Vertical quantum transport in magnetic topological insulator heterostructures

Magnetic topological insulator (TI) thin films with time-reversal symmetry broken by proximity to magnetic layers support the quantum anomalous Hall effect (QAHE) and the topological magneto-electric (TME) effect. We investigate theoretically a heterostructure composed of a TI thin film sandwiched between two antiferromagnetic (AFM) metal layers. The magnetic structure in the AFM films consists of alternating planes with ferromagnetic order parallel and antiparallel to the surface normal direction. We show that quantum transport along the vertical direction of this heterostructure is strongly influenced by whether the exchange fields on the top and bottom surface of the TI are parallel or antiparallel. In the first case, QAHE conducting topological edge states on the sidewall surfaces contribute significantly to a leak conductance. In the case of opposite magnetization, the system is in the axion-insulator state, all the edge states are gapped, and the vertical conductance is small. By using a microscopic tight-binding model combined with a non-equilibrium Green’s function approach, we explore the dependence of the sidewall electronic structure and transport on the profile of the exchange coupling, and the TME effect in the presence of external electric and magnetic fields.