Electrical transport studies in the topological insulator Bi$_{2}$Se$_{3}$ with exchange induced ferromagnetism*

The proximity-induced ferromagnetic order in topological insulator (TI)/ferromagnetic insulator (FI) heterostructures induces ferromagnetism in TI, which breaks local time reversal symmetry that can lead to many exotic properties, such as image magnetic monopole, topological magneto-electric effects, etc.[1] We achieved this novel ferromagnetic order in a TI Bi$_{2}$Se$_{3}$ through Bi$_{2}$Se$_{3}$/EuS bi-layer structures. Electric transport studies show a dramatic suppression of the weak anti-localization (WAL) effect in Bi$_{2}$Se$_{3}$/EuS compared to controlled Bi$_{2}$Se$_{3}$ samples. In contrast to the case of surface doping a TI with magnetic atoms (i.e. Fe), here the WAL cannot be quenched even with a full coverage EuS capping layer, which points that its origin can be the opening of a surface gap rather than a reduction of the magnetic scattering length. The results are analyzed with a theoretical model providing a value for the induced surface exchange gap. Other experimental results, such as the anomalous Hall effect that support the proximity induced ferromagnetism in Bi$_{2}$Se$_{3}$ will be discussed.\\[4pt] [1] Qi, X.-L. {\&} Zhang, S.-C., Rev Mod Phys 83, 1057-1110, (2011).