Superconductivity and local-moment magnetism in Eu(Fe$_{0.89}$Co$_{0.11}$)$_{2}$As$_{2}$

TL;DR

This study investigates the coexistence of superconductivity and ferromagnetism in Eu(Fe$_{0.89}$Co$_{0.11}$)$_{2}$As$_{2}$, revealing anisotropic critical fields, magnetic reentrance phenomena, and a transition from helimagnetic to ferromagnetic order under magnetic fields.

Contribution

It provides the first detailed analysis of how magnetic order influences superconductivity in Eu(Fe$_{0.89}$Co$_{0.11}$)$_{2}$As$_{2}$, highlighting the coexistence of superconductivity with ferromagnetic Eu$^{2+}$ spins.

Findings

Superconductivity appears at $T_c \,\sim\, 21$ K with anisotropic upper critical fields.

Resistivity reentrance at 17 K coincides with Eu$^{2+}$ magnetic ordering.

External magnetic fields induce a transition from helimagnetism to ferromagnetism, suppressing resistivity reentrance.

Abstract

We report the measurements of resistivity and magnetization under magnetic fields parallel and perpendicular to the basal plane, respectively, on a cobalt-doped Eu(Fe$_{0.89}$Co$_{0.11}$)$_{2}$As$_{2}$ single crystal. We observed a resistivity drop at $T_c\sim$ 21 K, which shifts toward lower temperatures under external fields, suggesting a superconducting transition. The upper critical fields near $T_c$ show large anisotropy, in contrast with those of other '122' FeAs-based superconductors. Low-field magnetic susceptibility data also show evidence of superconductivity below 21 K. Instead of expected zero-resistance below $T_c$, however, a resistivity reentrance appears at 17 K under zero field, coincident with the magnetic ordering of Eu$^{2+}$ moments. Based on the temperature and field dependences of anisotropic magnetization, a helical magnetic structure for the Eu$^{2+}$ spins is proposed. External magnetic fields easily changes the helimagnetism into a ferromagnetism with fully polarized Eu$^{2+}$ spins, accompanying by disappearance of the resistivity reentrance. Therefore, superconductivity coexists with ferromagnetic state of Eu$^{2+}$ spins under relatively low magnetic field. The magnetic and superconducting phase diagrams are finally summarized for both $H\parallel ab$ and $H\parallel c$.