Magnetic and structural properties of the iron oxychalcogenides La2O2Fe2OM2(M=S,Se)

We present the results of structural and magnetic phase comparisons of the iron oxychalcogenides ${\mathrm{La}}_{2}{\mathrm{O}}_{2}{\mathrm{Fe}}_{2}\mathrm{O}M{}_{2}(M=\text{S},\text{Se})$. Elastic neutron scattering reveals that $M=\text{S}$ and Se have similar nuclear structures at room and low temperatures. Our neutron diffraction data reveals that both materials obtain antiferromagnetic ordering at a N\'eel temperature ${T}_{N}90.1\phantom{\rule{4pt}{0ex}}\ifmmode\pm\else\textpm\fi{}0.16$ K and $107.2\ifmmode\pm\else\textpm\fi{}0.06$ K for $M=\text{Se}$ and S, respectively. The magnetic arrangements for both $M=\text{S}$, Se compounds are obtained through Rietveld refinement. We find the order parameter exponent $\ensuremath{\beta}$ to be 0.$129\ifmmode\pm\else\textpm\fi{}0.006$ for $M=\text{Se}$ and $0.133\ifmmode\pm\else\textpm\fi{}0.007$ for $M=\text{S}$. Each of these values is near the Ising symmetry value of 1/8. This suggests that although lattice and electronic structural modifications result from chalcogen replacement, the nature of the magnetic interactions is similar in these materials.