Giant Orbital Magneto-electric effect and Current-driven Magnetization Switching in Twisted Bilayer Graphene

TL;DR

This paper reveals how twisted bilayer graphene's symmetry reduction enables charge currents to generate significant orbital magnetization, leading to low-energy switching of magnetization states in quantum anomalous Hall systems.

Contribution

It uncovers the mechanism by which current-driven orbital magnetization occurs in TBGs due to symmetry breaking and Berry phase effects, explaining experimental magnetization switching.

Findings

Charge currents can generate large orbital magnetization in TBGs.

Symmetry breaking enhances the magnetoelectric effect in twisted bilayer graphene.

Small currents can switch magnetization states in quantum anomalous Hall phases.

Abstract

Recently, signatures of quantum anomalous Hall states with spontaneous ferromagnetism were observed in twisted bilayer graphenes (TBGs) near 3/4 filling [1, 2]. Importantly, it was demon-strated that an extremely small current can switch the direction of the magnetization. This offers the prospect of realizing low energy dissipation magnetic memories. However, the mechanism of the current-driven magnetization switching is poorly understood as the charge currents in graphene layers are generally believed to be non-magnetic. In this work, we demonstrate that, in TBGs, the twist-induced reduction of lattice symmetry allows a charge current to generate net orbital magnetization at a general filling factor through magnetoelectric effects. Substrate-induced strain and sublattice symmetry breaking further reduce the symmetry such that an out-of-plane orbital magnetization can be generated. Due to the large non-trivial Berry phase of the flat bands, the orbital magnetization of a Bloch state can be as large as tens of Bohr magnetons and therefore a small current would be sufficient to generate a large orbital magnetization. We further demonstrate how the charge current with orbital magnetization can switch the magnetization of the quantum anomalous Hall state near 3/4 filling as observed in the experiments [1, 2].