Highly tunable magnetic coupling in ultrathin topological insulator films due to impurity resonances

Abstract

We theoretically investigate the carrier-free exchange interaction between magnetic impurities in ultrathin Bi2Se3 topological insulator films by taking into account the low-energy states produced by the impurities. To match with experimental observations of magnetism on the surface of ultrathin topological insulator films, we restrict the calculations to having the chemical potential within the energy gap, with then interband processes mediating the exchange interaction. We find that the locally induced impurity resonances strongly influence the exchange interaction between magnetic moments. In particular, we find a noncollinear alignment to be more favorable than the collinear ferromagnetic alignment preferred when impurity states are ignored and only the pristine topological insulator band structure is considered. As a result, chiral ferromagnetism can easily become favorable over the ferromagnetic phase in these materials. Moreover, we show that by applying an electric field perpendicular to the ultrathin film, the exchange interaction can be drastically enhanced. This generates the possibility of highly tunable magnetism by electric field.