  Many exotic physics anticipated in topological insulators require a gap to be
opened for their topologica surface states by breaking time reversal symmetry.
The gap opening has been achieved by doping magnetic impurities, which however
inevitably create extra carriers and disorder that undermine the electronic
transport. In contrast, the proximity to a ferromagnetic/ferrimagnetic
insulator may improve the device quality, thus promises a better way to open
the gap while minimizing the side-effects. Here, we grow thin single-crystal
Sb1.9Bi0.1Te3 micro flakes on insulating ferrimagnet BaFe12O19 by using the van
der Waals epitaxy technique. The micro flakes show a negative magnetoresistance
in weak perpendicular fields below 50 K, which can be quenched by increasing
temperature. The signature implies the weak localization effect as its origin,
which is absent in intrinsic topological insulators, unless a surface state gap
is opened. The surface state gap is estimated to be 10 meV by using the theory
of the gap-induced weak localization effect. These results indicate that the
magnetic proximity effect may open the gap for the topological surface attached
to BaM insulating ferrimagnet. This heterostructure may pave the way for the
realization of new physical effects as well as the potential applications of
spintronics devices.
