In recent years, two-dimensional (2D) materials have revolutionized the landscape of condensed matter physics and materials science. These materials, consisting of atomic layers held together by weak van der Waals forces, offer an exceptional platform to explore quantum phenomena that are otherwise obscured in bulk three-dimensional crystals. A prime example is MnBi₂Te₄, the first intrinsic antiferromagnetic topological insulator, which uniquely integrates magnetism with nontrivial electronic band topology. This duality unlocks a plethora of exotic states such as the quantum anomalous Hall effect and the axion insulator phase, promising groundbreaking advances in quantum electronics and spintronics.
Despite the tantalizing theoretical and experimental developments, practical challenges hinder the widespread utilization of MnBi₂Te₄. The brittle nature of this material complicates the exfoliation process traditionally performed using Scotch tape, often resulting in fractured flakes too small or defective for device fabrication. Moreover, exposure to ambient conditions and fabrication residues degrade its intricate quantum states, making reproducibility a daunting task. Such obstacles have fueled the quest for more refined exfoliation and encapsulation techniques to reliably produce large, high-quality flakes that preserve their delicate topological and magnetic properties.
Addressing these critical challenges, the team from Tsinghua University in collaboration with Renmin University of China has developed an innovative wax-assisted exfoliation method for MnBi₂Te₄ crystals. This approach harnesses the thermomechanical properties of Crystalbond 509, a widely used thermoplastic adhesive, which softens at elevated temperatures and resolidifies into a robust, transparent shell upon cooling. By adhering MnBi₂Te₄ crystals onto heated softened wax, then allowing the wax to solidify, the researchers generated a rigid, protective platform. This wax substrate enabled repeated exfoliation cycles, producing large-area, atomically smooth flakes that remain intact without the common fracture issues faced in conventional methods.
The ingenuity of this wax-assisted strategy lies in its dual role as both a support and a shield. The softened wax molds intimately to the crystal surface during adhesion, maintaining crystal integrity throughout exfoliation. Once hardened, the transparent shell protects the sensitive flakes from mechanical disruption and environmental contaminants during subsequent manipulation and device assembly. This contrasts with previous auxiliary methods utilizing gold or aluminum oxide layers, which, while supportive, introduced complexity and potentially affected the underlying quantum states due to metal or oxide-induced interface effects.
Building on their prior findings demonstrating the positive influence of single-layer aluminum oxide capping on MnBi₂Te₄’s magnetic properties, the researchers extended this concept by developing dual-surface encapsulation. Both the top and bottom surfaces of the exfoliated MnBi₂Te₄ flakes were uniformly capped with AlOₓ layers, forming AlOₓ–MnBi₂Te₄–AlOₓ heterostructures. This encapsulation not only acts as an effective barrier against organic and particulate contamination during device fabrication but also enhances perpendicular magnetic anisotropy. The improved magnetic anisotropy strengthens the antiferromagnetic order intrinsic to MnBi₂Te₄, fortifying its topological states against perturbations.
Experimental devices fabricated using this wax-assisted dual-encapsulation approach delivered unprecedentedly robust quantum phenomena. In even-layered MnBi₂Te₄ devices, researchers observed a pronounced axion insulator phase characterized by a broad regime exhibiting zero Hall conductivity amid strongly insulating longitudinal resistance. This clear signature marks a significant advancement in the experimental realization of axion electrodynamics, which has implications for future topological quantum computing and novel magnetoelectric devices. Conversely, odd-layered devices displayed spectacular quantum anomalous Hall effects with nearly perfect rectangular hysteresis loops, signifying stable chiral edge state conduction and markedly enhanced coercive fields.
Notably, the quantum anomalous Hall effect in these devices exhibited further enhancement under applied in-plane magnetic fields, corroborating previously reported complex magnetic behaviors exclusive to MnBi₂Te₄. These enhanced magnetic responses suggest improved control over spin configurations and domain dynamics, offering an exciting avenue for finely tuning topological states via external fields. The robustness and reproducibility of these quantum effects underscore the transformative impact of the wax-assisted exfoliation method combined with dual AlOₓ encapsulation on experimental condensed matter research.
