  Several experimental implementations of cavity-magnon systems are presented.
First an Yttrium Iron Garnet (YIG) block is placed inside a re-entrant cavity
where the resulting hybrid mode is measured to be in the ultra strong coupling
regime. When fully hybridised the ratio between the coupling rate and uncoupled
mode frequencies is determined to be $g/\omega=0.46$. Next a thin YIG cylinder
is placed inside a loop gap cavity. The bright mode of this cavity couples to
the YIG sample and is similarly measured to be in the ultra strong coupling
regime with ratio of coupling rate to uncoupled mode frequencies as
$g/\omega=0.34$. A larger spin density medium such as lithium ferrite (LiFe) is
expected to improve couplings by a factor of 1.46 in both systems as coupling
strength is shown to be proportional to the square root of spin density and
magnetic moment. Such strongly coupled systems are potentially useful for
cavity QED, hybrid quantum systems and precision dark matter detection
experiments. The YIG disc in the loop gap cavity, is, in particular, shown to
be a strong candidate for dark matter detection. Finally, a LiFe sphere inside
a two post re-entrant cavity is considered. In past work it was shown that the
magnon mode in the sample has a turnover point in frequency. Additionally, it
was predicted that if the system was engineered such that it fully hybridised
at this turnover point the cavity-magnon polariton (CMP) transition frequency
would become insensitive to both first and second order magnetic bias field
fluctuations, a result useful for precision frequency applications. This work
implements such a system by engineering the cavity mode frequency to near this
turnover point, with suppression in sensitivity to second order bias magnetic
field fluctuations shown.
