Theory of Curie temperature enhancement in electron-doped EuO

TL;DR

This paper provides a theoretical analysis of how doping influences the Curie temperature in EuO, highlighting the role of Gd doping and oxygen deficiency in enhancing ferromagnetic properties.

Contribution

It introduces a comparative theoretical framework explaining the different effects of Gd doping and oxygen deficiency on EuO's Curie temperature, emphasizing the role of spin fluctuations and dopant activation.

Findings

Gd doping enhances $T_C$ due to Kondo-like spin fluctuations.

Oxygen deficiency does not produce the same $T_C$ enhancement.

Saturation of $T_C$ at high doping levels is due to reduced dopant electron activation.

Abstract

We present a comparative, theoretical study of the doping dependence of the critical temperature $T_C$ of the ferromagnetic insulator-metal transition in Gd-doped and O-deficient EuO, respectively. The strong $T_C$ enhancement in Eu$_{1-x}$Gd$_x$O is due to Kondo-like spin fluctuations on the Gd sites, which are absent in EuO$_{1-x}$. Moreover, we find that the $T_C$ saturation in Eu$_{1-x}$Gd$_x$O for large $x$ is due to a reduced activation of dopant electrons into the conduction band, in agreement with experiments, rather than antiferromagnetic long-range contributions of the RKKY interaction. The results shed light on possibilities for further increasing $T_C$.