Making EuO multiferroic by epitaxial strain engineering

Veronica Goian,Yakun Yuan,Phillipe Ghosez,Venkatraman Gopalan,Darrell G Schlom,Hua Zhou,Eric Bousquet,Alexander Melville,Rainer Held,Stanislav Kamba,Nicola A Spaldin

doi:10.1038/s43246-020-00075-1

Abstract

Multiferroics are materials exhibiting the coexistence of ferroelectricity and ideally ferromagnetism. Unfortunately, most known magnetoelectric multiferroics combine ferroelectricity with antiferromagnetism or with weak ferromagnetism. Here, following previous theoretical predictions, we provide clear experimental indications that ferroelectricity can be induced by epitaxial tensile strain in the ferromagnetic simple binary oxide EuO. We investigate the ferroelectric phase transition using infrared reflectance spectroscopy, finding that the frequency of the soft optical phonon reduces with increasing tensile strain and decreasing temperature. We observe such a soft mode anomaly at 100 K in (EuO)2/(BaO)2 superlattices grown epitaxially on (LaAlO3)0.29-(SrAl1/2Ta1/2O3)0.71 substrates, which is a typical signature for a displacive ferroelectric phase transition. The EuO in this superlattice is nominally subjected to 6.4% biaxial tensile strain, i.e., 50% more than believed needed from previously published calculations. We interpret our results with new first-principles density functional calculations using a hybrid functional, which provides a better quantitative agreement with experiment than the previously used local-density approximation and generalized gradient approximation functionals.

Highlights

Multiferroics are materials exhibiting the coexistence of ferroelectricity and ideally ferromagnetism
Other recent successful examples are the cases of CaMnO315 in which incipient ferroelectricity was observed below 25 K under 2.4% tensile strain[16], or SrMnO36, in which a phase transition to a polar state at 380 K has been observed for 1.7% tensile strain[17], while the antiferromagnetic transition temperature is simultaneously shifted from the bulk value of 230 K18,19 to 180 K in the strained film[19]
Another interesting case is strained NaMnF3, which was predicted to be ferroelectric with a weak ferromagnetic moment from density functional theory (DFT) simulations[8] and the multiferroic behavior was subsequently confirmed experimentally in films grown on SrTiO3 substrates[9]

Summary

Introduction

Multiferroics are materials exhibiting the coexistence of ferroelectricity and ideally ferromagnetism. Other recent successful examples are the cases of CaMnO315 in which incipient ferroelectricity was observed below 25 K under 2.4% tensile strain[16], or SrMnO36, in which a phase transition to a polar state at 380 K has been observed for 1.7% tensile strain[17], while the antiferromagnetic transition temperature is simultaneously shifted from the bulk value of 230 K18,19 to 180 K in the strained film[19] Another interesting case is strained NaMnF3, which was predicted to be ferroelectric with a weak ferromagnetic moment from DFT simulations[8] and the multiferroic behavior was subsequently confirmed experimentally in films grown on SrTiO3 substrates[9]. These calculations show that the predicted critical biaxial tensile strain to destabilize the polar order is higher (+5.8%) than the previously reported prediction (+4%)[7], and more consistent with our experimental observations

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Methods

Results