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-(SrAl_1/2Ta_1/2O₃)_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.

多元铁磁材料是铁电和理想的铁磁材料共存的材料。不幸的是，大多数已知的磁电多铁磁将铁电与反铁磁或弱铁磁结合。在此，根据先前的理论预测，我们提供了明确的实验指示，即铁磁简单二元氧化物EuO中的外延拉伸应变可以诱发铁电。我们使用红外反射光谱研究铁电相变，发现软光学声子的频率随着拉伸应变和温度的降低而降低。我们在（LaAlO ₃）上外延生长的（EuO）₂ /（BaO）₂超晶格中在100 K处观察到这种软模式异常_0.29-（SrAl _1/2 Ta _1/2 O ₃）_0.71衬底，这是位移铁电相变的典型特征。该超晶格中的EuO名义上承受6.4％的双轴拉伸应变，即比先前公布的计算所认为的需要高50％。我们使用混合函数通过新的第一原理密度函数计算来解释我们的结果，与先前使用的局部密度近似和广义梯度近似函数相比，该函数与实验提供了更好的定量一致性。