Electronic structure and magnetic properties of Gd-doped and Eu-rich EuO

TL;DR

This study uses first-principles calculations and Monte Carlo simulations to analyze how Gd doping and oxygen vacancies influence the magnetic properties and Curie temperature of EuO, revealing optimal doping levels and defect concentrations for enhanced magnetism.

Contribution

It provides a detailed theoretical investigation of the electronic and magnetic effects of Gd doping and oxygen vacancies in EuO, including the construction of magnetostructural cluster expansions.

Findings

Maximum Curie temperature of about 150 K for Gd-doped EuO.

Curie temperature increases to 120 K at 3.125% vacancy concentration.

Impurity states broaden into bands at higher vacancy levels.

Abstract

The effects of Gd doping and O vacancies on the magnetic interaction and Curie temperature of EuO are studied using first-principles calculations. Linear response calculations in the virtual crystal approximation show a broad maximum in the Curie temperature as a function of doping, which results from the combination of the saturating contribution from indirect exchange and a decreasing contribution from the f-d hopping mechanism. Non-Heisenberg interaction at low doping levels and its effect on the Curie temperature are examined. The electronic structure of a substitutional Gd and of an O vacancy in EuO are evaluated. When the 4f spins are disordered, the impurity state goes from single to double occupation, but correlated bound magnetic polarons are not ruled out. At higher vacancy concentrations typical for Eu-rich EuO films, the impurity states broaden into bands and remain partially filled. To go beyond the homogeneous doping picture, magnetostructural cluster expansions are constructed, which describe the modified exchange parameters near Gd dopants or O vacancies. Thermodynamic properties are studied using Monte Carlo simulations. The Curie temperature for Gd-doped EuO agrees with the results of the virtual crystal approximation and shows a maximum of about 150 K. At 3.125% vacancy concentration the Curie temperature increases to 120 K, consistent with experimental data for Eu-rich film samples.