  Static (DC) and dynamic (AC, at 14 MHz and 8 GHz) magnetic susceptibilities
of single crystals of a ferromagnetic superconductor,
$\textrm{EuFe}_{2}(\textrm{As}_{1-x}\textrm{P}_{x})_{2}$ (x = 0.23), were
measured in pristine state and after different doses of 2.5 MeV electron or 3.5
MeV proton irradiation. The superconducting transition temperature, $T_{c}(H)$,
shows an extraordinarily large decrease. It starts at
$T_{c}(H=0)\approx24\:\textrm{K}$ in the pristine sample for both AC and DC
measurements, but moves to almost half of that value after moderate irradiation
dose. Our results suggest that in
$\textrm{EuFe}_{2}(\textrm{As}_{1-x}\textrm{P}_{x})_{2}$ superconductivity is
affected by local-moment ferromagnetism mostly via the spontaneous internal
magnetic fields induced by the FM subsystem. Another mechanism is revealed upon
irradiation where magnetic defects created in ordered $\text{Eu}^{2+}$ lattice
act as efficient pairbreakers leading to a significant $T_{c}$ reduction upon
irradiation compared to other 122 compounds. On the other hand, the exchange
interactions seem to be weakly screened by the superconducting phase leading to
a modest increase of $T_{m}$ (less than 1 K) after the irradiation drives
$T_{c}$ to below $T_{m}$. The results suggest that FM and SC phases coexist
microscopically in the same volume.
