Quantum Imaging of Magnetic Phase Transitions in Intrinsic Magnetic Topological Nanoflakes*

Recently, the family of intrinsic topological magnet MnBi₂Te₄(Bi₂Te₃)_n has emerged as an attractive material platform to explore topologically protected quantum spin and charge transport behaviors in a broad magnetic field and temperature regime. Examples include the high-temperature quantum anomalous Hall effect, layer Hall effect, axion insulator phase, and many others. Establishment of a detailed knowledge of the local magnetic properties of MnBi₂Te₄(Bi₂Te₃)_n contributes to a comprehensive understanding of the observed exotic quantum transport behaviors. Here, we introduce nitrogen-vacancy (NV) centers as single-spin sensors to directly image the local static and dynamic spin properties of MnBi₂Te₄(Bi₂Te₃)_n nanoflakes, revealing the detailed process of the magnetic phase transition in nanometer length scale. We also reveal the intriguing physics underlying the spin transport in MnBi₂Te₄(Bi₂Te₃)_n. Our results illustrate the unique capability enabled by NV centers for investigating the interplay between magnetism and topology potentially in broad range of quantum materials.