  The indirect exchange interaction between local magnetic moments via surface
electrons has been long predicted to bolster the surface ferromagnetism in
magnetic topological insulators (MTIs), which facilitates the quantum anomalous
Hall effect. This unconventional effect is critical to determining the
operating temperatures of future topotronic devices. However, the experimental
confirmation of this mechanism remains elusive, especially in intrinsic MTIs.
Here we combine time-resolved photoemission spectroscopy with time-resolved
magneto-optical Kerr effect measurements to elucidate the unique
electromagnetism at the surface of an intrinsic MTI MnBi2Te4. Theoretical
modeling based on 2D Ruderman-Kittel-Kasuya-Yosida interactions captures the
initial quenching of a surface-rooted exchange gap within a factor of two but
over-estimates the bulk demagnetization by one order of magnitude. This
mechanism directly explains the sizable gap in the quasi-2D electronic state
and the nonzero residual magnetization in even-layer MnBi2Te4. Furthermore, it
leads to efficient light-induced demagnetization comparable to state-of-the-art
magnetophotonic crystals, promising an effective manipulation of magnetism and
topological orders for future topotronics.
