Tunable Orbital Ferromagnetism at Noninteger Filling of a Moiré Superlattice

The flat bands resulting from moiré superlattices exhibit fascinating correlated electron phenomena such as correlated insulators, ( 2018, 556 (7699), 80-84), ( 2019, 15 (3), 237) superconductivity, ( 2018, 556 (7699), 43-50), ( 2019, 572 (7768), 215-219) and orbital magnetism. ( 2019, 365 (6453), 605-608), ( 2020, 579 (7797), 56-61), ( 2020, 367 (6480), 900-903) Such magnetism has been observed only at particular integer multiples of , the density corresponding to one electron per moiré 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 moiré 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 = -2.3. 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.