Beyond their immediate results, these methodological advances open new horizons for other challenging 2D materials exhibiting subtle quantum phenomena. The simplicity and effectiveness of wax-assisted exfoliation overcome several longstanding obstacles, presenting a scalable and reproducible route to fabricate large-area, high-quality flakes essential for both fundamental studies and device engineering. Moreover, the dual-surface AlOₓ encapsulation technique can be adapted to rival sensitive quantum materials where environmental vulnerability limits practical applications, such as magnetic topological insulators, superconductors, and ultrathin semiconductors.
The dual combination of improved mechanical exfoliation with superior chemical and magnetic surface protection embodies an integrated materials engineering approach crucial to realizing the full potential of emerging quantum matter. By enabling the fabrication of atomically flat, magnetically stable MnBi₂Te₄ flakes, this research lays the groundwork for next-generation quantum devices exploiting topological magnetism, quantum phase transitions, and spintronic functionalities. These devices promise unprecedented performance in low-power electronics, quantum information processing, and novel sensing technologies, driving a paradigm shift in materials-driven innovation.
This breakthrough exemplifies the vital intersection of materials science, condensed matter physics, and innovative fabrication technologies. It demonstrates how finely tuned interfaces and meticulous sample preparation can unlock hidden physical states, previously inaccessible due to technical limitations. The integration of thermoplastic wax as a temporary yet effective exfoliation medium, combined with strategic oxide encapsulation, reflects a wider trend in leveraging unconventional approaches to overcome traditional materials barriers.
Looking forward, further optimization of the wax-assisted technique, possibly integrating in situ cleaning or doping strategies, could yield even more precise control over flake quality and interfacial properties. Complementary spectroscopic and microscopy investigations will provide deeper insights into the atomistic mechanisms by which the AlOₓ layers reinforce magnetic anisotropy and protect topological order. Such understanding will pave the way for tailored heterostructures with engineered quantum phases and innovative functionalities.
Furthermore, this novel fabrication paradigm may stimulate renewed interest in exploring more exotic magnetic topological phases predicted for MnBi₂Te₄ and related compounds under various external perturbations like pressure, strain, or electric field. The availability of large, high-quality, stably encapsulated flakes significantly enhances experimental flexibility and device integration potential, accelerating knowledge discovery and technology transfer in quantum materials science.
In summary, the wax-assisted exfoliation approach developed by the Tsinghua–RUC team represents a significant leap forward in the pursuit of viable quantum devices based on MnBi₂Te₄. By combining the benefits of a low-cost, accessible supportive wax medium with high-performance dual AlOₓ encapsulation, the researchers established a versatile platform that preserves crystal integrity, enhances magnetic properties, and stabilizes topological quantum states. This study not only advances fundamental understanding but also propels quantum material fabrication toward scalable and reproducible device production, ushering in a new era of quantum-enabled technologies.
Subject of Research: Experimental study of magnetic topological insulator MnBi₂Te₄ using advanced exfoliation and encapsulation methods.
Article Title: Wax-Assisted Exfoliation Enables High-Quality MnBi₂Te₄ Devices with Enhanced Topological and Magnetic Properties
Web References: http://dx.doi.org/10.1016/j.scib.2025.08.005
References:
- Science 367, 895 (2020) – Observation of quantum anomalous Hall effect in MnBi₂Te₄
- Nat. Mater. 19, 522 (2020) – Axion insulator state in MnBi₂Te₄
- Nat. Commun. 11, 2453 (2020) – Gold-assisted exfoliation methods
- Nature 563, 94 (2018) – AlOₓ-assisted exfoliation methodologies
- Nat. Commun. 16, 1727 (2025); Nature 641, 70 (2025) – AlOₓ capping effects on MnBi₂Te₄
Image Credits: ©Science China Press
Keywords: Two-dimensional materials, magnetic topological insulators, MnBi₂Te₄, quantum anomalous Hall effect, axion insulator, exfoliation methods, thermoplastic wax, aluminum oxide encapsulation, quantum materials, magnetic anisotropy, device fabrication, condensed matter physics
