Tracking magnon propagation with space/time resolved polarimetry

The propagation of spin waves in magnetically ordered systems has emerged as a potential means to shuttle quantum information over large distances. An important subset of these systems are "easy-plane" magnets in which the spins are oriented parallel to the planes but without a preferred direction within the plane. In this talk, I describe an experimental and theoretical study of the easy plane ferromagnet Fe₃Sn₂, in which magnetism arises from a Kagome lattice of Fe ions. Our measurements utilize temporal and spatially resolved optical techniques to launch and detect spin wavepackets, providing quantitative information on their amplitude, frequency, and phase. Conventionally, the arrival time of a spin wavepacket at a distance, d, is assumed to be determined by its group velocity, v_g. Surprisingly, we observe the arrival of spin information at times significantly less than d/v_g. We show that this spin wave "precursor" phenomenon originates from the interaction of light with the unusual spectrum of magnetostatic modes in Fe₃Sn₂. Related effects may have far-reaching consequences toward realizing long-range, ultrafast spin wave transport in both ferromagnetic and antiferromagnetic systems.