Magnetic and superconducting phase diagram of Eu(Fe${}_{1-x}$Ni${}_x$)As${}_2$

Abstract

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${}_2$ ($0\le x\le 0.15$) 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 $\le x\le$ 0.1 where a maximum of the superconducting transition temperature $T_{\mathrm{c}}^{\max }=$ 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.

10 pages, 7 figures; supplemental materials available on request

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