Evidence of orbital ferromagnetism in twisted bilayer graphene aligned to hexagonal boron nitride

TL;DR

This paper provides evidence that the ferromagnetism observed in twisted bilayer graphene aligned with hexagonal boron nitride is likely of orbital origin, showing high anisotropy and a transition to a new phase under strong in-plane magnetic fields.

Contribution

It demonstrates that the ferromagnetism in twisted bilayer graphene is highly anisotropic and likely orbital in nature, a novel finding in the context of two-dimensional magnetic materials.

Findings

Ferromagnetism in tBLG is highly anisotropic.

The magnetic state is a Chern insulator with quantized Hall resistance.

Strong in-plane magnetic fields destroy out-of-plane magnetization.

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 negligible 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 $5\ \mathrm{T}$, the out-of-plane magnetization is destroyed, suggesting a transition to a new phase.