John Wilde, Iowa State University

The square-lattice cobalt arsenides ACo₂As₂ (A =Ca, Sr, Ba, Eu) are itinerant magnets with competing intra-plane ferromagnetic (FM) and AFM fluctuations. With neutron and x-ray diffraction we show that this competition leads to fragile magnetic ordering within Sr(Co_1-x Ni_x)₂As₂ and Sr_xCa_1-xCo₂As₂. SrCo₂As₂ does not magnetically order down to 50 mK likely due to the competition FM and AFM ground states. This balance is broken by Ni-substitution on the Co site in Sr(Co_1-x Ni_x)₂As₂, which magnetically orders with FM planes of Co with magnetic moment in the Co-square-layer AFM stacked in a helix structure. CaCo₂As₂ shows similar fluctuations as SrCo₂As₂ but magnetically orders with FM-aligned planes stacked with simple AFM (+ - + -)and magnetic moment perpendicular to the Co-square-layer. With Sr-substitution Sr_xCa_1-xCo₂As₂ shows a cascade of different AFM structures with FM-aligned magnetic moments both in and out of the Co-square-layer. For both compounds FM intraplane coupling obviously dominates, but this doesn't explain the different AFM stackings. The different AFM stacking can be described as the result of competition between nearest and next-nearest-neighbor interlayer couplings between Co-square planes.