Effect of electron irradiation on the magnetic properties of EuFe2(As1−xPx)2 ferromagnetic superconductor

Radio- frequency AC magnetic susceptibility (10 MHz, 20 mOe AC field) and DC magnetization were measured in single crystals of a ferromagnetic superconductor EuFe2(As1−xPx)2 (x = 0.23) before and after 2.5 MeV electron irradiation with a total fluence of ≈ 2 × 10^19 electrons/cm2. The ferromagnetic transition T_FM ≈ 18 K remains practically unchanged with the applied external DC magnetic field and is unaffected by irradiation, as expected from local moment ferromagnet. The T_SC(H) line starts at ≈ 24 K in the pristine sample in zero DC field for both AC and DC measurements, practically independent of frequency. Remarkably, after the irradiation it moves significantly below the FM transition, to ≈ 16 K in AC measurements and ≈ 12 K in a DC regime in small external field of 20 Oe. We attribute such large difference to appearance of the internal field below FM transition, so that the measurements are now conducted in the mixed vortex state where the onset of diamagnetism is known to be frequency-dependent. Our results suggest that the main coupling between superconductivity and local-moment ferromagnet is via the internal field.