  SrCo2As2 is a peculiar itinerant magnetic system that does not order
magnetically, but inelastic neutron scattering experiments observe the same
stripe-type antiferromagnetic (AF) fluctuations found in many of the Fe-based
superconductors along with evidence of magnetic frustration. Here we present
results from neutron diffraction measurements on single crystals of
Sr(Co1-xNix)2As2 that show the development of long-range AF order with
Ni-doping. However, the AF order is not stripe-type. Rather, the magnetic
structure consists of ferromagnetically-aligned (FM) layers (with moments
laying in the layer) that are AF arranged along c with an incommensurate
propagation vector of (0 0 tau), i.e. a helix. Using high-energy x-ray
diffraction, we find no evidence for a temperature-induced structural phase
transition that would indicate a collinear AF order. This finding supports a
picture of competing FM and AF interactions within the square transition-metal
layers due to flat-band magnetic instabilities. However, the composition
dependence of the propagation vector suggests that far more subtle Fermi
surface and orbital effects control the interlayer magnetic correlations.
