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

The Mn lattice in olivine chalcogenide MnSiX (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 MnSiX olivines using magnetic susceptibility, and X-ray and neutron diffraction. We have determined the average and local crystal structures of MnSiS and MnSiSe 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 MnSiS and MnSiSe. The temperature evolution of magnetic susceptibility of MnSiS and MnSiSe shows anomalies below 83 K and 70 K, respectively, associated with magnetic ordering. From the neutron powder diffraction measurements the magnetic space groups of MnSiS and MnSiSe are found to be and '', respectively. We find that the Mn spins adopt a ferromagnetic alignment on the sawtooth in both MnSiS and MnSiSe 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 (S) = 83(2) K and (Se) = 70.0(5) K. Broad diffuse magnetic peaks are observed in both the compounds, and are prominently seen close to , 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.