Exploring the intrinsic limit of the charge-carrier-induced increase of the Curie temperature of Lu- and La-doped EuO thin films

R Held,A Melville,D Hodash,J A Mundy,Z Wang,M E Holtz,S T Dacek,T Mairoser,J T Heron,B Holländer,D G Schlom,D A Muller

doi:10.1103/physrevmaterials.4.104412

Abstract

Raising the Curie temperature ${T}_{\mathrm{C}}$ of the highly spin-polarized semiconductor EuO by doping it with rare-earth elements is a strategy to make EuO more technologically relevant to spintronics. The increase of ${T}_{\mathrm{C}}$ with free carrier density $n$ and the surprisingly low dopant activation $p$, found in Gd-doped EuO thin films [Mairoser et al., Phys. Rev. Lett. 105, 257206 (2010)], raised the important question of whether ${T}_{\mathrm{C}}$ could be considerably enhanced by increasing $p$. Using a low-temperature growth method for depositing high-quality Lu-doped EuO films we attain high dopant activation ($p$) values of up to 67%, effectively more than doubling $p$ as compared to adsorption-controlled growth of Lu- and Gd-doped EuO. Relating $n, p$, and lattice compression of La- and Lu-doped EuO films grown at different temperatures to the ${T}_{\mathrm{C}}$ of these samples allows us to identify several different mechanisms influencing ${T}_{\mathrm{C}}$ and causing an experimental maximum in ${T}_{\mathrm{C}}$. In addition, scanning transmission electron microscopy in combination with electron energy loss spectroscopy measurements on La-doped EuO indicate that extensive dopant clustering is one, but not the sole reason for dopant deactivation in rare-earth doped EuO films.

Highlights

Many of the remarkable properties of the semiconductor EuO, including strong ferromagnetism, large Faraday rotation, and a giant insulator-to-metal transition, were discovered more than 40 years ago [1,2,3,4]
The increasing structural perfection of EuO films grown directly on silicon may allow fabricating efficient spin filter contacts to silicon providing an alternative route for integrating spin filter functionality into silicon [8,9,10]
P, and TC of La- and Lu-doped EuO films grown at different temperatures, we show that increasing n beyond ∼2 − 4 × 1020 cm−3 has only a very limited effect on TC in these samples

Summary

Introduction

Many of the remarkable properties of the semiconductor EuO, including strong ferromagnetism, large Faraday rotation, and a giant insulator-to-metal transition, were discovered more than 40 years ago [1,2,3,4]. The increasing structural perfection of EuO films grown directly on silicon may allow fabricating efficient spin filter contacts to silicon providing an alternative route for integrating spin filter functionality into silicon [8,9,10]. These qualities render EuO an outstanding material for spintronic studies and proof-ofconcept devices. Deposition of the overoxidized half-metallic semiconductor EuO does not always require expensive ultrahigh-vacuum equipment: By employing topotactic transformation EuO thin films with excellent quality can be fabricated using just high-vacuum means [11].

Methods

Results

Conclusion