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 spin-based 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 (T_d) site on the sub-picosecond timescale, removing the Pauli blocking that initially inhibits electron delocalization and thereby enabling 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.