Evidence of Orbital Ferromagnetism in Twisted Bilayer Graphene Aligned to Hexagonal Boron Nitride

Abstract

We have previously reported ferromagnetism evinced by a large hysteretic anomalous Hall effect in twisted bilayer graphene (tBLG). Subsequent measurements of a quantized Hall resistance and small longitudinal resistance confirmed that this magnetic state is a Chern insulator. Here, we report that when tilting the sample in an external magnetic field, the ferromagnetism is highly anisotropic. Because spin−orbit coupling is weak in graphene, such anisotropy is unlikely to come from spin but rather favors theories in which the ferromagnetism is orbital. We know of no other case in which ferromagnetism has a purely orbital origin. For an applied in-plane field larger than 5 T, the out-of-plane magnetization is destroyed, suggesting a transition to a new phase.

Anomalous Hall effect at half filling in twisted bilayer graphene

“…Prior measurements of the magnetism underlying the AHE in tBLG indicate that it is driven primarily or exclusively by orbital magnetic moments [26,28,29]. Owing to the extraordinarily weak spin-orbit coupling (SOC) in graphene, the AHE we observe here is almost certainly also driven by orbital magnetism.…”

Spin-orbit–driven ferromagnetism at half moiré filling in magic-angle twisted bilayer graphene

“…S4). At the same time, direct coupling between the valley order and B || is shown to be absent in tBLG samples without SOC (31), which rules out the orbital effect as a possible origin for the observed B || -dependence. Taken together, we conclude that B⊥ and B || couple to the magnetic order through different mechanisms: B⊥ directly controls the magnetic ground state through valley Zeeman coupling,…”

Colloquium: Quantum anomalous Hall effect

“…In many cases, moiré materials can be tuned into regimes in which correlations are strong and broken symmetries are common. The broken symmetry states that have been realized include superconductors (Balents et al, 2020;Cao et al, 2018b;Lu et al, 2019;Yankowitz et al, 2019), Mott insulators (Cao et al, 2018a), Wigner crystals (Li et al, 2021c;Xu et al, 2020b) and -of particular interest to this review -unusual orbital ferromagnets with Chern insulator ground states (Chen et al, 2020a,a,b;Li et al, 2021e;Polshyn et al, 2020;Serlin et al, 2020;Sharpe et al, 2019Sharpe et al, , 2021Tschirhart et al, 2021). In the following, we explain the QAH effects seen in graphene and TMD moiré materials, which are alike in that they rely on spontaneous valley polarization, and distinct in that their non-trivial topologies are related respectively to sublattice and layer degrees of freedom.…”