Dynamically Tunable Magnon-Magnon Coupling in a Perpendicular Anisotropy Magnetic Garnet-Ferromagnet Bilayer

We experimentally explore the interfacial coherent spin pumping in the magnon-magnon hybridized regime of a perpendicular anisotropy ferrimagnetic insulator (Bi-substituted yttrium iron garnet, or BiYIG) and ferromagnetic metal (permalloy, or Py) bilayer. In addition to the interfacial exchange coupling-induced avoided crossings between the uniform modes of BiYIG and Py, we observe that the gap opening can be tailored by changing the relative orientations of the BiYIG and Py magnetizations due to a purely dynamic interaction. The avoided-crossing gap opening is small (large) when BiYIG and Py magnetizations are nearly perpendicular (partially collinear) to each other, indicating that interfacial fieldlike torque from spin pumping plays a dominant role. Macrospin simulations with a sizable fieldlike spin-pumping torque agree with the gap opening trend observed in the experiments.