  Transport signatures of exchange gap opening because of magnetic proximity
effect (MPE) are reported for bilayer structures of Bi2Se3 thin films on
yttrium iron garnet (YIG) and thulium iron garnet (TmIG) of perpendicular
magnetic anisotropy (PMA). Pronounced negative magnetoresistance (MR) was
detected, and attributed to an emergent weak localization (WL) effect
superimposing on a weak antilocalization (WAL). Thickness-dependent study shows
that the WL originates from the time-reversal-symmetry breaking of topological
surface states by interfacial exchange coupling. The weight of WL declined when
the interfacial magnetization was aligned toward the in-plane direction, which
is understood as the effect of tuning the exchange gap size by varying the
perpendicular magnetization component. Importantly, magnetotransport study
revealed anomalous Hall effect (AHE) of square loops and anisotropic
magnetoresistance (AMR) characteristic, typifying a ferromagnetic conductor in
Bi2Se3/TmIG, and the presence of an interfacial ferromagnetism driven by MPE.
Coexistence of MPE-induced ferromagnetism and the finite exchange gap provides
an opportunity of realizing zero magnetic-field dissipation-less transport in
topological insulator/ferromagnetic insulator heterostructures.
