revealed by single-crystal neutron diffraction*

SrCo₂As₂ has the same tetragonal lattice as the 122 Fe-based high Tc superconductors. It does not order magnetically, but inelastic neutron scattering experiments find antiferromagnetic (AF) fluctuations found in many of the 122 superconductors. Here we show results from neutron diffraction and magnetization experiments on single-crystals of Sr(Co_1-xNi_x)₂As₂. However, the AF order is not stripe-type. Instead, the AF structure consists of ferromagnetic layers (with moments in the layer) that are stacked along c with an incommensurate propagation vector (0, 0, q_z). The propagation vector is doping dependent, suggesting spin density wave order. Using high-energy x-ray diffraction, we also find no evidence for a structural phase transition accompanying the AF order.