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

TL;DR

This study reveals that Sr$_2$F$_2$Fe$_2$OS$_2$ exhibits quasi-two-dimensional non-collinear magnetic order below 106 K, characterized by a complex two-k structure and Mott insulating behavior driven by electronic correlations.

Contribution

It provides the first detailed neutron diffraction and DFT analysis of the non-collinear magnetism and Mott insulating state in Sr$_2$F$_2$Fe$_2$OS$_2$, highlighting its unique magnetic structure.

Findings

Quasi-two-dimensional magnetic order develops below 106 K.

Magnetic structure involves orthogonal AFM chains with a two-k pattern.

Electronic correlations lead to a Mott insulating state.

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 \AA\ and an out-of-plane correlation length of just 17(3) \AA. 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.