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.