Europium c-axis ferromagnetism in Eu(Co_1-xNi_x)_2-yAs_2: A single-crystal neutron diffraction study

We report neutron-diffraction results for the body-centered-tetragonal series Eu(Co_1-xNi_x)_2-yAs_2, x=0.10, 0.20, 0.42, and 0.82, y≤0.08, that detail changes to the magnetic ordering with nominal hole doping. We report the antiferromagnetic (AFM) propagation vectors, magnetic transition temperatures, and values for 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 parasitic FM ordering of the Eu spins, which in turn provides the anisotropy for the lower-temperature c-axis-cone order.