Formation of binary magnon polaron in a two-dimensional artificial magneto-elastic crystal

We observed strong tripartite magnon-phonon-magnon coupling in a two-dimensional periodic array of magnetostrictive nanomagnets deposited on a piezoelectric substrate, forming a 2D magnetoelastic “crystal”; the coupling occurred between two Kittel-type spin wave (magnon) modes and a (non-Kittel) magnetoelastic spin wave mode caused by a surface acoustic wave (SAW) (phonons). The strongest coupling occurred when the frequencies and wavevectors of the three modes matched, leading to perfect phase matching. We achieved this condition by carefully engineering the frequency of the SAW, the nanomagnet dimensions and the bias magnetic field that determined the frequencies of the two Kittel-type modes. The strong coupling (cooperativity factor exceeding unity) led to the formation of a new quasi-particle, called a binary magnon-polaron, accompanied by nearly complete (~100%) transfer of energy from the magnetoelastic mode to the two Kittel-type modes. This coupling phenomenon exhibited significant anisotropy since the array did not have rotational symmetry in space. The experimental observations were in good agreement with the theoretical simulations. This article reveals a study on magnon-phonon coupling in two-dimensional artificial magneto-elastic crystals. Researchers fabricated a 2D periodic array of magnetostrictive nanomagnets on a piezoelectric substrate and observed strong tripartite coupling involving two magnons and a phonon. This coupling transfers all or nearly all of the power from a magneto-elastic mode caused by surface acoustic waves (SAWs) to two Kittel-type spin wave modes. The findings highlight the importance of engineering SAW frequency, magnetic field, and nanomagnet dimensions to ensure near-perfect phase matching between all modes. This discovery could pave the way for future developments in energy-efficient computing, communications, and data storage. A two-dimensional array of magnetostrictive nanomagnets was used to demonstrate strong coupling between two different magnons (kM1′ and kM1′′) mediated by a phonon (kph). The coupling is strong, leading to the formation of a new quasi-particle – binary magnon-polaron. These two different magnons show 180° phase difference which is reminiscent of dark magnon modes. We show that it is possible to engineer this magnon-phonon coupling by choosing the frequency and wavelength of the acoustic wave to match the frequency and wavelength of the spin wave, the latter being controlled by a magnetic field.