The Motivation Behind This Research

The quantum anomalous Hall (QAH) effect has been a significant research frontier in condensed matter physics over the past decade, with profound scientific implications for exploring novel quantum phenomena and low-energy-consumption devices. Recent discoveries of various novel two-dimensional (2D) antiferromagnetic (AFM) materials have further advanced studies of the QAH effect. Among these, MnBi₂Te₄, identified as the first experimentally realized quantum material that simultaneously possesses 2D nature, intrinsic magnetism, and nontrivial topology, has received significant attention in recent years. Theoretically, the interplay between AFM order and band topology in MnBi₂Te₄ is predicted to enable the realization of high-temperature QAH effects [see Figs. 1a and 1b]. However, since the first observation of the QAH effect in this material by Yuanbo Zhang’s group from Fudan University [Science 367, 895 (2020)], no other groups have successfully reproduced the QAH effect in MnBi₂Te₄. The difficulty in fabricating high-quality devices has become a major bottleneck hindering the progress of this field.

Key Findings in This Research

In this work, we developed a novel fabrication method by introducing amorphous AlO_x into the nano-fabrication process for topological transport devices. This approach successfully enabled the realization of the QAH effect in multiple few-layer MnBi₂Te₄ devices. Our previous experiments revealed that direct contact between the photoresist PMMA and MnBi₂Te₄ during standard electron-beam-lithography (EBL) processes can significantly damage the surface and finally reduce the effective thickness of the device [Nat. Commun. 15, 3399 (2024)]. To address this issue, we devised a revised fabrication method by thermally depositing a 3-nm amorphous AlO_x on the sample surface prior to EBL. This layer physically isolates the photoresist from the surface states. Additionally, to mitigate the influence of the insulating nature of AlO_x on electrical contacts, an argon ion etching process was incorporated after EBL, ensuring good ohmic contact between the Cr/Au electrodes and the sample. As shown in Figs. 1e to 1g, optical measurements of 50 devices showed that all AlO_x-capped devices exhibited consistent optical contrast (O_c) before and after fabrication. This consistency indicates that AlO_x can effectively protect the samples throughout the fabrication process.

Next, we investigated the effect of AlO_x on the quantum transport behavior of odd-layer MnBi₂Te₄ devices. As displayed in Figs. 2a-2d, the transport measurement clearly demonstrated that for the sample without AlO_x capping layer, its σ_xy showed a zero Hall plateau near zero magnetic field, indicating a reduction of the effective thickness of the 7-SL sample to 6 SLs. In stark contrast, the AlO_x-capped device displayed a pronounced Hall hysteresis loop at zero magnetic field, suggesting the presence of long-range AFM order combined with band topology. Transport measurements on 17 MnBi₂Te₄ devices further confirmed these results. As displayed in Figs. 2e-2g, all the devices showed σ_xy = e²/h at the high μ₀H Chern insulator state, indicating the high quality of our devices. However, they exhibited dramatically different behaviors in the AH effect. Remarkably, among the 9 devices without AlO_x protection, the maximum σ_xy at zero fields reached only 0.3 e²/h, and some devices even exhibit almost indiscernible hysteresis. In contrast, all the other 8 devices with AlO_x capping layer showed significantly enhanced σ_xy and square-shaped hysteresis loops, with two devices almost quantized at e²/h.

Implications for Future Research

Achieving quantized transport in the absence of magnetic field is foundational for constructing and manipulating novel topological quantum states. Our research not only achieved zero-magnetic-field quantization in MnBi₂Te₄ devices, but also introduced a straightforward and universal method for fabricating high-quality devices. Compared to the previously reported Al₂O₃-assisted mechanic exfoliation and stencil mask method, this new method is simpler and compatible with the standard EBL process, enabling the fabrication of specific nanoscale devices with smaller sample sizes. Furthermore, since the AlO_x capping layer is deposited directly on the surface, it not only provides effective protection but also serves as a dielectric layer for top-gate tunability. Our work resolves a long-standing challenge in the field of magnetic topological materials, paving the way for investigating the interplay between band topology and 2D magnetism. However, while the use of AlO_x capping layers to achieve the QAH effect is promising, it remains in its early stages. Future studies focusing on precise control of AlO_x growth parameters and the interface of AlO_x/MnBi₂Te₄ may further optimize the QAH effect, making this an exciting direction for continued exploration.