  The quantum anomalous Hall effect (QAHE) that emerges under broken
time-reversal symmetry in topological insulators (TI) exhibits many fascinating
physical properties for potential applications in nano-electronics and
spintronics. However, in transition-metal doped TI, the only experimentally
demonstrated QAHE system to date, the effect is lost at practically relevant
temperatures. This constraint is imposed by the relatively low Curie
temperature (Tc) and inherent spin disorder associated with the random magnetic
dopants. Here we demonstrate drastically enhanced Tc by exchange coupling TI to
Tm3Fe5O12, a high-Tc magnetic insulator with perpendicular magnetic anisotropy.
Signatures that the TI surface states acquire robust ferromagnetism are
revealed by distinct squared anomalous Hall hysteresis loops at 400 K.
Point-contact Andreev reflection spectroscopy confirms that the TI surface is
indeed spin-polarized. The greatly enhanced Tc, absence of spin disorder, and
perpendicular anisotropy are all essential to the occurrence of the QAHE at
high temperatures.
