  The possible realization of dissipationless chiral edge current in a
topological insulator / magnetic insulator heterostructure is based on the
condition that the magnetic proximity exchange coupling at the interface is
dominated by the Dirac surface states of the topological insulator. Here we
report a polarized neutron reflectometry observation of Dirac electrons
mediated magnetic proximity effect in a bulk-insulating topological insulator
(Bi$_{0.2}$Sb$_{0.8}$)$_{2}$Te$_{3}$ / magnetic insulator EuS heterostructure.
We are able to maximize the proximity induced magnetism by applying an
electrical back gate to tune the Fermi level of topological insulator to be
close to the charge neutral point. A phenomenological model based on
diamagnetic screening is developed to explain the suppressed proximity induced
magnetism at high carrier density. Our work paves the way to utilize the
magnetic proximity effect at the topological insulator/magnetic insulator
hetero-interface for low-power spintronic applications.
