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We use neutron scattering to investigate spin excitations inSr(Co$_{1-x}$Ni$_{x})_2$As$_2$, which has a $c$-axis incommensurate helicalstructure of the two-dimensional (2D) in-plane ferromagnetic (FM) orderedlayers for $0.013\leq x \leq 0.25$. By comparing the wave vector and energydependent spin excitations in helical orderedSr(Co$_{0.9}$Ni$_{0.1}$)$_2$As$_2$ and paramagnetic SrCo$_2$As$_2$, we findthat Ni-doping, while increasing lattice disorder inSr(Co$_{1-x}$Ni$_{x})_2$As$_2$, enhances quasi-2D FM spin fluctuations.However, our band structure calculations within the combined density functionaltheory and dynamic mean field theory (DFT+DMFT) failed to generate a correctincommensurate wave vector for the observed helical order from nested Fermisurfaces. Since transport measurements reveal increased in-plane and $c$-axiselectrical resistivity with increasing Ni-doping and associated latticedisorder, we conclude that the helical magnetic order inSr(Co$_{1-x}$Ni$_{x})_2$As$_2$ may arise from a quantum order-by-disordermechanism through the itinerant electron mediated Ruderman-Kittel-Kasuya-Yosida(RKKY) interactions.

Magnetic order and fluctuations in quasi-two-dimensional planar magnet Sr(Co$_{1-x}$Ni$_x$)$_2$As$_2$