Magnetic and superconducting phase diagram of Eu(Fe1−xNix)As2

We report the Ni-doping effect on magnetism and superconductivity (SC) in an Eu-containing 112-type system Eu(Fe$_{1-x}$Ni$_{x})$As$_{2}$ ($0\leq x\leq 0.15$) by the measurements of resistivity, magnetization, and specific heat. The undoped EuFeAs$_2$ undergoes a spin-density-wave (SDW) transition at $T_\mathrm{SDW}\sim$ 105 K in the Fe sublattice and a magnetic ordering at $T_\mathrm{m}\sim$ 40 K in the Eu sublattice. Complex Eu-spin magnetism is manifested by a spin-glass reentrance at $T_\mathrm{SG}\sim$ 15 K and an additional spin reorientation at $T_\mathrm{SR}\sim$ 7 K. With Ni doping, the SDW order is rapidly suppressed, and SC emerges in the Ni-doping range of 0.01 $\leq x\leq$ 0.1 where a maximum of the superconducting transition temperature $T_\mathrm{c}^{\mathrm{max}}=$ 17.6 K shows up at $x$ = 0.04. On the other hand, $T_\mathrm{m}$ decreases very slowly, yet $T_\mathrm{SG}$ and $T_\mathrm{SR}$ hardly change with the Ni doping. The phase diagram has been established, which suggests a very weak coupling between SC and Eu spins. The complex Eu-spin magnetism is discussed in terms of the Ruderman-Kittel-Kasuya-Yosida interactions mediated by the conduction electrons from both layers of FeAs and As surrounding Eu$^{2+}$ ions.