Tunable Orbital Ferromagnetism at Noninteger Filling of a Moire Superlattice

The flat bands resulting from moire superlattices exhibit fascinating correlated electron phenomena such as correlated insulators, (Nature 2018, 556 (7699), 80-84), (Nature Physics 2019, 15 (3), 237) superconductivity, (Nature 2018, 556 (7699), 43-50), (Nature 2019, 572 (7768), 215-219) and orbital magnetism. (Science 2019, 365 (6453), 605-608), (Nature 2020, 579 (7797), 56-61), (Science 2020, 367 (6480), 900-903) Such magnetism has been observed only at particular integer multiples of n(0), the density corresponding to one electron per moire superlattice unit cell. Here, we report the experimental observation of ferromagnetism at noninteger filling (NIF) of a flat Chern band in a ABC-TLG/hBN moire superlattice. This state exhibits prominent ferromagnetic hysteresis behavior with large anomalous Hall resistivity in a broad region of densities centered in the valence miniband at n = -2.3n(0). We observe that, not only the magnitude of the anomalous Hall signal, but also the sign of the hysteretic ferromagnetic response can be modulated by tuning the carrier density and displacement field. Rotating the sample in a fixed magnetic field demonstrates that the ferromagnetism is highly anisotropic and likely purely orbital in character.

Automated summary

The study reports the experimental observation of ferromagnetism at noninteger filling of a flat Chern band in a ABC-TLG/hBN moire superlattice. The state exhibits prominent ferromagnetic hysteresis behavior with large anomalous Hall resistivity in a broad region of densities centered in the valence miniband at n = -2.3n(0). The magnitude and sign of the anomalous Hall signal can be modulated by tuning the carrier density and displacement field. The ferromagnetism is highly anisotropic and likely purely orbital in character.