A microwave technique suitable for investigating the AC magnetic susceptibility of small samples in the GHz frequency range is presented. The method-which is based on the use of a coplanar waveguide resonator, within the resonator perturbation approach-allows one to obtain the absolute value of the complex susceptibility, from which the penetration depth and the superfluid density can be determined. We report on the characterization of several iron-based superconducting systems, belonging to the 11, 122, 1144, and 12442 families. In particular, we show the effect of different kinds of doping for the 122 family, and the effect of proton irradiation in a 122 compound. Finally, the paradigmatic case of the magnetic superconductor EuP-122 is discussed, since it shows the emergence of both superconducting and ferromagnetic transitions, marked by clear features in both the real and imaginary parts of the AC susceptibility.
Keywords: high-frequency AC susceptibility; iron-based superconductors; microwave superconductivity.