Large orbital moment and spin-orbit enabled Mott transition in the Ising Fe honeycomb lattice BaFe2(PO4)2

TL;DR

This study reveals how spin-orbit coupling and large orbital moments induce a Mott transition in BaFe2(PO4)2, an Ising ferromagnet with complex electronic and structural behavior, challenging traditional correlation-driven gap opening models.

Contribution

It demonstrates the critical role of spin-orbit coupling in enabling a Mott transition and large orbital moments in a Fe-based honeycomb lattice, highlighting mechanisms beyond standard Hubbard interactions.

Findings

Large orbital moment (>0.7 μB) due to SOC and correlations.

SOC-induced degeneracy lifting enables the Mott gap.

Structural transitions linked to orbital occupation changes.

Abstract

BaFe2(PO4)2 is an unusual Ising insulating ferromagnet based on the Fe$^{2+}$ spin $S$ = 2 ion, the susceptibility of which suggests a large orbital component to the Fe local moment. We apply density functional theory based methods to obtain a microscopic picture of the competing interactions and the critical role of spin-orbit coupling (SOC) in this honeycomb lattice system. The low-temperature ferromagnetic phase displays a half-semimetallic Dirac point pinning the Fermi level and preventing gap opening before consideration of SOC, presenting a case in which correlation effects modeled by a repulsive Hubbard $U$ fail to open a gap. Simultaneous inclusion of both correlation and SOC drives a large orbital moment in excess of 0.7 $\mu_B$ (essentially $L$ = 1) for spin aligned along the $\hat{c}$ axis, with a gap comparable with the inferred experimental value. The large orbital moment accounts for the large Ising anisotropy, in spite of the small magnitude of the SOC strength on the 3$d$ (Fe) ion. Ultimately, the Mott-Hubbard gap is enabled by degeneracy lifting by SOC and the large Fe moments, rather than by standard Hubbard interactions alone. We suggest that competing orbital occupations are responsible for the structural transitions involved in the observed re-entrant rhombohedral-triclinic-rhombohedral sequence.