Microwaves impinging on matter usually interact with charges as well as with spins (via electric and magnetic field components, respectively), with the charge response typically much stronger than the spin response. But in the case of magnetic resonance, spins can be directly probed using microwaves. For paramagnetic materials, this technique is called electron spin resonance (ESR) and for ferromagnetic materials ferromagnetic resonance (FMR).
In the paramagnetic case, such an experiment probes the Zeeman splitting, with a linear relation between the static external magnetic field and the frequency of the probing microwave field. A popular combination, as implemented in commercial X-band ESR spectrometers, is approximately 0.3 T (static field) and 10 GHz (microwave frequency) for a typical material with electron g-factor close to 2.