Magnetic order and fluctuations in the quasi-two-dimensional planar magnet .

We use neutron scattering to investigate spin excitations in , which has a -axis incommensurate helical structure of the two-dimensional (2D) in-plane ferromagnetic (FM) ordered layers for . By comparing the wave vector and energy dependent spin excitations in helical ordered and paramagnetic , we find that Ni doping, while increasing lattice disorder in , 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 -axis electrical resistivity with increasing Ni doping and associated lattice disorder, we conclude that the helical magnetic order in may arise from a quantum order-by-disorder mechanism through the itinerant electron mediated Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions.