Epitaxial strain can induce displacive ferroelectric phase transitions in incipient ferroelectrics, which can be recognized by observation of anomalies in the frequencies of polar soft phonons. For that reason, our group developed a unique method of investigating phonons in ultrathin films using IR reflectance and THz transmission spectroscopy. In the past we demonstrated it in the tensile strained SrTiO3, Srn+1TinO3n+1, EuTiO3 and EuO. In superlattices of EuO/BaO we achieved 6.3 % tensile strain in EuO which has induced the ferroelectric phase transition near 100 K. In this way we prepared a new ferroelectric ferromagnet with possible high magnetoelectric coupling. We also investigated compressively strained MnO and NiO thin films, where a strain-dependent phonon splitting was observed in the antiferromagnetic phase. The effect was successfully explained by the strain dependence of the magnetic exchange coupling coefficient J1. In progress are studies of the strained Sr1-xBaxTiO3 with perovskite and Ruddlesden-Popper structure, where we observed an anomalously low microwave losses and high electric field tunability of the permittivity. Our experiments confirmed that the soft phonon behavior is responsible for the unique microwave properties.
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