Twisted magnetic topological insulators

Abstract

Motivated by the discovery of quantum anomalous Hall effect in Cr-doped Bi 2 Se 3 , we study the generic states for magnetic topological insulators and explore the physical properties for both magnetism and itinerant electrons. First-principles calculations are exploited to investigate the magnetic interactions between magnetic Co atoms adsorbed on the Bi 2 Se 3 (111) surface. Due to the absence of inversion symmetry on the surface, there are Dzyaloshinskii-Moriya like twisted spin interactions between the local moments of Co ions. These non-ferromagnetic interactions twist the collinear spin configuration of the ferromagnet and generate various magnetic orders beyond a simple ferromagnet. Among them, the spin spiral state generates alternating counterpropagating modes across each period of spin states, and the skyrmion lattice even supports a chiral mode around the core of each skyrmion. The skyrmion lattice opens a gap at the surface Dirac point, resulting in anomalous Hall effect. These results would inspire further experimental investigation of magnetic topological insulators.