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Imaging orbital ferromagnetism in a moiré Chern insulator
C L Tschirhart1, M Serlin1, H Polshyn1
1Department of Physics, University of California, Santa Barbara, CA 93106, USA.
Science (New York, N.Y.)
|May 28, 2021
Summary
Researchers observed orbital magnetism in twisted bilayer graphene, leading to a quantized anomalous Hall effect. This magnetism, primarily orbital, forms reproducible domains linked to structural imperfections.
Area of Science:
- Condensed Matter Physics
- Materials Science
- Quantum Phenomena
Background:
- Moiré flat band systems exhibit spontaneous time-reversal symmetry breaking.
- This symmetry breaking can lead to the quantized anomalous Hall effect (QAHE).
- Understanding the nature and origin of magnetism in these systems is crucial.
Purpose of the Study:
- To investigate the magnetic properties of twisted bilayer graphene (TBG) aligned to hexagonal boron nitride (hBN).
- To determine the nature (orbital vs. spin) of the observed magnetism.
- To explore the relationship between magnetism, QAHE, and material structure.
Main Methods:
- Utilized a superconducting quantum interference device (SQUID) to image stray magnetic fields.
- Measured magnetization in TBG/hBN heterostructures.
- Mapped the spatial evolution of field-driven magnetic reversal.
Main Results:
- Observed significant magnetization (several Bohr magnetons per charge carrier), indicating primarily orbital magnetism.
- Found a large change in magnetization near the QAHE gap, consistent with orbital Chern insulator theory.
- Identified reproducible micrometer-scale magnetic domains linked to structural disorder.
Conclusions:
- The study confirms orbital magnetism as the source of QAHE in TBG/hBN.
- Chiral edge states contribute to the magnetization, supporting the orbital Chern insulator model.
- Structural disorder plays a key role in pinning magnetic domains and influencing magnetic reversal.
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