Imaging orbital ferromagnetism in a moire Chern insulator

Electrons in moire flat band systems can spontaneously break time-reversal symmetry, giving rise to a quantized anomalous Hall effect. In this study, we use a superconducting quantum interference device to image stray magnetic fields in twisted bilayer graphene aligned to hexagonal boron nitride. We find a magnetization of several Bohr magnetons per charge carrier, demonstrating that the magnetism is primarily orbital in nature. Our measurements reveal a large change in the magnetization as the chemical potential is swept across the quantum anomalous Hall gap, consistent with the expected contribution of chiral edge states to the magnetization of an orbital Chern insulator. Mapping the spatial evolution of field-driven magnetic reversal, we find a series of reproducible micrometer-scale domains pinned to structural disorder.

Automated summary

Researchers have found that electrons in moire flat band systems can break time-reversal symmetry, leading to a quantized anomalous Hall effect, with magnetism primarily orbital in nature. The study also reveals a significant change in magnetization as the chemical potential crosses the quantum anomalous Hall gap, consistent with the expected contribution of chiral edge states to the magnetization in an orbital Chern insulator. Additionally, mapping the spatial evolution of field-driven magnetic reversal shows reproducible micrometer-scale domains pinned to structural disorder.