  Interfacing topological insulators (TIs) with magnetic insulators (MIs) has
been widely used to study the interaction between topological surface states
and magnetism. Previous transport studies typically interpret the suppression
of weak antilocalization or appearance of the anomalous Hall effect as
signatures of magnetic proximity effect (MPE) imposed to TIs. Here, we report
the observation of nonlinear planar Hall effect (NPHE) in Bi2Se3 films grown on
MI thulium and yttrium iron garnet (TmIG and YIG) substrates, which is an order
of magnitude larger than that in Bi2Se3 grown on nonmagnetic gadolinium gallium
garnet (GGG) substrate. The nonlinear Hall resistance in TmIG/Bi2Se3 depends
linearly on the external magnetic field, while that in YIG/Bi2Se3 exhibits an
extra hysteresis loop around zero field. The magnitude of the NPHE is found to
scale inversely with carrier density. We speculate the observed NPHE is related
to the MPE-induced exchange gap opening and out-of-plane spin textures in the
TI surface states, which may be used as an alternative transport signature of
the MPE in MI/TI heterostructures.
