Ultrafast phonon-mediated melting of magnetism via itinerant spins in MnBi₂Te₄

A comprehensive understanding of spins away from equilibrium, including their interplay with lattice and electronic excitations is key to achieving fundamental breakthroughs in understanding how to manipulate topological phases with light. To this end, we study the ultrafast melting of magnetic order in the layered antiferromagnetic topological insulator MnBi₂Te₄. Here we use a multimodal experimental approach with ultrafast electron diffuse scattering to probe of nonequilibrium phonon dynamics, and magneto-optic Kerr rotation and resonant soft X-ray scattering are used to directly measure ultrafast dynamics of conduction and localized spins. Our experiments reveal that ultrafast demagnetization occurs through a previously unidentified p-like itinerant spin subsystem, with optical phonons acting as the primary channel for the dissipation of spin angular momentum. Localized Mn 3d⁵ spins disorder by a distinct, much slower thermalization mechanism, due to the quenched orbital angular momentum. The disparate demagnetization timescales in both spin subsystems provides evidence of a strong exchange coupling between the 3d­ localized spins and the p-like itinerant bands, which sheds light on the interatomic exchange pathways that enable magnetic topological phases.