  The tunability of magnons enables their interaction with various other
quantum excitations, including photons, paving the route for novel hybrid
quantum systems. Here, we study magnon-photon coupling using a high-quality
factor split-ring resonator and single-crystal yttrium iron garnet (YIG)
spheres at room temperature. We investigate the dependence of the coupling
strength on the size of the sphere and find that the coupling is stronger for
spheres with a larger diameter as predicted by theory. Furthermore, we
demonstrate strong magnon-photon coupling by varying the position of the YIG
sphere within the resonator. Our experimental results reveal the expected
correlation between the coupling strength and the rf magnetic field. These
findings demonstrate the control of coherent magnon-photon coupling through the
theoretically predicted square-root dependence on the spin density in the
ferromagnetic medium and the magnetic dipolar interaction in a planar
resonator.
