Making EuO multiferroic by epitaxial strain engineering

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.

Ferroelectricity and multiferroicity in anti–Ruddlesden–Popper structures

“…This natural strain induces ferroelectricity, as previously highlighted in strained BaO. We note that such a level of strain (6%) is difficult to reach within epitaxial films of rock salt (55). In Ba4Sb2O, however, polarization increases much more slowly with strain than in BaO, due to the presence of Sb atoms which limit the deformation of octahedra in the FE phase (see SI Appendix).…”

“…Eu4Sb2O, similar to its parent rock salt EuO, is a ferromagnetic insulating oxide (70). The coexistence of ferromagnetism and ferroelectricity has just been confirmed experimentally in epitaxially strained EuO films (55) and is naturally appearing in Eu4Sb2O anti-Ruddlesden-Popper phase. The magnetic space group I 4m m is compatible with linear magnetoelectric coupling, and the magnetoelectric tensor has the following form (71):…”

Chaos at Interface Brings Order into Oxide/Silicon Structure

“…To optimize the quality of the EuO/Si interface, we turn to LT conditions, employed recently for EuO synthesis on various substrates to enhance the carrier concentration [ 17 ] or make EuO multiferroic by strain engineering. [ 18 ] This type of EuO growth is more challenging, requiring Eu flux to match that of oxygen. In the present study, a number of EuO films of 6 nm thickness have been synthesized; particular details of the syntheses are given in the Experimental Section.…”

“…To test this hypothesis, we study here LT synthesis of the ferromagnetic semiconductor EuO on Si. The oxide is known for advanced properties tunable by doping and strain engineering; [ 17,18 ] it enjoys record magneto‐optical [ 19 ] and magnetotransport [ 20 ] characteristics. Recently, EuO has drawn attention as a component of oxide structures generating a 2D superconducting electron gas.…”

Growth and Characterisation Studies of Eu3O4 Thin Films Grown on Si/SiO2 and Graphene

“…Eu-chalcogenides and Eu-oxides attracted great attention as promising materials for magneto-optical device applications [ 18 , 19 , 20 ]. Among these materials, a special interest was given to EuO due to its desirable properties for potential spintronics applications such as spin-filter [ 14 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ]. On the other hand, only a few papers are found on the Eu O compound [ 2 , 3 , 4 , 5 , 27 , 28 , 29 , 30 , 31 ], making it an interesting metamagnetic compound to be studied as there are still many aspects to be explored on this material.…”