Controlling magnon-photon coupling in a planar geometry

Magnons, the collective excitations in magnetically ordered media, have a highly tunable dispersion making them promising candidates for novel hybrid dynamic systems. Magnons efficiently couple to other excitations including phonons, photons and magnon modes. In this work, we study the coupling of magnons with microwave photons in a planar geometry consisting of a microwave split-ring resonator (SRR). The coupling of magnons in yttrium iron garnet (YIG) spheres was studied by placing the spheres on top of the SRR and sweeping the magnetic field. The observed applied field-dependent transmission parameter reveals an avoided level crossing between magnon and microwave photon resonance signifying efficient magnon-photon coupling. We demonstrate control of the coupling by varying the position of the sphere within the vicinity of the SRR and changing the size of the sphere. The coupling is the strongest at the location where the microwave-magnetic field is maximum as confirmed by finite element modeling. Moreover, we find that the coupling increases with the dimension of the sphere as we would expect since the coupling strength scales with the square root of the total number of spins. We compare the magnon-photon coupling of two SRRs with different resonant frequencies and find that the coupling is stronger with the SRR resonating at higher frequency. Our results pave the way for an on-chip integration of hybrid magnonic systems based on planar high-quality resonators and low loss materials.