Antiferromagnetism and the emergence of frustration in the sawtooth lattice chalcogenide olivines Mn2SiS4-x,Sex, (x=0-4)

The magnetism in the sawtooth lattice of Mn in the olivine chalcogenides, Mn2SiS4-x,Se-x (x = 1-4), is studied in detail by analyzing their magnetization, specific heat, and thermal conductivity properties and complemented with density functional theory calculations. The air-stable chalcogenides are antiferromagnets and show a linear trend in the transition temperature T-N as a function of Se content (x), which shows a decrease from T-N approximate to 86 K for Mn2SiS4 to 66 K for Mn2SiSe4. Additional magnetic anomalies are revealed at low temperatures for all the compositions. Magnetization irreversibilities are observed as a function of x. The specific heat and the magnetic entropy indicate the presence of short-range spin fluctuations in Mn2SiS4-x,Se-x. A spin-flop antiferromagnetic phase transition in the presence of applied magnetic field is present in Mn2SiS4-x,Se-x, where the critical field for the spin flop increases from x = 0 towards 4 in a nonlinear fashion. Density functional theory calculations show that an overall antiferromagnetic structure with ferromagnetic coupling of the spins in the ab plane minimizes the total energy. The band structures calculated for Mn2SiS4 and Mn2SiSe4 reveal features near the band edges similar to those reported for Fe-based olivines suggested as thermoelectrics; however the experimentally determined thermal transport data do not support superior thermoelectric features. The transition from long-range magnetic order in Mn2SiS4 to short-range order and spin fluctuations in Mn2SiSe4 is explained using the variation of the Mn-Mn distances in the triangle units that constitutes the sawtooth lattice upon progressive replacement of sulfur with selenium. Overall, the results presented here point towards the role played by magnetic anisotropy and geometric frustration in the antiferromagnetic state of the sawtooth olivines.