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