Magnetism and superconductivity in single crystals $Eu_{1-x}Sr_xFe_{2-y}Co_{y}As_2$

TL;DR

This study explores how magnetic ordering affects superconductivity in Eu-based iron pnictides, revealing that ferromagnetism can coexist with superconductivity while antiferromagnetism suppresses it, with external magnetic fields controlling this interplay.

Contribution

It demonstrates that external magnetic fields can suppress resistivity reentrance by inducing ferromagnetism, and shows that partial Sr substitution eliminates Eu antiferromagnetism, allowing superconductivity to coexist with ferromagnetism.

Findings

Magnetic field induces a transition from antiferromagnetism to ferromagnetism in Eu spins.

Superconductivity is suppressed by Eu antiferromagnetism but coexists with ferromagnetism.

Partial Sr substitution removes Eu antiferromagnetic order, enabling superconductivity without resistivity reentrance.

Abstract

We systematically studied the transport properties of single crystals of $Eu_{1-x}Sr_xFe_{2-y}$Co$_{y}As_2$. Co doping can suppress the spin-density wave (SDW) ordering and induces a superconducting transition, but a resistivity reentrance due to the antiferromagnetic ordering of $Eu^{2+}$ spins is observed, indicating the competition between antiferromagnetism (AFM) and superconductivity. It is striking that the resistivity reentrance can be completely suppressed by external magnetic field (H) because a metamagnetic transition from antiferromagnetism to ferromagnetism for $Eu^{2+}$ spins is induced by magnetic field. Superconductivity without resistivity reentrance shows up by partial substitution of Eu$^{2+}$ with non-magnetic Sr$^{2+}$ to completely destroy the AFM ordering of $Eu^{2+}$ spins. These results suggest that the antiferromagnetism destroys the superconductivity, while the ferromagnetism can coexist with the superconductivity in the iron-based high-$T_c$ superconductors.