Exchange coupling in Eu monochalcogenides from first principles

TL;DR

This study uses first-principles density functional calculations to analyze exchange interactions and magnetic ordering temperatures in Eu monochalcogenides, revealing pressure effects and the role of virtual f-state processes.

Contribution

It provides a quantitative first-principles analysis of exchange interactions and magnetic transitions in EuX compounds, including pressure effects and the influence of unoccupied f states.

Findings

Curie temperatures increase significantly with pressure.

Ferromagnetic tendency is enhanced from EuTe to EuO.

Virtual processes involving unoccupied f states contribute notably to exchange.

Abstract

Using a density functional method with explicit account for strong Coulomb repulsion within the 4f shell, we calculate effective exchange parameters and the corresponding ordering temperatures of the (ferro)magnetic insulating Eu monochalcogenides (EuX; X=O,S,Se,Te) at ambient and elevated pressure conditions. Our results provide quantitative account of the many-fold increase of the Curie temperatures with applied pressure and reproduce well the enhancement of the tendency toward ferromagnetic ordering across the series from telluride to oxide, including the crossover from antiferromagnetic to ferromagnetic ordering under pressure in EuTe and EuSe. The first and second neighbor effective exchange are shown to follow different functional dependencies. Finally, model calculations indicate a significant contribution of virtual processes involving the unoccupied f states to the effective exchange.