  Magnetism in topological materials creates phases exhibiting quantized
transport phenomena with applications in spintronics and quantum information.
The emergence of such phases relies on strong interaction between localized
spins and itinerant states comprising the topological bands, and the subsequent
formation of an exchange gap. However, this interaction has never been measured
in any intrinsic magnetic topological material. Using a multimodal approach,
this exchange interaction is measured in MnBi2Te4, the first realized intrinsic
magnetic topological insulator. Interrogating nonequilibrium spin dynamics,
itinerant bands are found to exhibit a strong exchange coupling to localized Mn
spins. Momentum-resolved ultrafast electron scattering and magneto-optic
measurements reveal that itinerant spins disorder via electron-phonon
scattering at picosecond timescales. Localized Mn spins, probed by resonant
X-ray scattering, disorder concurrently with itinerant spins, despite being
energetically decoupled from the initial excitation. Modeling the results using
atomistic simulations, the exchange coupling between localized and itinerant
spins is estimated to be >100 times larger than superexchange interactions.
This implies an exchange gap of >25 meV should occur in the topological surface
states. By directly quantifying local-itinerant exchange coupling, this work
validates the materials-by-design strategy of utilizing localized magnetic
order to create and manipulate magnetic topological phases, from static to
ultrafast timescales.
