Superconductivity and ferromagnetism in EuFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$

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 $0.2leq xleq0.4$, 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.

Self doping effect and successive magnetic transitions in superconducting Sr$_2$VFeAsO$_3$

We have studied a quinary Fe-based superconductor Sr$_2$VFeAsO$_3$ by the measurements of x-ray diffraction, x-ray absorption, M{o}ssbauer spectrum, resistivity, magnetization and specific heat. This apparently undoped oxyarsenide is shown to be self doped via electron transfer from the V$^{3+}$ ions. We observed successive magnetic transitions within the VO$_2$ layers: an antiferromagnetic transition at 150 K followed by a weak ferromagnetic transition at 55 K. The spin orderings within the VO$_2$ planes are discussed based on mixed valence of V$^{3+}$ and V$^{4+}$.

Narrow superconducting window in Ni-doped LaFeAsO

We have studied Ni-substitution effect in LaFe$_{1-x}$Ni$_{x}$AsO ($0leq x leq0.1$) by the measurements of x-ray diffraction, electrical resistivity, magnetic susceptibility, and heat capacity. The nickel doping drastically suppresses the resistivity anomaly associated with spin-density-wave ordering in the parent compound. Superconductivity emerges in a narrow region of $0.03leq x leq0.06$ with the maximum $T_c$ of 6.5 K at $x$=0.04, where enhanced magnetic susceptibility shows up. The upper critical field at zero temperature is estimated to exceed the Pauli paramagnetic limit. The much lowered $T_c$ in comparison with LaFeAsO$_{1-x}$F$_{x}$ system is discussed.

Superconductivity induced by cobalt doping in iron-based oxyarsenides

Chemical doping has recently become a very important strategy to induce superconductivity especially in complex compounds. Distinguished examples include Ba-doped La$_2$CuO$_4$ (the first high temperature superconductor), K-doped BaBiO$_3$, K-doped C$_{60}$ and Na$_{x}$CoO$_{2}cdot y$H$_{2}$O. The most recent example is F-doped LaFeAsO, which leads to a new class of high temperature superconductors. One notes that all the above dopants are non-magnetic, because magnetic atoms generally break superconducting Cooper pairs. In addition, the doping site was out of the (super)conducting structural unit (layer or framework). Here we report that superconductivity was realized by doping magnetic element cobalt into the (super)conducting-active Fe$_2$As$_2$ layers in LaFe$_{1-x}$Co$_{x}$AsO. At surprisingly small Co-doping level of $x$=0.025, the antiferromagnetic spin-density-wave transition in the parent compound is completely suppressed, and superconductivity with $T_csim $ 10 K emerges. With increasing Co content, $T_c$ shows a maximum of 13 K at $xsim 0.075$, and then drops to below 2 K at $x$=0.15. This result suggests essential differences between previous cuprate superconductor and the present iron-based arsenide one.