Strongly enhanced topological quantum phases in dual-surface AlOx-encapsulated MnBi2Te4

The topological quantum phases in antiferromagnetic topological insulator MnBi₂Te₄ hold promise for next-generation spintronics, but their experimental realization has been constrained by challenges in preparing high-quality devices. In this work, we report a new wax-assisted exfoliation and transfer method that enables the fabrication of MnBi₂Te₄ heterostructures with both surfaces encapsulated by AlO_x. This strategy strongly enhances the transport performances of the topological quantum phases in MnBi₂Te₄ flakes, which is attributed to the enhancement of magnetism by the AlO_x layer. We observe the robust axion insulator state in even-layer device with wide zero Hall plateau and high longitudinal resistivity, and the quantum anomalous Hall effect in odd-layer device with large hysteresis and sharp plateau transition. These results demonstrate that the combination of wax exfoliation and AlO_x encapsulation provides great potentials for exploring novel topological quantum phenomena and potential applications in MnBi₂Te₄ and other two-dimensional materials.