Magnetic order and fluctuations in the quasi-two-dimensional planar magnet Sr( Co 1 − x Ni x ) 2 As 2

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