Modulating Super-Exchange Strength to Achieve Robust Ferromagnetic Couplings in Two-Dimensional Semiconductors

TL;DR

This paper demonstrates that ligand substitution with lower electronegativity elements can strengthen ferromagnetic super-exchange interactions in two-dimensional semiconductors, potentially enabling higher-temperature ferromagnetism for spintronic applications.

Contribution

It introduces a ligand modulation strategy to enhance super-exchange couplings and Curie temperature in 2D ferromagnets, supported by theoretical analysis and application to various materials.

Findings

Ligand substitution with halides strengthens ferromagnetic super-exchange.

Enhanced electron hopping and reduced energy gaps improve magnetic coupling.

Ligand modulation applies broadly to other 2D ferromagnetic materials.

Abstract

Low-dimensional semiconducting ferromagnets have attracted considerable attention due to their promising applications as nano-size spintronics. However, realizing robust ferromagnetic couplings that can survive at high temperature is restrained by two decisive factors: super-exchange couplings and anisotropy. Despite widely explored low-dimensional anisotropy, strengthening super-exchange couplings has rarely been investigated. Here, we found that ligands with lower electronegativity can strengthen ferromagnetic super-exchange couplings and further proposed the ligand modulation strategy to enhance the Curie temperature of low-dimensional ferromagnets. Based on the metallic CrX2 (X = S, Se, Te) family, substituting ligand atoms by halides can form stable semiconducting phase as CrSeCl, CrSeBr and CrTeBr. It is interesting to discover that, the nearest ferromagnetic super-exchange couplings can be strengthened when substituting ligands from S to Se and Te. Such evolution originates from the enhanced electron hopping integral and reduced energy intervals between d and p orbits. While the second nearest anti-ferromagnetic couplings are also benefitted due to delocalized p-p interactions. Finally, ligand modulation strategy is applied in other ferromagnetic monolayers, further verifying our theory and providing a fundamental understanding on controlling super-exchange couplings in low-dimension.