The effects of Ni doping in Eu(${\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x{)}_{2-y}{\mathrm{As}}_2$ single crystals with $x=0$ to 1 grown out of self-flux are investigated via crystallographic, electronic transport, magnetic, and thermal measurements. All compositions adopt the body-centered-tetragonal ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$ structure with space group $I4/mmm$. We also find 3%–4% of randomly distributed vacancies on the Co/Ni site. Anisotropic magnetic susceptibility ${\chi }_{\alpha }(\alpha =ab,c)$ data versus temperature $T$ show clear signatures of an antiferromagnetic (AFM) $c$-axis helix structure associated with the ${\mathrm{Eu}}^{+2}$ spins $\frac{7}{2}$ for $x=0$ and 1 as previously reported. The ${\chi }_{\alpha }\left(T\right)$ data for $x=0.03$ and 0.10 suggest an anomalous $2q$ magnetic structure containing two helix axes along the $c$ axis and in the $ab$ plane, respectively, whereas for $x=0.75$ and 0.82 a $c$-axis helix is inferred as previously found for $x=$ 0 and 1. At intermediate compositions $x=0.2$, 0.32, 0.42, 0.54, and 0.65, a magnetic structure with a large ferromagnetic (FM) $c$-axis component is found from magnetization versus field isotherms, suggested to be an incommensurate FM $c$-axis cone structure associated with the Eu spins, which consists of both AFM and FM components. In addition, the $\chi \left(T\right)$ and heat capacity $C_{\mathrm{p}}\left(T\right)$ data for $x=0.2$–0.65 indicate the occurrence of itinerant FM order associated with the Co/Ni atoms with Curie temperatures from 60 to 25 K, respectively. Electrical resistivity $\rho \left(T\right)$ measurements indicate metallic character for all compositions with abrupt increases in slope on cooling below the Eu AFM transition temperatures. In addition to this panoply of magnetic transitions, ${}^{151}\mathrm{Eu}$ Mössbauer measurements indicate that ordering of the Eu moments proceeds via an incommensurate sine amplitude-modulated structure with additional transition temperatures associated with this effect.

• Received 15 May 2020
• Accepted 21 July 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.084407