Abstract
Strong coupling between two quanta of different excitations leads to the formation of a hybridized state which paves a way for exploiting new degrees of freedom to control phenomena with high efficiency and precision. 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. The observation of magnon polarons in the time domain has remained extremely challenging because the weak interaction of magnons and phonons and their short lifetimes jeopardize the strong coupling required for the formation of a hybridized state. Here, we overcome these limitations by spatial matching of magnons and phonons in a metallic ferromagnet with a nanoscale periodic surface pattern. The spatial overlap of the selected phonon and magnon modes formed in the periodic ferromagnetic structure results in a high coupling strength which, in combination with their long lifetimes allows us to find clear evidence of an optically excited magnon polaron. We show that the symmetries of the localized magnon and phonon states play a crucial role in the magnon polaron formation and its manifestation in the optically excited magnetic transients.
Highlights
Magnons are collective spin excitations in magnetically-ordered materials
Summarizing the main experimental results, we clearly observe magnon polaron formation for the fundamental magnon (FM)-quasi-transverse acoustic (QTA) resonance through the normal mode splitting, but no avoided crossing is detected for the FMQLA resonance
The avoided crossing effect is observed for the lower FM-QTA* resonance mode while for the upper FM-quasi-longitudinal acoustic (QLA) resonance the splitting of the hybridized state is masked by strong driving of the FM mode by resonant phonons
Summary
Magnons are collective spin excitations in magnetically-ordered materials. Nowadays the manipulation of coherent high-frequency magnons on the nanoscale is one of the most prospective concepts for information technologies, in the quantum regime. The experimental observation of the avoided crossing for magnon polarons is possible when the energy splitting at the magnon-phonon resonance exceeds the spectral widths of the interacting modes. By surface patterning of a ferromagnetic film, we achieve magnon and phonon modes with significantly long lifetimes Spatial matching of their wave distributions determines the coupling strength and selects particular modes in the phonon and magnon spectra for hybridization. This allows us to solve the main problem of quick dephasing of the interacting excitations, and we achieve C ≈ 8 sufficient for pronounced magnon polaron formation. The estimated errors (the standard mean errors of the fit parameters) of fj and γj are 0.002 GHz and 0.02 GHz, respectively
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