  The development of a new era of quantum devices requires an understanding of
how paramagnetic dopants or impurity spins behave in crystal hosts. Here, we
describe a new spectroscopic technique which uses traditional Electron Spin
Resonance (ESR) combined with the measurement of a large population of
electromagnetic Whispering Gallery (WG) modes. This allows the characterization
of the physical parameters of paramagnetic impurity ions in the crystal at low
temperatures. We present measurements of two ultra-high purity sapphires cooled
to 20 mK in temperature, and determine the concentration of Fe3+ ions and their
frequency sensitivity to a DC magnetic field. Our method is different to ESR in
that it is possible to track the resonant frequency of the ion from zero
applied magnetic field to any arbitrary value, allowing excellent measurement
precision. This high precision reveals anisotropic behaviour of the Zeeman
splitting, which has not been previously reported. In both crystals, each
Zeeman component demonstrates a different g-factor.
