Study of the paramagnetic properties of Fe3+ ions in sapphire towards realizing an ultra-stable cryogenic maser oscillator

In a preceding publication, we reported the experimental demonstration of maser oscillation at 12.04 GHz in a cryogenic sapphire whispering-gallery-mode resonator, where the resonator´s dielectric ring contained a low concentration of paramagnetic Fe³⁺ ions within its monocrystalline sapphire lattice. Our preliminary measurements revealed a frequency stability of the order of 2.5×10^-14 in a non-optimized design. In this paper, we report new measurements made on this same resonator to determine more precisely the main parameters affecting the dynamics of the Fe³⁺ ions when exposed to a microwave signal within their ESR bandwidth.