Excitation and detection of coherent magnon polarons in a ferromagnetic nanograting

A magnon polaron is the hybridized state of a phonon and a magnon, the elementary quanta of lattice vibrations and spin waves in a magnetically-ordered material. A magnon polaron can be formed at the intersection of the magnon and phonon dispersions, where their frequencies coincide. In spite of recent reports of magnon polarons in spintronic experiments, the observation of coherent magnon polarons in the time domain has remained extremely challenging because the weak interaction of magnons and phonons and their short lifetime jeopardize the strong coupling required for formation of a hybridized state. Here, we overcome these limitations by localizing magnons and phonons in a nanograting fabricated from a metallic ferromagnet, Galfenol, which possesses strong magnon-phonon interaction. The long lifetimes of the localized phonon and magnon modes allow us to find clear evidence of an optically excited magnon polaron in the magnetic and elastic coherent response of the nanograting. We show also that the symmetry of the localized modes governs the magnon-phonon hybridization and is a decisive factor in its direct observation.