Evidence for competing magnetic and superconducting phases in superconducting Eu1-xSrxFe2-yCoyAs2 single crystals

In single crystals of Eu1-xSrxFe2-yCoyAs2, Co doping suppresses spin-density wave (SDW) ordering and induces a superconducting transition. A resistivity reentrance due to the antiferromagnetic ordering of Eu2+ spins is observed, indicating the competition between antiferromagnetism (AFM) and superconductivity (SC). It is striking that the resistivity reentrance can be completely suppressed by a small magnetic field due to a field-induced metamagnetic transition from AFM to ferromagnetism (FM). The resistivity reentrance can also be suppressed by the substitution of Eu2+ ions with nonmagnetic Ba2+/Sr2+ to completely destroy the AFM ordering. These results indicate that the AFM order appears destructive to SC, while FM can coexist with the superconductivity. Further we find that magnon excitation exists in AFM ordering and can be suppressed by an applied field. Coexistence of SC from the FeAs layer and the inner field produced by the ferromagnetic Eu2+ layer suggest a possible p-wave component in the superconducting order parameter.