Covalent bonds against magnetism in transition metal compounds

TL;DR

This paper reveals that covalent metal-metal bonds can suppress magnetism in transition metal compounds, especially in 4d and 5d systems, challenging the traditional isolated ion approach and highlighting a new intersite effect.

Contribution

It introduces a novel mechanism where covalent bonding between metal sites affects magnetic properties, especially in noninteger electron systems, and demonstrates its relevance through analytical and numerical studies.

Findings

Covalent bonds can suppress double exchange and ferromagnetism.

The effect is significant in 4d and 5d compounds like Nb₂O₂F₃ and Ba₅AlIr₂O₁₁.

This mechanism may also influence 3d oxides such as CrO₂ under pressure.

Abstract

Magnetism in transition metal compounds is usually considered starting from a description of isolated ions, as exact as possible, and treating their (exchange) interaction at a later stage. We show that this standard approach may break down in many cases, especially in $4d$ and $5d$ compounds. We argue that there is an important intersite effect -- an orbital-selective formation of covalent metal-metal bonds, which leads to an "exclusion" of corresponding electrons from the magnetic subsystem, and thus strongly affects magnetic properties of the system. This effect is especially prominent for noninteger electron number, when it results in suppression of the famous double exchange, the main mechanism of ferromagnetism in transition metal compounds. We study this novel mechanism analytically and numerically and show that it explains magnetic properties of not only several $4d-5d$ materials, including Nb$_2$O$_2$F$_3$ and Ba$_5$AlIr$_2$O$_{11}$, but can also be operative in $3d$ transition metal oxides, e.g. in CrO$_2$ under pressure. We also discuss the role of spin-orbit coupling on the competition between covalency and magnetism. Our results demonstrate that strong intersite coupling may invalidate the standard single-site starting point for considering magnetism, and can lead to a qualitatively new behaviour.