  This work demonstrates strong coupling regime between an Erbium ion spin
ensemble and microwave Hybrid Cavity-Whispering Gallery Modes in a Yttrium
Aluminium Garnet dielectric crystal. Coupling strengths of $220$~MHz and mode
quality factors in excess of $10^6$ are demonstrated. Moreover, the magnetic
response of high-Q modes demonstrates behaviour which is unusual for
paramagnetic systems. This behaviour includes hysteresis and memory effects.
Such qualitative change of the system's magnetic field response is interpreted
as a phase transition of Rare Earth ion impurities. This phenomenon is similar
to the phenomenon of dilute ferromagnetism in semiconductors. The clear
temperature dependence of the phenomenon is demonstrated.
