Large Itinerant Electron Exchange Coupling in the Magnetic Topological Insulator MnBi2Te4

TL;DR

This study measures and analyzes the strong exchange interaction between localized Mn spins and itinerant electrons in the magnetic topological insulator MnBi2Te4, revealing ultrafast spin dynamics and large exchange gaps.

Contribution

It provides the first direct measurement of exchange coupling in an intrinsic magnetic topological insulator, demonstrating its magnitude and ultrafast dynamics.

Findings

Strong exchange coupling >100 times larger than superexchange

Itinerant spins disorder via electron-phonon scattering at picosecond timescales

Exchange gap >25 meV in topological surface states

Abstract

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.