Long-distance coherent propagation of magnon polarons in a ferroelectric-ferromagnetic heterostructure

We experimentally demonstrate the coherent propagation of magnon polarons, the hybridized excitations formed by the coupling of spin waves and Rayleigh/Love surface acoustic waves, in a heterostructure consisting of ferroelectric ${\mathrm{BaTiO}}₃$ and ferromagnetic ${\mathrm{La}}_0.67{\mathrm{Sr}}_0.33{\mathrm{MnO}}₃$. By systematically adjusting external magnetic field strengths and angles with respect to the wave vector, the wavelengths of coherent magnon polarons can be precisely controlled and maintained over distances of up to $80\phantom{\rule{4pt}{0ex}}\textmu{}\mathrm{m}$. Their excitation was obtained using nanoscale interdigital transducer antennas. Our theoretical model, based on effective interlayer magnetoelastic coupling, reproduces the experimental transmission spectra, in particular the anticrossing features, allowing us to estimate the magnetoelastic coupling strength in this system to be of the order of $2\ifmmode\times\else\texttimes\fi{}{10}⁵\phantom{\rule{4pt}{0ex}}\mathrm{J}/{\mathrm{m}}³$. Our findings provide valuable insights into the coherent properties of magnon polarons and open up possibilities for the design of functional devices in the fields of spintronics and magnonics.