We report bulk superconductivity induced by an isovalent doping of phosphorus in BaFe(2)(As(1-x)P(x))(2). The P-for-As substitution results in shrinkage of the lattice, especially for the FeAs block layers. The resistivity anomaly associated with the spin-density-wave (SDW) transition in the undoped compound is gradually suppressed by the P doping. Superconductivity with a maximum T(c) of 30 K emerges at x = 0.32, coinciding with a magnetic quantum critical point (QCP) which is shown by the disappearance of SDW order and the linear temperature-dependent resistivity in the normal state. The T(c) values were found to decrease with further P doping and no superconductivity was observed down to 2 K for x≥0.77. The appearance of superconductivity in the vicinity of QCP hints at the superconductivity mechanism in iron-based arsenides.

We report the measurements of resistivity and magnetization under magnetic fields parallel and perpendicular to the basal plane, respectively, on a cobalt-doped $\text{Eu}{\left({\text{Fe}}_{0.89}{\text{Co}}_{0.11}\right)}_2{\text{As}}_2$ single crystal. We observed a resistivity drop at $T_c\sim 21\text{K}$, 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 the expected zero resistance below $T_c$, however, a resistivity re-entrance appears at 17 K under zero field, coincident with the magnetic ordering of ${\text{Eu}}^{2+}$ moments. Based on the temperature and field dependences of anisotropic magnetization, a helical magnetic structure for the ${\text{Eu}}^{2+}$ spins is proposed. External magnetic fields easily change the helimagnetism into ferromagnetism with fully polarized ${\text{Eu}}^{2+}$ spins, accompanying by disappearance of the resistivity re-entrance. Therefore, superconductivity coexists with ferromagnetic state of ${\text{Eu}}^{2+}$ spins under relatively low magnetic field. The magnetic and superconducting phase diagrams are finally summarized for both $H\parallel ab$ and $H\parallel c$.