Magnetically-controlled Vortex Dynamics in a Ferromagnetic Superconductor

Abstract

Ferromagnetic superconductors are exceptionally rare because the strong ferromagnetic exchange field usually destroys singlet superconductivity. EuFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$, an iron-based superconductor with a maximum critical temperature of 25 K, uniquely exhibits full coexistence with ferromagnetic order below $T_{\mathrm{FM}}$ $\simeq$ $19$ 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 $T_{\mathrm{FM}}$ we observe a pronounced peak in the creep activation energy, and magnetic force microscopy measurements reveal the presence of very closely-spaced ($w\ll \lambda$) 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.

13 + 8 pages, 5 + 6 figures

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Cited by in corpus (1)

• Breakdown of the critical state in the ferromagnetic superconductor EuFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$