  CaCo2As2 is a unique itinerant system having strong magnetic frustration.
Here we report the effect of electron doping on the physical properties
resulting from Ni substitutions for Co. The A-type antiferromagnetic transition
temperature TN = 52 K for x = 0 decreases to 22 K with only 3 percent Ni
substitution and is completely suppressed for x > 0.11. For 0.11 < x < 0.52
strong ferromagnetic (FM) fluctuations develop as revealed by magnetic
susceptibility chi(T) measurements. Heat-capacity Cp(T) measurements reveal the
presence of FM quantum spin fluctuations for 0.11 < x < 0.52. Our
density-functional theory (DFT) calculations confirm that FM fluctuations are
enhanced by Ni substitutions for Co. The Sommerfeld electronic heat-capacity
coefficient is enhanced for x = 0, 0.21, and 0.42 by about a factor of two
compared to DFT calculations of the bare density of states at the Fermi energy.
The crystals with x > 0.52 do not exhibit FM spin fluctuations or magnetic
order, which was found from the DFT calculations to arise from a Stoner
transition. Neutron-diffraction studies of crystals with x = 0.11 and 0.16
found no evidence of A-type ordering as observed in CaCo2As2 or of other common
magnetic structures.
