Single Crystals*

EuCo_2-yAs₂ crystallizes in the uncollapsed-tetragonal ThCr₂Si₂-type structure and exhibits c-axis helical antiferromagnetic (AF) ordering of the Eu below T_N=47 K with an AF propagation vector of t=(0,0,0.73(7)). EuNi_2-yAs₂ is the electron-doped analogue which has the collapsed-tetragonal structure and shows c-axis helical-AF ordering below T_N=15 K with t=(0,0,0.92).  Here, we present results from neutron diffraction measurements on single-crystal samples that reveal changes to the magnetic ordering of Eu(Co_1-xNi_x)_2-yAs₂ across the doping series. We find good agreement with the magnetic phase diagram derived from previous thermodynamic and transport measurements. For x=0.1, we find a c-axis helical structure with the Eu ferromagnetically aligned within the ab plane and the planes AF stacked along c consistent with t~(0,0,0.67).  With more Ni doping, upon cooling, ferromagnetic ordering of the Co/Ni sets in prior to AF ordering of the Eu.  This creates a complex ground-state magnetic structure with a superposition of the in-plane helix structure and c-axis ferromagnetism. t changes with increasing x, reaching ~(0, 0, 0.85) at x=0.8.