Imaging orbital ferromagnetism in a moir\'e Chern insulator

TL;DR

This study visualizes orbital ferromagnetism in a moiré Chern insulator using SQUID microscopy, revealing large orbital magnetization, chiral edge states, and domain structures in twisted bilayer graphene aligned with hBN.

Contribution

First direct imaging of orbital ferromagnetism and chiral edge states in a moiré Chern insulator using SQUID microscopy.

Findings

Large orbital magnetization per charge carrier.

Magnetization change across the quantum anomalous Hall gap.

Reproducible micron-scale magnetic domains with chiral edge states.

Abstract

Electrons in moir\'e 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 one such system composed of 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 whose boundaries host chiral edge states.