Superconductivity and local-moment magnetism in Eu(Fe${}_{0.89}$Co${}_{0.11}$)${}_2$As${}_2$

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 $H\parallel c$.

8 pages, 10 figures