Magnon polaron formed by selectively coupled coherent magnon and phonon modes of a surface patterned ferromagnet

TL;DR

This paper demonstrates the formation of magnon polarons in a surface-patterned ferromagnet by spatially matching magnon and phonon modes, enabling strong coupling and clear observation of hybridized states.

Contribution

It introduces a method to achieve strong magnon-phonon coupling in a nanoscale patterned ferromagnet, overcoming previous limitations of weak interaction and short lifetimes.

Findings

High coupling strength achieved through spatial mode matching.

Clear evidence of optically excited magnon polarons.

Symmetry of localized modes influences polaron formation.

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.