Small-angle neutron scattering (SANS) measurements are performed on the ferromagnetic superconductor ${\mathrm{EuFe}}_2{\left({\mathrm{As}}_{0.8}{\mathrm{P}}_{0.2}\right)}_2$ ($T_{\mathrm{sc}}=22.5\mathrm{K}$) to probe the delicate interplay between ferromagnetism and superconductivity. A clear signature of large ferromagnetic domains is found below the ferromagnetic ordering temperature $T_{\mathrm{C}}=18.5\mathrm{K}$. In a small temperature interval of $\sim 1.5\mathrm{K}$ below $T_{\mathrm{C}}$, additional SANS signal is observed, of which the indirect Fourier transform reveals characteristic length scales in between $\sim 80\mathrm{nm}$ and $\sim 160\mathrm{nm}$. These nanometer-scaled domain structures are identified to result from an intermediate inhomogeneous Meissner effect denoted the domain Meissner state, which was recently observed on the surface of ${\mathrm{EuFe}}_2{\left({\mathrm{As}}_{0.79}{\mathrm{P}}_{0.21}\right)}_2$ crystals by means of magnetic force microscopy [V. S. Stolyarov et al., Sci. Adv. 4, 1061 (2018)], ascribed to the competition between ferromagnetism and superconductivity. Our measurements clearly render the domain Meissner state as a bulk phenomenon and provide a key solution to the mystery regarding the intriguing coexistence of strong ferromagnetism and bulk superconductivity in these compounds.

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• Ferromagnetism
• Superconductivity
• Ferromagnetic superconductors
• Iron-based superconductors
• Neutron diffraction
• Small angle neutron scattering