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. Here 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 micron scale domains,
pinned to structural disorder.