Ultrafast magnetization and magnon dynamics in a two-dimensional topological antiferromagnet MnBi₂Te₄

Atomically thin van der Waals magnetic materials have not only provided a fertile playground to explore basic physics in the two-dimensional (2D) limit but also created vast opportunities for novel ultrafast functional devices. Here we systematically investigate ultrafast magnetization dynamics and spin wave dynamics in few-layer topological antiferromagnetic MnBi₂Te₄ crystals as a function of layer number, temperature, and magnetic field. We observe laser-induced (de)magnetization processes that can be used to accurately track the distinct magnetic states in different magnetic field regimes, including showing clear odd-even layer number effects. In addition, strongly field-dependent antiferromagnetic magnon modes with tens of gigahertz frequencies are optically generated and directly observed in the time domain. Surprisingly, the magnetic state dependence and magnons are observed not only in time-resolved Kerr rotation and but also in time-resolved reflectivity measurements, indicating a strong correlation between the magnetic state and the electronic structure. These measurements present the first comprehensive overview of ultrafast spin dynamics in this novel 2D antiferromagnet, paving the way for potential applications in 2D antiferromagnetic spintronics and magnonics as well as further studies of ultrafast control of both magnetization and topological quantum states.