Generalized calculation of magnetic coupling constants for Mott-Hubbard insulators: Application to ferromagnetic Cr compounds

TL;DR

This paper derives analytic formulas for magnetic coupling constants in Mott-Hubbard insulators using perturbation theory, confirming some classical rules and exploring conditions favoring ferromagnetism, with applications to chromium compounds.

Contribution

It provides a generalized analytical method for calculating super-exchange coupling constants in Mott-Hubbard insulators, extending understanding beyond traditional rules.

Findings

Short-path interactions align with Goodenough-Kanamori-Anderson rules.

Less than half-filled d shells favor ferromagnetic coupling.

Ferromagnetism increases with longer exchange paths.

Abstract

Using a Rayleigh-Schr\"odinger perturbation expansion of multi-band Hubbard models, we present analytic expressions for the super-exchange coupling constants between magnetic transition metal ions of arbitrary separation in Mott-Hubbard insulators. The only restrictions are i) all ligand ions are closed shell anions and ii) all contributing interaction paths are of equal length. For short paths, our results essentially confirm the Goodenough-Kanamori-Anderson rules, yet in general there does not exist any simple rule to predict the sign of the magnetic coupling constants. The most favorable situation for ferromagnetic coupling is found for ions with less than half filled d shells, the (relative) tendency to ferromagnetic coupling increases with increasing path length. As an application, the magnetic interactions of the Cr compounds Rb$_2$CrCl$_4$, CrCl$_3$, CrBr$_3$ and CrI$_3$ are investigated, all of which except CrCl$_3$ are ferromagnets.