Magnetically-controlled Vortex Dynamics in a Ferromagnetic Superconductor

Ferromagnetic superconductors are exceptionally rare because the strong ferromagnetic exchange field usually destroys singlet superconductivity. EuFe ₂(As _1−xP _x) ₂, an iron-based superconductor with a maximum critical temperature of 25 K, uniquely exhibits full coexistence with ferromagnetic order below T _FM ≃ 19 K. The interplay leads to narrowing of ferromagnetic domains at higher temperatures and spontaneous nucleation of vortices/antivortices at lower temperatures. Here we demonstrate how the underlying magnetic structure controls the superconducting vortex dynamics in applied magnetic fields. Just below T _FM we observe a pronounced peak in the creep activation energy, and magnetic force microscopy measurements reveal the presence of very closely spaced (w ≪ λ) vortex clusters. We attribute these observations to the formation of vortex polarons, for which we present a theoretical description. In contrast, we link strong magnetic irreversibility at low temperatures to a critical current governed by giant flux creep over an activation barrier for vortex-antivortex annihilation near domain walls. Our work suggests new routes for the magnetic enhancement of vortex pinning with important applications in high-current conductors. (Figure presented.)