Double Exchange Model at Low Densities: Magnetic Polarons and Coulomb Suppressed Phase Separation

TL;DR

This paper investigates the formation of magnetic polarons and the suppression of phase separation in the low-density double exchange model, linking theoretical predictions with experimental observations in europium hexaboride.

Contribution

It introduces an independent polaron model to analyze magnetic polaron formation and demonstrates how electrostatic effects suppress phase separation, connecting these phenomena to experimental data.

Findings

Electrostatic and localization effects suppress phase separation at low densities.

Magnetic polarons form ferromagnetic droplets containing single electrons.

The model reproduces the stability region of polaronic phases in europium hexaboride.

Abstract

We consider the double exchange model at very low densities. The conditions for the formation of self-trapped magnetic polarons are analyzed using an independent polaron model. The issue of phase separation in the low density region of the temperature-density phase diagram is discussed. We show how electrostatic and localization effects can lead to the substantial suppression of the phase separated regime. By examining connections between the resulting phase and the polaronic phase, we conclude that they reflect essentially the same physical situation of a ferromagnetic droplet containing one single electron. In the ultra diluted regime, we explore the possible stabilization of a Wigner crystal of magnetic polarons. Our results are compared with the experimental evidence for a polaronic phase in europium hexaboride, and we are able to reproduce the experimental region of stability of the polaronic phase. We further demonstrate that phase-separation is a general feature expected in metallic ferromagnets whose bandwidth depends on the magnetization.