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Coherent coupling between spin wave excitations (magnons) and microwavephotons in a cavity may disclose new paths to unconventional phenomena as wellas for novel applications. Here, we present a systematic investigation on YIG(Yttrium Iron Garnet) films on top of coplanar waveguide resonators made ofsuperconducting YBCO. We first show that spin wave excitations with frequencyhigher than the Kittel mode can be excited by putting in direct contact a5~$\mu$m thick YIG film with the YBCO coplanar resonator (cavity frequency$\omega_c/2 \pi = 8.65$~GHz). With this configuration, we obtain very largevalues of the collective coupling strength $\lambda/2 \pi \approx 2$~GHz andcooperativity $C=5 \times 10^4$. Transmission spectra are analyzed by amodified Hopfield model for which we provide an exact solution that allows usto well reproduce spectra by introducing a limited number of free parameters.It turns out that the coupling of the dominant magnon mode with photons exceeds0.2 times the cavity frequency, thus demonstrating the achievement of theultrastrong coupling regime with this architecture. Our analysis also shows avanishing contribution of the diamagnetic term which is a peculiarity of purespin systems.

Ultrastrong Magnon-Photon Coupling Achieved by Magnetic Films in Contact with Superconducting Resonators