Magnetically-controlled Vortex Dynamics in a Ferromagnetic Superconductor

TL;DR

This study explores how magnetic structures influence vortex behavior in a rare ferromagnetic superconductor, revealing vortex polaron formation and potential for magnetic vortex pinning enhancement.

Contribution

It provides the first detailed experimental and theoretical analysis of vortex dynamics controlled by magnetic order in EuFe$_2$(As$_{1-x}$P$_x$)$_2$, a ferromagnetic superconductor.

Findings

Observation of vortex clusters with very small spacing near $T_ ext{FM}$

Identification of vortex polaron formation through magnetic imaging and theory

Link between magnetic irreversibility and vortex-antivortex flux creep

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.