Europium c-axis ferromagnetism in Eu(Co₁₋ₓNiₓ)2-yAs₂: A single-crystal neutron diffraction study

We report neutron diffraction results for the body-centered-tetragonal series Eu(Co₁₋ₓNiₓ)2-yAs₂, $x=0.10$, $0.20$, $0.42$, and $0.82$, y≤0.10, that detail changes to the magnetic ordering with nominal hole doping. We report the antiferromagnetic (AFM) propagation vectors, magnetic transition temperatures, and the ordered magnetic moments. We find a nonmonotonic change of the AFM propagation vector with x, with a minimum occurring at the tetragonal to collapsed-tetragonal phase crossover. For $x=0.10$ and $0.82$ we find c-axis helix ordering of the Eu magnetic moments (spins) similar to $x=0$ and $1$, with the spins oriented within the $ab$-plane. For $x=0.20$ and $0.42$ we find higher-temperature c-axis FM order and lower-temperature c-axis cone order. Using the extinction conditions for the space group, we discovered that the Eu spins are ordered in the higher-temperature c-axis FM phase for intermediate values of x, contrary to a previous report suggesting only Co/Ni spin ordering. Although we cannot directly confirm that the Co/Ni spins are also ordered, we suggest that c-axis itinerant-FM ordering of the Co/Ni spins could provide a molecular field that drives FM ordering of the Eu spins, which in turn provides the anisotropy for the lower-temperature c-axis cone order.