Wax-assisted exfoliation and dual-surface AlOx encapsulation: significant enhancement of topological phases in MnBi2Te4
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Two-dimensional materials are composed of atomic layers coupled by van der Waals force. This weak interlayer bonding allows for the isolation from bulk crystals into single or few-layer flakes through exfoliation methods, giving rise to exotic physical phenomena. The most widely used method is Scotch-tape mechanical exfoliation onto SiO₂/Si substrates, where the strong adhesion of the substrate facilitates the cleavage of the surface layers.
MnBi 2 Te 4 is the first intrinsic antiferromagnetic topological insulator, which has attracted intense attention in recent years. It hosts both layered magnetic order and nontrivial band topology, making it a unique platform for studying topological quantum states. Experiments have already demonstrated the quantum anomalous Hall (QAH) effect in odd-layer MnBi 2 Te 4 devices ( Science 367, 895 (2020)) and the axion insulator state in even-layer devices ( Nat. Mater. 19, 522 (2020)). However, the challenges of fabricating high-quality MnBi 2 Te 4 devices and achieving reproducible quantum states in them have constrained the experimental realization of these quantum phenomena, thereby underscoring the need for innovative device fabrication techniques.
Conventional Scotch-tape exfoliation is often ineffective for materials that are difficult to exfoliate, as the resulting flakes typically crack and break up into small pieces. To address these issues, researchers previously introduced gold-assisted ( Nat. Commun. 11, 2453 (2020)) or AlO x -assisted ( Nature 563, 94 (2018)) exfoliation methods, where a deposited metal or oxide layer acts as a supporting layer. Researchers continued to ask whether a more convenient auxiliary layer could be found—one that could support the sample and at the same time protect its delicate quantum properties.
MnBi2Te4 crystals adhere to softened wax at elevated temperatures; upon cooling, the wax solidifies into a rigid and transparent protective shell. Repeated exfoliation on the wax surface yields large-area, smooth flakes suitable for transfer and device fabrication.
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Research Breakthrough
In this study, the Tsinghua–RUC team developed a wax-assisted exfoliation method (patent no. CN202311150854.3) to fabricate high-quality MnBi 2 Te 4 devices with dual-surface AlO x encapsulation .
Crystalbond 509, a commonly used thermoplastic adhesive, becomes soft and viscous upon heating, while cooling transforms it into a rigid and transparent solid. Taking advantage of this property, the research group attached MnBi 2 Te 4 crystals onto heated softened wax; upon cooling, the wax formed a hard transparent “protective shell.” Exfoliation on the wax substrate not only preserved the crystal integrity but also enabled the preparation of large-area, atomically flat flakes—well-suited for subsequent transfer and device fabrication.
Building on their earlier work demonstrating the beneficial effects of single AlO x capping layer on MnBi 2 Te 4 ( Nat. Commun. 16, 1727 (2025); Nature 641, 70 (2025)), the research group fabricated AlO x –MnBi 2 Te 4 –AlO x heterostructures , with both surfaces of MnBi 2 Te 4 flakes capped by AlO x layers. The AlO x serves dual roles:
- as a protective barrier against organic contamination during fabrication, and
- as an interface that enhances perpendicular magnetic anisotropy, thereby strengthening the magnetic order in MnBi 2 Te 4 .
This dual encapsulation approach significantly improved the robustness of the topological phases in MnBi 2 Te 4 . In even-layer devices, a well-developed axion insulator state was observed, featuring a broad zero-Hall plateau and highly insulating longitudinal resistance. In odd-layer devices, the QAH effect exhibited nearly rectangular hysteresis loops with enhanced coercivity (or spin flip magnetic field). Moreover, the effect was further amplified by in-plane magnetic fields, corroborating previous observations of peculiar magnetic responses unique to MnBi 2 Te 4 .
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