We use neutron scattering to investigate spin excitations in Sr(Co1-xNix)2As2, which has a c-axis incommensurate helical structure of the two-dimensional (2D) in-plane ferromagnetic (FM) ordered layers for 0.013=x=0.25. By comparing the wave vector and energy dependent spin excitations in helical ordered Sr(Co0.9Ni0.1)2As2 and paramagnetic SrCo2As2, we find that Ni doping, while increasing lattice disorder in Sr(Co1-xNix)2As2, 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(Co1-xNix)2As2 may arise from a quantum order-by-disorder mechanism through the itinerant electron mediated Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions.
Publication Source (Journal or Book title)
Physical Review B
Xie, Y., Li, Y., Yin, Z., Zhang, R., Wang, W., Stone, M., Cao, H., Abernathy, D., Harriger, L., Young, D., Ditusa, J., & Dai, P. (2020). Magnetic order and fluctuations in the quasi-two-dimensional planar magnet Sr(Co1-xNix)2As2. Physical Review B, 102 (21) https://doi.org/10.1103/PhysRevB.102.214431