Spin-Orbital States and Strong Antiferromagnetism of Layered Eu2SrFe2O6 and Sr3Fe2O4Cl2.

The insulating iron compounds Eu2SrFe2O6 and Sr3Fe2O4Cl2 have high-temperature antiferromagnetic (AF) order despite their different layered structures. Here, we carry out density functional calculations and Monte Carlo simulations to study their electronic structures and magnetic properties aided with analyses of the crystal field, magnetic anisotropy, and superexchange. We find that both compounds are Mott insulators and in the high-spin (HS) Fe2+ state (S = 2) accompanied by the weakened crystal field. Although they have different local coordination and crystal fields, the Fe2+ ions have the same level sequence and ground-state configuration (3z2-r2)2(xz, yz)2(xy)1(x2-y2)1. Then, the multiorbital superexchange produces strong AF couplings, and the (3z2-r2)/(xz, yz) mixing via the spin-orbit coupling (SOC) yields a small in-plane orbital moment and anisotropy. Indeed, by tracing a set of different spin-orbital states, our density functional calculations confirm the strong AF couplings and the easy planar magnetization for both compounds. Moreover, using the derived magnetic parameters, our Monte Carlo simulations give the Néel temperature TN = 420 K (372 K) for the former (the latter), which well reproduce the experimental results. Therefore, the present study provides a unified picture for Eu2SrFe2O6 and Sr3Fe2O4Cl2 concerning their electronic and magnetic properties.