  Topological Hall effect (THE), an electrical transport signature of systems
with chiral spin textures like skyrmions, has been observed recently in
topological insulator (TI)-based magnetic heterostructures. However, the
intriguing interplay between the topological surface state and THE is yet to be
fully understood. In this work, we report a large THE of ~10 ohm (~4
micro-ohm*cm) at 2 K with an electrically reversible sign in a top-gated 4 nm
TI (Bi0.3Sb0.7)2Te3 (BST) grown on a ferrimagnetic insulator (FI) europium iron
garnet (EuIG). Temperature, external magnetic field angle, and top gate bias
dependences of magnetotransport properties were investigated and consistent
with a skyrmion-driven THE. Most importantly, a sign change in THE was
discovered as the Fermi level was tuned from the upper to the lower parts of
the gapped Dirac cone and vice versa. This discovery is anticipated to impact
technological applications in ultralow power skyrmion-based spintronics.
