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Condensed matter seminar: Competing Magnetic Interactions in Doped SrCo2As2

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(Co1-x Nix)2As2 and SrxCa1-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(Co1-x Nix)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 SrxCa1-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.