  Strong coupling in cavity-magnon systems has shown great potential for use in
spintronics and information processing technologies due to the low damping
rates and long coherence times. Although such systems are conceptually similar
to those coupled by magnon-polaritons (MPs), the link between magnon-cavity
coupling and MPs has not been explicitly defined. In this work we establish
such a connection by studying the frequency-wavevector dispersion of a strongly
coupled magnon-cavity system, using a height-adjustable microwave cavity, and
by modelling the observed behaviour through the system's effective
permeability. A polariton gap between the upper and lower coupled modes of the
magnon-cavity system is defined, and is seen to be dependent on the system's
effective filling factor. This gap is equal to the MP polariton gap in the
limit where filling factor = 1, corresponding to the removal of the microwave
cavity. Thus, our work clarifies the connection between magnon-cavity and MP
coupling, improving our understanding of magnon-photon interactions in coupled
systems.
