Superconductivity and ferromagnetism in EuFe2_{2}(As1−x_{1-x}Px_{x})2_{2}

Superconductivity and ferromagnetism are two antagonistic cooperative phenomena, which makes it difficult for them to coexist. Here we demonstrate experimentally that they do coexist in EuFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ with $0.2\leq x\leq0.4$, in which superconductivity is associated with Fe-3$d$ electrons and ferromagnetism comes from the long-range ordering of Eu-4$f$ moments via Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. The coexistence is featured by large saturated ferromagnetic moments, high and comparable superconducting and magnetic transition temperatures, and broad coexistence ranges in temperature and field. We ascribe this unusual phenomenon to the robustness of superconductivity as well as the multi-orbital characters of iron pnictides.Comment: 9 pages, 5 figures, submitted to JPCM: special issue on Superconducting Materials. The main result of this paper was presented in the 12th National Conference on Low Temperature Physics held in Qingdao in July 2009, and the 1st Hangzhou Workshop on Quantum Matter held in October 200