Microwave analysis of the interplay between magnetism and superconductivity in EuFe_{2}(As_{1−x}P_{x})_{2} single crystals

We report on the microwave analysis of the interplay between magnetism and superconductivity in single crystals of EuFe_{2}(As_{1−x}P_{x})_{2}, accomplished by means of a coplanar waveguide resonator technique. The bulk complex magnetic susceptibility χ_{m} extracted through a cavity perturbation approach is demonstrated to be highly sensitive to the magnetic structure and dynamics, revealing two distinct magnetic transitions below the superconducting critical temperature. By a comparison with magnetic force microscopy maps, we ascribe the χ_{m}^{′′} peak observed at about 17 K to the transition from the ferromagnetic domain Meissner phase to the domain vortex-antivortex state, with the subsequent evolution of the domain structure at lower temperatures. The second χ_{m}^{′′} peak observed at 11 K reflects a specific high-frequency feature, connected to vortex-antivortex dynamics and eventual spin reorientation transition of the Eu^{2+} canted ferromagnetic subsystem. The two peaks merge and vanish upon application of an in-plane magnetic field, which is compatible with the presence of a quantum critical point below 1 T.