Ferromagnetic superconductors are exceptionally rare because the strong ferromagnetic exchange field usually destroys singlet superconductivity. EuFe2(As1−xPx)2, an iron-based superconductor with a maximum critical temperature of 25 K, uniquely exhibits full coexistence with ferromagnetic order below TFM≃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 TFM 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.
@article{arxiv.2412.04098,
title = {Magnetically-controlled Vortex Dynamics in a Ferromagnetic Superconductor},
author = {Joseph Alec Wilcox and Lukas Schneider and Estefani Marchiori and Vadim Plastovets and Alexandre Buzdin and Pardis Sahafi and Andrew Jordan and Raffi Budakian and Tong Ren and Ivan Veschunov and Tsuyoshi Tamegai and Sven Friedemann and Martino Poggio and Simon John Bending},
journal= {arXiv preprint arXiv:2412.04098},
year = {2025}
}