  We experimentally study the magnon-photon coupling in a system consitsing of
the compensating ferrimagnet gadolinium iron garnet (GdIG) and a
three-dimensional microwave cavity. The temperature is varied in order to tune
the GdIG magnetization and to observe the transition from the weak coupling
regime to the strong coupling regime. By measuring and modelling the complex
reflection parameter of the system the effective coupling rate g eff and the
magnetization M eff of the sample are extracted. Comparing g eff with the
magnon and the cavity decay rate we conclude that the strong coupling regime is
easily accessible using GdIG. We show that the effective coupling strength
follows the predicted square root dependence on the magnetization.
