain behavior.

A 4‑SL device, also made from low‑defect material, similarly reaches a high‑field Chern‑insulating state, but unlike the pristine exfoliated 4‑SL flakes, it exhibits a spin‑flip transition. This discrepancy suggests that the device fabrication process modifies the magnetic ground state.

The authors then perform systematic “operando heating” experiments: as‑exfoliated flakes are heated stepwise up to 90 °C inside the cryostat, and RMCD hysteresis loops are recorded after each step. Remarkably, even modest heating (45 °C) induces significant changes. In odd‑layer flakes, the spin‑flip field H_c1 shifts to higher values while the spin‑flop field H_c2 moves to lower values; the opposite trend occurs in even‑layer flakes. The remanent RMCD signal (proportional to out‑of‑plane magnetization) decreases with temperature for odd layers but increases for even layers. At temperatures ≳90 °C, the hysteresis loops of odd and even layers become nearly indistinguishable, erasing the characteristic layer‑parity signature.

These heating‑induced changes reproduce the magnetic behavior observed in fabricated devices, leading the authors to conclude that the thermal load during Au evaporation (estimated ≈90 °C) is the primary cause of the defect‑related magnetic anomalies in devices. When Ti is deposited as a wetting layer before Au, the effective heating exceeds 190 °C, further altering the magnetic response. The authors propose that heating promotes the formation or migration of anti‑site defects, especially near the exposed surface, thereby increasing the defect density that drives the observed magnetic evolution.

In summary, the study identifies anti‑site defect density as a hidden but decisive factor governing the magnetic transitions and, consequently, the topological quantum states of few‑layer MnBi₂Te₄. Low‑defect crystals display the ideal odd‑even layer magnetic dichotomy predicted by theory, whereas higher defect densities (or thermal processing) blur this distinction and introduce unwanted spin‑flip hysteresis. The work underscores the extreme sensitivity of MnBi₂Te₄ to modest temperature excursions, highlighting the need for careful thermal management and defect engineering during device fabrication. The extracted parameters (critical fields H_c1, H_c2, and remanent RMCD intensity) provide practical metrics for assessing sample quality and guiding the development of robust MnBi₂Te₄‑based quantum devices.