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 ferromagnetic (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.