Recently, quantum anomalous Hall effect with spontaneous ferromagnetism was observed in twisted bilayer graphenes (TBG) near 3/4 filling. Importantly, it was observed 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 graphenes are generally believed to be non-magnetic. In this work, we demonstrate that in TBG, the twisting and substrate induced symmetry breaking allow an out of plane orbital magnetization to be generated by a charge current. Moreover, the large Berry curvatures of the flat bands give the Bloch electrons large orbital magnetic moments so that a small current can generate a large orbital magnetization. We further demonstrate how the charge current can switch the magnetization of the ferromagnetic TBG near 3/4 filling as observed in the experiments.

最近，在 3/4 填充附近的扭曲双层石墨烯 (TBG) 中观察到具有自发铁磁性的量子反常霍尔效应。重要的是，观察到极小的电流可以切换磁化方向。这为实现低能量耗散磁存储器提供了前景。然而，电流驱动磁化切换的机制知之甚少，因为石墨烯中的充电电流通常被认为是非磁性的。在这项工作中，我们证明了在 TBG 中，扭曲和衬底引起的对称性破坏允许充电电流产生平面外轨道磁化。此外，平带的大贝里曲率使布洛赫电子具有大的轨道磁矩，因此小电流可以产生大的轨道磁化强度。