Exchange interactions between topological and meta-magnetic insulators*

The hallmark of 3D topological insulators(TIs)are topological surface states(TSS)protected by the time-reversal symmetry.One of the important goals is the ability to manipulate surface electron states and to lift the topological protection of TSS for the emergence of novel physical phenomena.Gapping of TSS has been demonstrated in TIs with bulk doping using the magnetic impurities and via exchange coupling to insulating magnetic or antiferromagnetic materials.Here we report an experimental study of exchange coupling in EuSe/Bi₂Se₃ and EuSe/Bi₂Se₃/EuSe heterostructures grown by MBE.Bi₂Se₃ is a typical TI and EuSe is a meta-magnetic insulator with a rich phase diagram of paramagnetic,antiferromagnetic,ferrimagnetic and ferromagnetic phases at low temperatures.Polarized neutron scattering experiments indicate enhanced exchange interaction between EuSe and Bi₂Se₃ and we observe finite in-plane magnetization at the interface even above the Néel temperature (T_N~5K) of EuSe.In transport measurement,we observe modulation of longitudinal and Hall resistance as a function of the magnetic field,which may be explained by the formation and re-arrangement of magnetic domains at the interface.