Origin of insulating ferromagnetism in iron oxychalcogenide Ce${}_2$O${}_2$FeSe${}_2$

An insulating ferromagnetic (FM) phase exists in the quasi-one-dimensional iron chalcogenide Ce${}_2$O${}_2$FeSe${}_2$ but its origin is unknown. To understand the FM mechanism, here a systematic investigation of this material is provided, analyzing the competition between ferromagnetic and antiferromagnetic tendencies and the interplay of hoppings, Coulomb interactions, Hund’s coupling, and crystal-field splittings. Our intuitive analysis based on second-order perturbation theory shows that large entanglements between doubly-occupied and half-filled orbitals play a key role in stabilizing the FM order in Ce${}_2$O${}_2$FeSe${}_2$. In addition, via many-body computational techniques applied to a multi-orbital Hubbard model, the phase diagram confirms the proposed FM mechanism, in agreement with experiments.