Quasi-two-dimensional non-collinear magnetism in the Mott insulator Sr${}_2$F${}_2$Fe${}_2$OS${}_2$

Abstract

The magnetism of Sr${}_2$F${}_2$Fe${}_2$OS${}_2$ was examined through neutron powder diffraction and thermodynamic and transport measurements. Quasi-two-dimensional magnetic order develops below $T_N$ = 106(2) K with an in-plane correlation length exceeding 310 Å and an out-of-plane correlation length of just 17(3) Å. The data are consistent with a two-k structure with $k_1$ = (1/2, 0, 1/2) and $k_2$ = (0, 1/2, 1/2) and an ordered moment of 3.3(1) ${\mu }_B$ oriented along the in-plane components of k. This structure is composed of orthogonal AFM chains intersecting at super-exchange mediating O sites. Density Functional Theory (DFT) also points to this structure and a narrower Fe 3d band than for the iron pnictides from which electronic correlations produce a Mott insulator.