Bulk domain Meissner state in the ferromagnetic superconductor EuFe2(As0.8P0.2)(2): Consequence of compromise between ferromagnetism and superconductivity

Small-angle neutron scattering (SANS) measurements are performed on the ferromagnetic superconductor EuFe2(As0.8P0.2)2 (Tsc = 22.5 K) to probe the delicate interplay between ferromagnetism and superconductivity. A clear signature of large ferromagnetic domains is found below the ferromagnetic ordering temperature TC = 18.5 K. In a small temperature interval of -1.5 K below TC, additional SANS signal is observed, of which the indirect Fourier transform reveals characteristic length scales in between -80 nm and -160 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 EuFe2(As0.79P0.21)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.