Ultrafast Charge, Spin, and Lattice Coupling Facilitates Spin Transport as Magnon-Polarons in Yttrium Iron Garnet

Low spin damping and long spin-polarization lifetimes are essential for spintronic and spinbased energy-conversion technologies, yet their microscopic origin in magnetic insulators remains poorly understood. Yttrium iron garnet (YIG), a prototypical magnetic insulator exhibiting exceptionally low damping and long magnon lifetimes, provides an ideal platform to investigate how charge, spin, and lattice interactions stabilize spin-polarized carriers. Here we directly probe these coupled dynamics using ultrafast linearly and circularly polarized extreme-ultraviolet (XUV) spectroscopy, which provides femtosecond time resolution together with element-and site-specific sensitivity to electronic structure and spin polarization. Our measurements reveal the ultrafast formation of magnon–polaron quasiparticles mediated by strong charge–spin–lattice coupling. XUV magnetic circular dichroism (XUV-MCD) directly resolves a transient spin reversal at the Fe tetrahedral (Td) site on the sub-picosecond timescale, relaxing Pauli exclusion, and enabling electron delocalization and efficient spin-polarized transport. Coherent charge–lattice dynamics and phonon coherence transfer further indicate the formation of delocalized quasiparticles extending over several to tens of unit cells. These results establish a direct microscopic link between magnon–polaron formation, the long-lived spin polarization and efficient spin transport in YIG, providing a framework for understanding how coupled spin–charge–lattice excitations govern spin transport in magnetic insulators.