Crossover from ferromagnetic superconductor to superconducting ferromagnet in P-doped EuFe2(As1−xPx)2

In P-doped pnictide, ${\mathrm{EuFe}}_{2}{({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x})}_{2}$ superconductivity coexists with a ferromagnetism of $\mathrm{Eu}\text{\ensuremath{-}}4f$ spins. Above $x\ensuremath{\simeq}0.26$, superconductivity vanishes whereas ferromagnetism remains. Here, we focus on the crossover region and study two ${\mathrm{EuFe}}_{2}{({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x})}_{2}$ single crystals with $x=0.21$ and $x=0.25$; the superconducting critical temperature of the first one is slightly higher and of the second one is slightly lower than the Curie temperature. Despite similar compositions, characteristic temperatures and bulk magnetic properties of the two systems, their local magnetic structures and superconducting vortex-antivortex phases are found drastically different. We demonstrate that the interplay between superconductivity and magnetism is strongly dominated by the superconducting order in the first system, whereas it is mainly governed by ferromagnetism in the second one. Our discovery raises several fundamental questions on the vortex nucleation and dynamics in magnetic superconductors.