Chiral Stoner magnetism in Dirac bands

Abstract

Stoner magnetism in bands endowed with Berry curvature is shown to be influenced by the coupling between spin-chirality density $\mathit{s}·({\partial }_x\mathit{s}\times {\partial }_y\mathit{s})$ and Berry's orbital magnetization. The key effect is that carriers moving in the presence of a spin texture see it as a source of a geometric magnetic field coupled to the carrier's orbital motion through a spin-dependent Aharonov-Bohm effect. This interaction was recently predicted to enable chiral magnons propagating along system boundaries. Here we show that it also favors chiral spin textures such as skyrmions—the topologically protected objects with particle-like properties, stabilized in the ground state. Unlike previously studied systems, here skyrmion textures can arise in the absence of microscopic spin-dependent interactions such as spin-orbit coupling or Zeeman coupling. The threshold for Stoner instability is found to soften, rendering chiral spin-ordered phases accessible under realistic conditions. We present a detailed analysis of the chiral effect for Bernal bilayer graphene and discuss the unique properties of skyrmion textures in graphene multilayers.