Super-Exchange Theory for Polyvalent Anion Magnets

TL;DR

This paper introduces an extended super-exchange theory (ESET) that accurately predicts magnetic interactions in materials with polyvalent anions, surpassing traditional GKA rules, and clarifies magnetic ordering in CrOCl.

Contribution

The paper proposes ESET, a novel super-exchange theory that accounts for multiple valence states of anions, improving prediction of magnetic properties in complex materials.

Findings

ESET predicts magnetic exchange constants and ground states accurately.

CrOCl exhibits antiferromagnetic ordering, contrary to previous reports.

ESET's predictions align with first-principles calculations and experiments.

Abstract

The Goodenough-Kanamori-Anderson (GKA) rules have been widely applied for explaining the magnetic properties induced by super-exchange interaction. As conclusions of the super-exchange theory, they reveal the antiferromagnetic (ferromagnetic) ordering along with bond angle of 180 degrees (90 degrees) in the cation-anion-cation interaction path, in which the theory sets a pre-condition that the electronic states of cations in all paths are identical. We observed that the GKA rules are in fact not universal and even invalid to materials containing anions with different valence states, for example, the layered CrOCl crystal (with two valence states of anions: O2- and Cl-). In this study, we propose an extended super-exchange theory (ESET) related to superposed electronic states of cation in a specific path. ESET is capable of predicting not only the sign and relative magnitude of magnetic exchange constants in different cation-anion-cation paths, but also the magnetic ground state. Through our proposed theory, we conclude that the magnetic ordering along with bond angle of 90 degrees in Cr-Cl-Cr path is moderately antiferromagnetic and of 180 degrees in Cr-O-Cr path is strongly ferromagnetic, which are opposite to the contents of GKA rules. Moreover, we clarify that monolayer CrOCl has antiferromagnetic ordering rather than ferromagnetic as reported recently. The reliability of ESET is verified via first-principles calculation and previous experimental report as well, and its universality is also demonstrated. Thus, our theory is powerful to predict the magnetic properties, which makes it possible to design new high Curie temperature two-dimensional semiconducting ferromagnets with polyvalent anion materials.