Origin of ferromagnetism in Cs${}_2$AgF${}_4$: importance of Ag - F covalency

Abstract

The magnetic nature of Cs${}_2$AgF${}_4$, an isoelectronic and isostructural analogue of La${}_2$CuO${}_4$, is analyzed using density functional calculations. The ground state is found to be ferromagnetic and nearly half metallic. We find strong hybridization of Ag-$d$ and F-$p$ states. Substantial moments reside on the F atoms, which is unusual for the halides and reflects the chemistry of the Ag(II) ions in this compound. This provides the mechanism for ferromagnetism, which we find to be itinerant in character, a result of a Stoner instability enhanced by Hund's coupling on the F.