  The strong photon-magnon coupling between an electromagnetic cavity and two
yttrium iron garnet (YIG) spheres has been investigated in the context of a
strong mutual dipolar interaction between the spheres. A decrease in the
coupling strength between the YIG spheres and the electromagnetic cavity is
observed, along with an increase of the total magnetic losses, as the distance
between the spheres is decreased. A model of inhomogeneous broadening of the
ferromagnetic resonance linewidth, partly mitigated by the dipolar narrowing
effect, reproduces the reduction in the coupling strength observed
experimentally. These findings have important implications for the
understanding of strongly coupled photon-magnon system involving densely packed
magnetic objects, such as ferromagnetic nanowires arrays, in which the total
coupling strength with an electromagnetic cavity might become limited due to
mutual dipolar interactions.
