Magnetic structures and excitations in sawtooth olivine chalcogenides Mn₂SiX₄ (X = S, Se)

The Mn lattice in olivine chalcogenide Mn₂SiX₄ (X = S, Se) compounds forms a sawtooth, which is of special interest in magnetism owing to the possibility of realizing flat bands in magnon spectra, a key component in magnonics. In this work, we investigate the Mn₂SiX₄ olivines using magnetic susceptibility, and X-ray and neutron diffraction. We have determined the average and local crystal structures of Mn₂SiS₄ and Mn₂SiSe₄ using synchrotron X-ray, neutron diffraction, and X-ray total scattering data followed by Rietveld and pair distribution function analyses. It is found from the pair distribution function analysis that the Mn triangle that constitutes the sawtooth is isosceles in Mn₂SiS₄ and Mn₂SiSe₄. The temperature evolution of magnetic susceptibility of Mn₂SiS₄ and Mn₂SiSe₄ shows anomalies below 83 K and 70 K, respectively, associated with magnetic ordering. From the neutron powder diffraction measurements the magnetic space groups of Mn₂SiS₄ and Mn₂SiSe₄ are found to be Pnma and Pnm'a', respectively. We find that the Mn spins adopt a ferromagnetic alignment on the sawtooth in both Mn₂SiS₄ and Mn₂SiSe₄ but along different crystallographic directions for the S and the Se compounds. From the temperature evolution of Mn magnetic moments obtained from refining neutron diffraction data, the transition temperatures are accurately determined as T_N(S) = 83(2) K and T_N(Se) = 70.0(5) K. Broad diffuse magnetic peaks are observed in both the compounds, and are prominently seen close to T_N, suggesting the presence of a short-range magnetic order. The magnetic excitations studied using inelastic neutron scattering reveal a magnon excitation with an energy corresponding to approximately 4.5 meV in both S and Se compounds. Spin correlations are observed to persist up to 125 K much above the ordering temperature and we suggest the possibility of short-range spin correlations responsible for this.