  A dense system of independent oscillators, connected only by their
interaction with the same cavity excitation mode, will radiate coherently,
which effect is termed superradiance. In several cases, especially if the
density of oscillators is high, the superradiance may dominate the intrinsic
relaxation processes. This limit can be achieved, e.g., with cyclotron
resonance in two-dimensional electron gases. In those experiments, the
cyclotron resonance is coupled to the electric field of light, while the
oscillator density can be easily controlled by varying the gate voltage.
However, in the case of magnetic oscillators, to achieve the dominance of
superradiance is more tricky, as material parameters limit the oscillator
density, and the magnetic coupling to the light wave is rather small. Here we
present quasi-optical magnetic resonance experiments on thin films of yttrium
iron garnet. Due to the simplicity of experimental geometry, the intrinsic
damping and the superradiance can be easily separated in the transmission
spectra. We show that with increasing film thickness, the losses due to
coherent radiation prevail the system's internal broadening.
